APPLE
Malus sylvestris Mill., family RosaceaeIn 1969, about 6.7 billion pounds of apples, valued at $274.4 million, were produced in the United States. In the six States where almost two- thirds of the entire crop was produced, the volume, in million pounds, was: Washington, 1,675; New York, 855; Michigan, 720; California, 540; Pennsylvania, 525; and Virginia, 472.
Hedrick (1938*) stated that 4,000 to 5,000 cultivars of apples were described, but Henderson et al. (1969) showed that fewer than two dozen cultivars account for 95 percent of the total crop. The leading cultivar is 'Delicious', which accounts for 30 percent of the total production. 'Golden Delicious' ranks second and accounts for 13 percent. Other leading cultivars and their percentages of the total crop are: 'McIntosh', 10 percent; 'Rome Beauty', 8 percent; 'Jonathan', 6 percent; and 'York Imperial', 5 percent.
Plant:
The apple tree may reach a height of 40 feet or more; however, for various cultural reasons, commercial apple growers keep their trees of standard rootstock less than half that high. Trees on the recently developed dwarf (fig. 37) and semidwarf rootstock (Tydeman 1955) in the newer orchards and replants may be less than 10 feet. This development of dwarf apples is so changing apple production that much of the older information on culture, pollination, and harvest of this crop may no longer be applicable. An example of the difference in the size and planting rate of apple trees is given in table 7.
Many of the older trees were spaced 40 by 40 feet (27 per acre) and took 25 years to reach their maximum production of 500 bushels ( a bushel weighs about 44 pounds) per acre (Anonymous 1969). Snyder (1968) reported production of 113 to 377 bu/acre on 21 farms observed in western New York, where the number of trees ranged from 70 to 182 and averaged 91 per acre. Kelly ( n.d. ) reported 313 bu/acre on 18 farms in Pennsylvania, where over 50 percent of the trees were standard cultivars,. Henderson et al. (1969) reported an agerage of 592 bu/acre for California.
By using dwarf apple trees, the growers can have as many as 1,000 trees per acre, and expect a maximum production of 900 bushels in 6 years on 'Jonathan' trees, or as much as 1,300 bu/acre on 'Golden Delicious' (Anonymous 1971). The smaller trees yield more per acre, reach maximum production at a much earlier age, are more easily pruned and sprayed, and the fruit is much more accessible for thinning and harvesting (Shoemaker and Teskey 1959, Gaylord 1965).
Norton (1971) considered the density of the trees per acre as follows: Low 75 to 150 trees; medium, 200 to 300 trees; high, 400 to 800 trees; ultra-high, 1,000 or more trees.
[gfx] FIGURE 37. - Dwarf apple tree in blossom.
TABLE 7. - Difference in the size and planting rate of apple trees1
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Inflorescence:
The apple flower cluster, made up of about six flowers, is produced on a 1- to 3-year-old woody shoot, l/2 inch to 2 inches long, called a spur. The clusters are usually found at the tip of the spur in the axils of leaves, and are formed the previous summer (Bradford 1915, Latimer 1933). The primary or "king" bud opens first, and usually produces the choicest fruit. If the king bloom fails, the lateral blooms, which open a day or more later, can also produce fruit. Howlett (1926a) showed, however, that the lateral flowers are much more likely to shed, making the preservation of the king bud still more important. The five pinkish-white petals of the 1- to 1 1/2-inch broad and pleasantly scented blossom (fig. 38) shed a few days after they open, but the five green sepals persist in a dried shriveled state in the blossom end of the mature fruit.
The five stigmas, which unite into a common style that leads to the ovary, are surrounded by 20 to 25 erect pollen-bearing stamens. Nectar is secreted between the bases of the stamens and the style.
The ovary is divided into five compartments, each containing two ovules (four in the case of the cultivar 'Northern Spy') so that 10 (or 20 in 'Northern Spy') seeds may develop (Goff 1899, 1901).
The apple flower produces both nectar and pollen in abundance, more nectar than most of our other deciduous fruit trees produce (Smith and Bradt 1967*). Apple pollen and nectar are eagerly collected by honey bees, and are important contributors to spring buildup in honey bee colonies. Colonies usually arrive in the orchard low on stores and relatively weak, the period of bloom is short, and frequently the weather is unfavorable for bee activity. This prevents the storage of surplus honey, so that apple honey on the market is rare. The amount that the bees are able to collect is left in the hive for food reserves.
The average blossoming period for apples is about 9 days. Cool weather lengthens and warm or dry windy weather shortens this period (Morris 1921). Bee activity on apples during the day is usually greatest about 9 a.m. (Brittain 1933). Although numerous blossoms appear on the apple tree, a set of only 5 percent will produce a fair apple crop (McDaniels and Heinicke 1929, Brittain 1935).
[gfx] FIGURE 38. - Longitudinal section of 'Delicious' apple blossom x 6.
Pollination Requirements:
The pollination of apples has been of interest since Cooke (1745) stated that the "farina" (pollen) of one apple tree influenced the fruit of another. Eventually, Wicks (1918) showed that foreign pollen does not bestow a benefit to the fruit in either size, shape, color, or quality. The pollen stimulates development of the seed, which in turn produces an auxin that stimulates adjoining tissue to develop. Of course, the pollen influences the offspring that develops from the seed.
The fertilization of every ovule in the ovary is not essential to fruit development, but the larger the number fertilized the greater the likelihood that the fruit will succeed in the competition for the plant's nutrients and remain on the tree until harvest (Brittain 1933, Tydeman 1943). Usually, the more seeds that develop in the apple, the larger it is (Murneek and Schowengert 1935). About six or seven seeds are necessary for good fruit set (Hartman and Howlett 19S4). Some apple selections set seedless fruit without pollination, but no commercial cultivar has this characteristic (Chan and Cain 1967).
The research by Waite (1895, 1899) produced the first concrete evidence that apples and other pomaceous fruits benefit from the interplanting of and cross-pollination between cultivars, and that pollinating insects are essential for transferring the pollen between compatible cultivars. This research led scores of other scientists to study the pollination requirements of apples, both in the United States and abroad. These studies have been reviewed by Hutson (1926), Brittain (1933), and Free (1960, 1970*), who also conducted research on the subject.
Griggs (1970*) stated that all apple cultivars are self-incompatible to some degree. Some set no fruit at all when self-pollinated; others set various proportions of a commercial crop under favorable conditions. He also stated that the self-fuitfulness of an individual cultivar may vary in different parts of the country, but apple specialists generally agree that no apple cultivar is sufficiently self-fertile to be dependably productive when planted alone. The grower, then, has no choice except to interplant. His problem is to find the most satisfactory and profitable combination of cultivars to produce his crop.
Studies, in particular by Brittain (1933), Burrell and Parker (1931), Latimer (1931), MacDaniels and Heinecke (1929), and Overholser (1927), proved that interplanting of cultivars was necessary, but that all cultivars were not equally compatible. The best pollenizer cultivar is one that has the most compatible pollen, and it blooms at the same time as the main cultivar. Although numerous studies have been made on the pollination of apples, we may not have full information on these points for all major cultivars in all apple-growing regions.
In selecting appropriate cultivars for interplanting, the grower should choose those that flower at the same time. Way (1971) showed that, at least in New York, flowering of early, midseason, and late cultivars generally overlaps sufficiently for their use as pollinators of any commercial cultivar. In the southern section of the apple-growing regions, this difference between cultivars increases, and an overlapping of flowering dates is less likely to occur. This increases the importance of selecting cultivars that flower at the appropriate time. Compatible cultivars should, of course, also be chosen.
The importance of compatibility of cultivars, even when they flower at the right time, was shown by Overholser (1927). The cultivar 'Newtown' set 51.5 percent of its blossoms when cross-pollinated with 'Bellflower' (under a tent enclosing a colony of honey bees, which, presumably, provided maximum cross-pollination), but 'Bellflower' set only 4.3 percent of its flowers that were cross-pollinated with 'Newtown'.
Frost:
The damaging effect of frost is sometimes blamed for poor yields, when, actually, the problem is inadequate cross-pollination. However, blossoms that have been pollinated are believed to be less susceptible to frost damage than nonpollinated ones. The grower should strive, therefore, to get the flowers pollinated as soon as possible after they open. This increased effort to get the flowers pollinated may result in excessive set of fruit some seasons, but excess fruit can be thinned. There is no way to put fruit on the tree after flowering has ceased. As Rom (1970) stated, "Pollination is without question the most critical event in the yearly production cycle [of apples]."
Problems with Interplanting for Cross-Pollination:
In one planting pattern that has been used, every third tree in every third row is a pollenizer. This places every tree of the main cultivar next to a pollenizer. This plan was satisfactory, from both the pollination and the harvesting standpoint, with standard cultivars and separated trees.
In high-density orchards, the trees within the row frequently form a hedge. If pollenizers are planted within the row, the pickers or picking machines fail to separate the fruit from the two cultivars, which may be necessary for the packaging of uniform fruit. If the pollenizer trees are planted on separate rows, the bees, being inclined to forage only within the row rather than to cross the intervening space between rows, become ineffective.
In an attempt to solve this problem, some growers are seeking a small pyramidal crabapple selection that might serve within the row as a pollenizer, occupying little space, furnishing compatible pollen for the main cultivar, yet producing fruit unlikely to be harvested with that of the main cultivar. This should be a satisfactory solution, if the flowers are equally attractive, so that the bees will forage indiscriminately between flowers.
Beekeeper Problems with Dwarf Trees:
Beekeepers who provide colonies for the pollination of apples claim that the narrow spacing between rows of dwarf apple trees creates a maneuvering problem for large vehicles used in transporting bee colonies. Some beekeepers deliver the colonies to the edge of the orchard; then the grower, using a forklift or other small vehicle, distributes the colonies within the orchard.
Pollinators:
The need for an appropriate agent to transfer poller from one self- incompatible cultivar to another was established by Waite (1895, 1899), although growers had associated insect pollination with increased production for years.
Wind has been suggested and disproved at various times as a possible agent in the transfer of apple pollen (Lewis and Vincent 1909, Free 1966). It is no longer considered of significance for this task.
Various wild bees have been mentioned as important pollinators of apples, including the genera Andrena, Bombus, Halictus, and Osmia (Brittain 1933,1936; Free 1964; Glukhov 1955; Hutson 1926; Kitamura and Maeta 1969; Loken 1958; Phillips 1933; Horticultural Education Association 1967). Some wild bees, for, example Osmia, visit flowers at lower temperatures than do honey bees. At times and in some areas, wild bees are sufficiently abundant to set an apple crop. In general however, wild bees cannot be depended upon to adequately pollinate the blossoms of a commercial apple orchard in the United States.
Honey bees are easily handled, and they can be concentrated within the orchard the degree desired. As a result, commercial apple growers have come to depend upon the honey bee as their apple pollinating agent.
The precise method of utilizing honey bees on apples for maximum economic production is less well defined than the appropriate agent. Free and Spencer-Booth (1963) showed that bees were consistently fewer between groups of nine colonies in the center of 9-acre blocks but not when they were in groups of four or singly at one colony per acre. The strength, placement, and manipulation of colonies, the effects of competing plants, soil, and weather, and other factors both within the colony and in the environment contribute to the effectiveness of honey bees.
Smith and Bradt (1967*) mentioned, as had various others before them, that when the honey bee visits an apple blossom for nectar its proboscis is sometimes inserted at the base of the stamens, leaving the anthers and stigma untouched (fig. 39B). When this is done, little pollination occurs. By contrast, the larger bumble bee clambers over the anthers and stigma when foraging and cannot help but transfer pollen from flower to flower. Preston (1949) found that bees visited one cultivar four times as frequently as another. He associated this difference in visitation to the accessibility of nectar in the flowers. The filaments of the 'Delicious' apple are in a narrow upright cluster, more so, according to Roberts (1945), than other cultivars. This permits the bee to alight on the petal, insert its proboscis between the upright filaments, and collect nectar without touching the stigma. For this reason, he recommended that more colonies be used to pollinate 'Delicious' than would be needed on other cultivars. When honey bees are collecting apple pollen, their pollinating efficiency on apples is much greater than when they are collecting nectar.
Beekeepers also mention that dwarf trees have more blooms per acre than trees on standard rootstock; therefore, more bees are needed on the dwarf plantings.
Griggs (1970*) stated that growers who previously worried about overpollination now favor it, knowing that no adequate set can be otherwise obtained. Then, when there is too much fruit set, they thin with chemical sprays to the desired set of fruit, which prevents alternate bearing.
Viable, compatible pollen has been distributed by hand, airplane, or other mechanical means, even by pollen dispensers attached to the entrance of beehives (Bullock and Snyder 1946, Corner et al. 1964, Jaycox 1971, Snyder 1946). When pollen is applied by any of these methods, the grower expects the pollinating insect to pick up the pollen and redistribute it to flowers that were not directly applied with the pollen. Since insects are thus required, the grower would generally get more satisfactory pollination if he would utilize more pollinating insects. A study of pollen tube growth in relation to marginal temperatures (which frequently stimulate growers to use artificial means of pollination) would be of interest. If the tube does not grow at such temperatures, the grower would be wasting his investment in these methods.
[gfx] PN-3768 FIGURE 39. -honey bee on apple blossom. A, collecting nectar; B, collecting pollen.
Pollination Recommendations and Practices:
There are no recommendations for use of wild bees on apples in the United States, but scores of papers have recommended the use of honey bees. These recommendations have changed considerably since Doolittle (1893) first suggested that apiaries of 100 colonies should be placed every few miles. The recommended placement of the colonies now is near or distributed within the orchard (fig. 40), and the recommended number of colonies has increased. These have varied from (1) one colony per 2 to 4 acres (Hooper 1913, Howlett 1926b, Kelty 1929, Kurrenoi 1969, Luce and Morris 1928, West 1912); to (2) one colony per acre (Brittain 1933, Griggs 1953*, Hutson 1926, Jaycox 1968, Lundie 1927, Phillips 1930, Philp and Vansell 1932); to (3) two or more colonies per acre (Benson 1896, Burrell and MacDaniel 1930, Rom 1970).
Many of the recommendations are based more on grower experience with use of bees than precise experimental results. The recommendations stress "strong" colonies, but the growers often leave colony strength to the discretion of the beekeeper.
Woodrow (1933, 1934) and Gooderham (1950) showed that populous colonies of honey bees were much more effective in apple pollination than weaker ones, and overwintered colonies superior to packages of bees. MacDaniels (1929) supported the value of strong colonies particularly in the ability of such colonies to effectively pollinate an orchard when only a few hours of weather were favorable for bee flight.
Even the appropriate number of bees per blossom has not been established with certainty; however, Palmer Jones and Clinch (1968) indicated that there should be one bee for each 1,000 blossoms. Petkov and Panov (1967) reported that the percentage of 'Jonathan' flowers that set increased with bee visits up to six visits per flower. They also associated larger fruit with increased numbers of bee visits.
The effectiveness of the bee is determined by the cross-visits it makes between compatible varieties. If the visits are confined to one variety they are not effective. Repeated cross-pollination of the flowers must occur to produce the optimum set. If a sufficiently large bee population is created, it superimposes over the fixed population a number of wandering bees. These wanderers consist of a few old bees driven on by competition and a larger number of young bees that have not yet become fixed to any particular area of the crop. These wanderers, which are forced to "shop around" from tree to tree to obtain their load of food, are the most valuable to the grower.
When temperatures are marginal for bee flight, bees tend to visit only the blossoms that are near the hive, and also those blossoms on the warm or leeward side of the tree. This preferential visitation can be substantially overcome by the use of strong healthy colonies and by thorough distribution of the colonies in the orchard. If the weather is fair and calm and the temperatures range into the seventies or above, a single group of colonies might adequately pollinate an orchard of many acres in a single day. With cold, cloudy, or windy days, the bees are likely to visit only trees within a few hundred feet of the hives.
The grower should expect the best but prepare for the worst. This includes providing plenty of strong colonies, appropriately distributed for getting ample pollination and a maximum harvest of highest quality fruit even under unfavorable conditions.
[gfx] FIGURE 40.- Honey bee colonies in apple orchard.
LITERATURE CITED:
ANONYMOUS.
1969. ORCHARDS GOING COMPACT TO KEEP APPLES COMPETITIVE. Food and Life Sciences (New York). N.Y. (Cornell) Agr. Expt. Sta. 2(3): 21._____ 1971. DEVELOPING DWARF APPLE TREES. Mich. Agr. Expt. Sta. ''Science in Action," ser. 17,16 pp.
BENTON, F.
1896. THE HONEY BEE - A MANUAL OF INSTRUCTION IN APICULTURE U.S. Dept. Agr., Div. Ent. Bul. 1, n.s., rev., 118 pp.BRADFORD, F. C.
1915. THE POLLINATION OF THE POMACEOUS FRUITS. Oreg. Agr. Expt. Sta. Bul. 129,16 pp.BRITTAIN, W. H.
1933. APPLE POLLINATION STUDIES IN THE ANNAPOLIS VALLEY N.S., CANADA, 1928-1932. Canada Dept. Agr. Bul. 162, n.s., 198 pp.BRITTAIN, W. H.
1935. STUDIES IN BEE ACTIVITY DURING APPLE BLOOM. Jour. Econ. Ent. 28: 553-559.BULLOCK, R. M., and SNYDER J. C.
1946. SOME METHODS OF TREE FRUIT POLLINATION. Wash. State Hort. Assoc. 42d Ann. Mtg., pp. 215-226.BURRELL, A. B., and MACDANIELS, L. H.
1931. FURTHER POLLINATION STUDIES WITH THE MCINTOSH APPLE IN THE CHAMPLAIN VALLEY OF NEW YORK. Amer. Soc. Hort. Sci. Proc. (1930): 374-385._____ and PARKER, R. G.
1932. POLLINATION OF THE MCINTOSH APPLE IN THE CHAMPLAIN VALLEY. Third Progress Report. Amer. Soc. Hort. Sci. Proc. (1931) 28: 78-84.CHAN, B. G., and CAIN, J. C.
1967. THE EFFECT OF SEED FORMATION ON SUBSEQUENT FLOWERING IN THE APPLE. Amer. Soc. Hort. Sci. Proc. 91: 63 - 68.COOKE, B.
1745. CONCERNING THE EFFECT WHICH THE FARINA OF THE BLOSSOMS OF DIFFERENT SORTS OF APPLE-TREES HAD ON THE FRUIT OF A NEIGHBORING TREE. Phil. Trans. 43: 169.CORNER, J., LAPINS, K. O., and ARRAND, J. C.
1964. ORCHARD AND HONEY BEE MANAGEMENT IN PLANNED TREE-FRUIT POLLINATION. Min. Agr. Apiary Cir. 14, 18 pp. Victoria, Brit. Columbia.DOOLITTLE, G. M.
1893. FRUIT-BLOOM FERTILIZATION. Gleanings Bee Cult. 21: 427-428.FREE, J. B.
1960. THE POLLINATION OF FRUIT TREES. Bee World 41(6): 141-151; (7): 169-186.____ 1964. COMPARISON OF THE IMPORTANCE OF INSECT AND WIND POLLINATION OF APPLE TREES. Nature 201(4920): 726 - 727.
____ 1966. THE POLLINATING EFFICIENCY OF HONEY BEE VISITS TO APPLE FLOWERS. Jour. Hort. Sci. 41: 91 - 94.
____ and SPENCER-BOOTH, Y.
1963. THE FORAGING AREAS OF HONEY-BEE COLONIES IN FRUIT ORCHARDS. Jour. Hort. Sci. 38(2): 129 - 137.GAYLORD, P.
1965. HOW WE GROW SEMI-DWARF APPLES AND THE YIELDS OBTAINED. N.Y. State Hort. Soc. Proc., 110th Ann. Mtg.: 172-175.GLUKHOV, M. M.
1955. [HONEY PLANTS.] Izd. 6, Perer. i Dop. Moskva, Gos. Izd-vo Selkhoz Lit-ry. 512 pp. [In Russian.]GOFF, E. S.
1899-1901. THE ORIGIN AND EARLY DEVELOPMENT OF THE FLOWERS IN THE CHERRY, PLUM, APPLE AND PEAR. 1899,16th Ann. Rpt. Wis. Agr. Expt. Sta.: 289 - 303; 1900, 17th Ann. Rpt. Wis. Agr. Expt. Sta.: 266 - 285; 1901, 18th Ann. Rpt. Wis. Agr. Expt. Sta.: 304-316.GOFF, E. S.
1901. ORIGIN AND DEVELOPMENT OF THE APPLE BLOSSOM. Amer. Gard. 22: 330, 346-347.GOODERHAM, C. B.
1950. OVERWINTERED COLONIES VERSUS PACKAGE BEES FOR ORCHARD POLLINATION. Canada Dept. Agr. Dominion Apiarist Prog. Rpt., 1937-48. (Abs.) Bee World 31: 96.HARTMAN, F. O., and HOWLETT, F. S.
1954. FRUIT SETTING OF THE DELICIOUS APPLE. Ohio Agr. Expt. Sta. Bul. 745, 64 pp.HENDERSON, W. W., KTTTERMAN, J. M., and NELSON, G. A.
1969. COMMERCIAL APPLES, UNITED STATES APPLE PRODUCTION BY VARIETIES. Calif. Crop and Livestock Rptg. Serv., 2 pp. Sacramento.HOOPER, C. H.
1913. THE POLLINATION AND SETTING OF FRUIT BLOSSOMS AND THEIR INSECT VISITORS. Roy. Hort. Soc. Jour. 38: 238 - 248.HORTICULTURAL EDUCATION ASSOCIATION.
1967. THE POLLINATION OF FRUIT CROPS. 68 pp. Hort. Ed. Assoc. Fruit Com., Res. Sta., Long Ashton, Bristol, England. Reprinted from Sci. Hort. 14 126-150 (1960); 15: 82-122 (1961).HOWLETT. F. S.
1926a. SOME FACTORS OF IMPORTANCE IN FRUIT SETTING STUDIES WITH APPLE VARIETIES. Amer. Soc. Hort. Sci. Proc. 59: 307-315.____ 1926b. POLLINATION STUDIES IN OHIO. Ohio State Hort. SOc. Proc. 59: 197-200.
HUTCHINSON, A.
1964. ROOTSTOCKS FOR TREE FRUITS. Ontario Dept. Agr. Pub. 334, 18 pp., Toronto.HUTSON, R.
1926. RELATION OF THE HONEYBEE TO FRUIT POLLINATION IN NEW JERSEY. N.J. Agr. Expt. Sta. Bull 434, 32 pp.JAYCOX, E. R.
1968. EVALUATING HONEY BEE COLONIES FOR POLLINATION. Fruit Growing 20, 3 pp., rev.____ 1971. POLLEN INSERTS FOR APPLE POLLINATION. Fruit Growing 22, 3 pp., rev.
KELLY, B. W.
[n.d.] FACTORS RELATED TO THE COST OF PRODUCING APPLES IN PENNSYLVANIA, 1959-1963. Farm Mangt. 18 (Pa. Agr. Ext. Serv.), 20 pp.KELTY, R. H.
1929. RENTING OR KEEPING BEES FOR USE IN THE ORCHARD. Mich. Agr. Col. Ext. Bul. 56, 11 pp.KITAMURA T., and MAETA, Y.
1969. STUDIES ON THE POLLINATION OF APPLE BY OSMIA. III. PRELIMINARY REPORT ON THE HOMING ABILITY OF OSMIA CORNIFRONS (RADOSZKOWSKI) AND O. PEDICORNIS COCKERELL. Kontyu 37(1): 83 - 90. AA-80/71.KURRENOI, N. M.
1969. [THE ROLE OF HONEY BEES IN REGULAR FRUIT BEARING OF THE APPLE TREE.] In 22d Internatl. Apic. Cong. Proc., Munich, pp. 483-485. [In Russian, English abstract.]LATIMER, L. P.
1931. POLLINATION STUDIES WITH THE MCINTOSH APPLE IN NEW HAMPSHIRE. Amer. SOC. Hort. Sci. Proc. 1930: 386-396.____ 1933. POLLINATION AND FRUIT SETTING IN THE APPLE. N.H. Agr. Expt. Sta. Bull 274, 38 pp.
LEWIS, C. I., and VINCENT, C. C.
1909. POLLINATION OF THE APPLE. Oreg. Agr. Expt. Sta. BuL 104, 40 pp.LOKEN, A.
1958. POLLINATION STUDIES IN APPLE ORCHARDS OF WESTERN NORWAY. In 10th Internatl. Cong. Ent. Proc. 4: 961-965. Aug. 17-25, 1956, Montreal.LUCE, W. A., and MORRIS, O. M.
1928. POLLINATION OF DECIDUOUS FRUITS. Wash. Agr. Expt. Sta. Bul. 223, 22 pp.LUNDIE, A. E.
1927. THE HONEY-BEE AND THE FRUIT GROWER. HOW THE FLOWERS ARE FERTILIZED. Farming in So. Africa 1(10): 384 - 387.MACDANIELS, L. H.
1929. POLLINATION STUDIES IN NEW YORK STATE. Amer. Soc. Hort. Sci. Proc. 1928: 129-137.____ and HEINICKE, A. J.
1929. POLLINATION AND OTHER FACTORS AFFECTING THE SET OF FRUIT, WITH SPECIAL REFERENCE TO THE APPLE. N.Y. (Cornell) Agr. Expt. Sta. Bul. 497, 47 pp.MORRIS, O. M.
1921. STUDIES IN APPLE POLLINATION. Wash. Agr. Expt. Sta. Bul. 163, 32 pp.MURNEEK, A. E., and SCHOWENGERT, G. C.
1935. A STUDY OF THE RELATION OF SIZE OF APPLES TO NUMBER OF SEEDS AND WEIGHT OF SPUR LEAVES. Amer. Soc. Hort. Sci. Proc. 33: 4 - 6.NORTON, R. L.
1971. WHAT YOU MUST KNOW ABOUT HIGH-DENSITY PLANTINGS. Amer. Fruit Grower 91(5): 14-15, 24; (6): 20-23; (7): 10-11.OVERHOLSER, E. L.
1927. APPLE POLLINATION STUDIES IN CALIFORNIA. Calif. Agr. Expt. Sta. Bul. 426, 17 pp.PALMER-JONES, T., and CLINCH, P. G.
1968. HONEY BEES ESSENTIAL FOR POLLINATION OF APPLE TREES. New Zeal. Jour. Agr. 11(5): 32 - 33.PETKOV, V. G., and PANOV, V.
1967. STUDY ON THE EFFICIENCY OF APPLE POLLINATION BY BEES. In 21st InternatL Apic. Cong. Proc., College Park, Md., pp. 432- 436.PHILLIPS, E. F.
1930. HONEY BEES FOR THE ORCHARD. N.Y. (Cornell) Agr. Ext. Serv. Bul. 190, 24 pp._____ 1933. INSECTS COLLECTED ON APPLE BLOSSOMS IN WESTERN NEW YORK. Jour. Agr. Res. 46: 851 -862.
PHILP, G. L., and VANSELL, G. H.
1932. POLLINATION OF DECIDUOUS FRUITS BY BEES. Calif. Agr. Ext. Serv. Cir. 62, 27 pp.PRESTON A. P.
l949. AN OBSERVATION ON APPLE BL0SSOM MORPH0L0GY IN RELATION TO VISITS FROM HONEYBEES (APIS MELLIFERA). East Malling Res. Sta. Ann. Rpt. (1948), pp.64-67. (Abs.) Bee World 31: 24,1950.ROBERTS, R. H.
1945. BEE P0LLINATION OF DELICIOUS. Amer. Fruit Grower 65(4): 16.ROM, R. C.
1970. VARIETY AND CULTURAL CONSIDERATIONS NECESSARY TO ASSURE ADEQUATE P0LLINATION IN APPLE ORCHARDS. In The Indispensable Pollinators, Ark. Agr. Ext. Serv. Misc. Pub. 127, pp.219 - 225.SHOEMAKER, J. S., and TESKEY, B. J. E.
1959. TREE FRUIT PRODUCTION. 456 pp. John Wiley & Sons, Inc., New York.SNYDER, D. P.
1968. AN ECONOMIC STUDY OF APPLE PRODUCTION ON SIZE CONTR0LLED TREES. N.Y. (Cornell) Agr. Expt. Sta., Dept. Agr. Econ., Agr. Econ. Res. 233,20 pp.SYNDER. J. C.
1946. P0LLINATION OF TREE FRUITS AND NUTS. Wash. Agr. Ext. Serv. Bull 342,20 pp.TYDEMAN, H. M.
1943. THE INFLUENCE OF DIFFERENT P0LLENS ON THE GROWTH AND DEVEL0PMENT OF THE FRUIT IN APPLES AND PEARS. East Malling Res. Sta. Ann. Rpt., pp. 31 - 34.TYDEMAN, H. M.
1965. DESCRIPTION OF THE MALLING APPLE ROOTSTOCKS. East Malling Res. Sta. Ann. Rpt., pp. 64 - 66.WAITE M. B.
1895. THE P0LLINATION OF PEAR FL0WERS. U.S. Dept. Agr., Div. Veg. Path. Bul. 5,86 pp.____ 1899. P0LLINATION OF POMACEOUS FRUIT U.S. Dept. Agr. Yearbook 1898: 167 - 180.
WAY, R. D.
1971. APPLE CULTIVARS. Search - Agr. Pomol. I (1): 1-84.WEST, G. H.
1912. THE P0LLINATION OF APPLES AND PEARS. Kans. State Hort. Soc. Trans. 32: 38 - 50.WICKS W. H.
1918. THE EFFECT OF CROSS-P0LLINATION ON SIZE, C0L0R, SHAPE, AND QUALITY OF THE APPLE. Ark. Agr. Expt. Sta. Bul. 143,32 pp.WOODROW, A. W.
1933. THE COMPARATIVE VALUE OF DIFFERENT C0L0NIES OF BEES FOR FRUIT P0LLINATION. N.Y. (Cornell) Agr. Expt. Sta. Mem. 147,29 pp.____ 1934. THE EFFECT OF C0L0NY SIZE ON THE FLIGHT RATES OF HONEYBEES DURING THE PERIOD OF FRUIT BL00M. Jour. Econ. Ent. 27: 624-629.
APRICOT
Prunus armenica L., family RosaceaeApricots are produced primarily in California. In 1969, 223,000 tons were produced as compared with 3,050 tons in Washington and 4,500 tons in Utah. The estimated value of the total 1969 apricot crop was $33.5 million.
Plant:
In appearance, the apricot tree, fruit, and flower seem to be somewhat intermediate between the plum and the peach. The tree may be larger than a plum tree but spreads like the peach. The flowers are usually white like plum flowers, but they are not borne in clusters. The pit is smooth, somewhat like that of the plum but broader, flatter, and more winged, and intermediate in size between that of the peach and plum (fig. 41). The fruit is peach shaped (Cullinan 1937).
Inflorescence:
The white flower is borne either singly or doubly at a node on very short stems. There are about 30 stamens with one pistil, again like both the plum and the peach (fig. 42). The flowers are attractive to bees for both pollen and nectar. The cultivars of apricots were discussed by Coe (1934) and Hesse (1952).
[gfx] FIGURE 42.- Longitudinal section of 'Royal' apricot flower, x 6.
Pollination Requirements:
The literature on pollination of apricots is meager an not in complete agreement. Cook and Green (1894 reported that the best set of fruit was obtained from bagged flowers, with the next best from flowers in bags with honey bees, and the lowest set in the open. They did not comment on the activity of the bees either in the bags or on the open flowers. Cullinan (1937) stated that the apricot is self-fruitful. He did not indicate whether he meant the flowers would pollinate themselves or that they would set only if pollinated with their own pollen. Jusubov (1957) reported that some cultivars were self-fertile and some were completely self-sterile. Kostina
(1966) also found variation in degrees of fertility between cultivars. When Schultz (1948) bagged flowers on different cultivars, he reported good sets on 'Blenheim', 'Royal', 'Tilton', and 'Wenatchee Moorpark'. Schultz (1948) and Griggs (1970*) identified two self- incompatible cvs., 'Perfection' and 'Riland'. Slate (1970) stated that some cultivars are self-unfruitful. Luce and Morris (1928) stated that visits to blossoms by insects "seem to increase the set of fruit even in larger blocks of a single variety." Corner et al. (1964) reported that half of the Canadian cultivars were self-sterile. Hootman (1935) stated ". . . even self-fertile varieties produce better crops when interplanting is practiced."The rather meager data indicate that some apricot cultivars must be cross-pollinated and other cultivars are benefited by cross-pollination.
[gfx] FIGURE 41. - Harvesting apricots from fruit-laden tree.
Pollinators:
There seems to be little question as to which pollinating agents are effective on apricots. Jorgensen and Drage (1953) stated that wind is not an effective pollinating agent. Instead, they said that the sticky pollen needs the help of insects to carry it from the stamens to the stigma. Murneek (1937) also concluded that, whether a cultivar is self-sterile or self-fertile, insects are equally necessary for proper pollination and setting of fruit.
The chief pollinators are bees. Stark (1944) stated: "Other insects may be responsible for the pollination of an occasional flower but would not begin to do the job for a commercial crop of fruit."
These observations and statements show that insect pollination is required on self-sterile cultivars and is at least beneficial to the self- fertile cultivars. Honey bees are the primary pollinating agents.
Pollination Recommendations and Practices:
The available literature indicates that the apricot, like the peach and nectarine, depends upon pollinating insects to set a commercial crop on all cultivars. A heavy population of bees may be unnecessary, but they should be distributed throughout the orchard. Thus, since the bees are required but not in large numbers, the recommendation by Corner et al. (1964) of a colony of honey bees per acre would seem adequate, providing the colonies were distributed in small groups in the orchard.
LITERATURE CITED:
COE, E. M.
1934. APRICOT VARIETES. Utah Agr. Expt. Sta. Bul. 251,59 pp.COOK, A. J., and GREEN, E. C.
1894. SYMPOSIUM ON BEES AND FRUIT-FERTILIZATION, AGAIN. Gleanings Bee Cult. 22: 448-451.CORNER, J., LAPINS K O.. and ARRAND. J. C.
1964. ORCHARD AND HONEY BEE MANAGEMENT IN PLANNED TREE-FRUIT POLLINATION. Min. Agr., Victoria, British Columbia, Apiary Cir. 14,18 pp.CULLINAN, E. P.
1937. IMPROVEMENT OF STONE FRUITS. U.S. Dept. Agr. Yearbook 1937: 665-748.HESSE, C. O.
1952. APRICOT CULTURE IN CALIFORNIA Calif. Agr. Expt. Sta. Cir. 412,58 pp.HOOTMAN, H. D.
1935. IMPORTANCE OF POLLINATION AND THE HONEY-BEE IN FRUIT YIELDS. Midwest Fruitman 8(9): 3-4,9-10.JORGENSEN, C., and DRAGE, C. M.
1953. POLLINATION OF COLORADO FRUIT. Colo. Agr. Expt. Sta. and Ext. Serv. Bul. 427-A, 13pp.JUSUBOV, A. M.
1957. [POLLINATORS FOR NEW APRICOT VARIETIES IN THE CENTRAL BELT.] Sad i Ogorod 2: 47-48. [In Russian.] Plant Breeding Abs. 27(4): 4367, p.721, 1957.KOSTINA, K. F.
1966. [THE DEGREE OF SELF-FERTILIZATION IN APRICOT VARIETIES AND HYBRIDS OF DIFFERENT ECOLOGICAL-GEOGRAPHICAL GROUPS.] Sel'skokhoz. Biol. 1(3): 352-355. [In Russian, English summary.]LUCK, W. A., and MORRIS, O. M.
1928. POLLINATION OF DECIDUOUS FRUITS. Wash. Agr. Expt. Sta. Bul. 223,22 pp.MURNEEK, A. E.
1937. POLLINATION AND FRUIT SETTING. Midwest Fruitman 10(5): 8-9.SCHULTZ, J. H.
1948. SELF-INCOMPATIBILITY IN APRICOTS. Amer. Soc. Hort. Sci. Proc. 51: 171-174.SLATE, G. L.
1970. APRICOTS, NECTARINES AND ALMONDS. Horticulture 48(5): 42,47-48.STARK, A. L.
1944. FRUIT POLLINATION - A PROBLEM IN UTAH. Farm and Home Sci. 5(4): 5-6.
AVOCADO
Persea americana Mill., family LauraceaeThe avocado is grown primarily in California, to a lesser extent in Florida, and on only a few acres in Hawaii, Puerto Rico, and southern Texas. Crop production in 1970 amounted to 83,400 tons valued at $30 million. California produced 64,600 tons and Florida produced 18,800 tons.
On mature trees, about 2 tons of fruit per acre are harvested, although productive orchards will yield 3 to 6 tons. Year-to-year production varies, depending upon many factors, but a year of high production is frequently followed by a year of low production. Weather has a strong impact upon production. Prolonged cool weather, subfreezing weather, low humidity, strong winds at flowering time, or tornadoes can all result in low set of fruit and low production. The most critical effect of temperature occurs during flowering.
Plant:
The avocado is a tropical evergreen, upright shrub or tree that grows to 60 feet high, but usually between 15 and 30 feet in height (fig. 46). Its dark green leaves are 4 to 10 inches long and 2 to 3 inches wide. The plant may exhibit two or more growth flushes during the year in contrast to the single growth period of most deciduous plants. It may flower in summer or in winter, and may have a flowering period lasting 6 months. It is less tolerant of cold than lemons or navel oranges and prefers high humidity and calm weather. The fruit, which can remain on the tree for several months after maturity, is a nutritious, fresh food rich in oil and high in calories and vitamin E. A few seedling dooryard trees are estimated to be 100 years old, but commercial trees last about 35 years (Goodall et al. 1970).
Hundreds of cultivars have been tried in the United States, but about two dozen are of commercial importance (Rowland 1970).
The cv. 'Fuerte' has for years provided the bulk of the avocado crop (Bergh et al. 1966, Rock and Platt 1968, Rowland 1970). Its fruit weighs 8 to 16 ounces and contains 18 to 28 percent oil. It is cold resistant and ripens over a long period - December to May. By comparison, the Florida cv., 'Pollock', weighs 30 to 50 ounces and contains only 3 to 5 percent oil. The 'Haas' cv. is second in importance to 'Fuerte.' Its fruit weighs only 6 to 12 ounces. Other important California cvs. include the 'Bacon', 'Zutano', 'Rincon', 'Nabal', 'McArthur', 'Anaheim', 'Carlsbad', 'Dickinson', and 'Puebla'. In Florida, the most important cultivars include 'Booth 8', 'Lula', 'Booth 7', 'Waldin', 'Pollock', end 'Hickson' (Rowland 1970).
Avocados can be grown from seed, but the plants are usually propagated by grafting. They are set in the grove 20 to 40 feet apart depending upon whether the type of growth is spreading or upright. Sometimes they are set at 15 to 20 feet with the alternate plants removed after a few years. Older orchards with spreading trees may have as few as 40 trees per acre. Orchards with upright trees may have 150 trees per acre. About 90 trees per acre is average (Lee and Burns 1967). Fruit bearing begins at 3 to 6 years of age and may continue for 50 or more years.
The honey bee is attracted to the plant for both the nectar and the pollen, although citrus, mustard, and many other plants that flower at the same time as avocado are much more attractive to bees than are avocado flowers. Pellett (1926, 1947*) reported that bees collect only a small amount of avocado honey. Vansell (1931) stated that avocados are visited moderately by bees for nectar and pollen. In general beekeepers consider the plant as a source of buildup for their bees rather than as a source of surplus honey.
[gfx] FIGURE 46. - Avocado orchard in bloom.
FIGURE 47. - Closeup of avocado tree in full bloom.Inflorescence:
A full-grown avocado tree may bear a million flowers in a season, the flowers occurring in panicles of severe dozen to several hundred on the ends of the numerous branches (Robinson and Savage 1926) (fig. 47).
The relatively inconspicuous blossom is about one half inch in both width and depth. Three sepals and three similar-appearing green petals make up the perianth. The single pistil has a simple, bulbous, smooth ovary and a somewhat elongated style terminated by a slightly enlarged stigma. There are nine stamens inserted in two whorls. The inner whorl consists of three stamens, with three prominent, orange, nectar-producing staminodes (sterile or abortive stamens) alternating between them. Opposite each stamen and staminode of the inner whorl is one of the six stamens of the outer whorl. There is an orange nectary, slightly smaller than the staminode, on each side of each outer stamen.
The flower opens twice, on subsequent days or in two stages (fig. 48). In stage 1, the first day, the petals separate and bend outward. The stigma is whitish, fresh, and receptive to pollination (Hodgson 1930), but the stamens, bent out at right angles to the pistil, release no pollen. Some nectar appears on the staminodes. After a few hours, the flower closes.
In stage 2, the second day, the flower opens again. This time, nectar on the six true nectaries is secreted more profusely than occurred on the staminodes. The pistil is shriveled and dark and no longer receptive. The stamens are longer and larger, the inner three overtopping the stigma but facing away from it, and the outer stamens at about a 45 deg angle from the style and facing it, and both sets releasing sticky clumps of pollen. Each stamen has four pollen sacs, the valves of which hinge at the top.
When the flower closes the second day, it never reopens. It is therefore, structurally bisexual but functionally unisexual. This dichogamous condition was first noticed by Nirody (1922) and enlarged upon by Stout and Savage (1925) and Peterson (1955a, b, 1956).
The unusual part about the avocado flower is that in some cultivars stage 1 occurs in the morning of the first day and stage 2 in the afternoon of the second day. These cultivars are referred to as type A. In other cultivars, referred to as type B, stage 1 occurs in the afternoon, and stage 2 occurs the following morning. If cultivars of both types are interplanted within the same orchard, pollen should always be available when the stigmas are receptive (Stout 1932, Robinson 1930, 1933, Ward 1933, Bergh and Gustafson 1958, Bergh and Garber 1964). At least one cv., 'Collinson', produces no pollen; therefore, it is incapable of setting fruit unless pollen is transferred to it from other cultivars that release pollen when its stigmas are receptive (Anonymous 1930).
If the temperature is too low, some flowers, for example, those on the 'Fuerte' cv., may fail to open in the female stage, making fruit set impossible. On the other hand, hot weather and low humidity are not conducive to fruit set. Also, too much disturbance of the flowers by wind can cause shedding. A mild climate with calm humid days is best for the flower.
Bergh (1968) showed that trees set more fruit when there are flowers of different avocado cultivars nearby. This may not be true for all cultivars or all years, but such effects have been thoroughly demonstrated. For example, he showed that the 'Fuerte' and the 'MacArthur', which are considered to be self-fertile, increased production as much as 50 percent when exposed to pollen of other interplanted cultivars.
Avocado flowering may extend from one to several months depending upon conditions affecting fruit setting. A sufficient supply of pollinating agents will tend to shorten the period of flowering. The number of flowers that may set fruit has been variously estimated by different people. Purseglove (1968*) stated that only one in 5,000 flowers produces a fruit. Gustafson and Bergh (1966) considered that a set of less than 1 percent of the flowers is usually sufficient for a good fruit crop. Chandler (1958*) stated that flower clusters containing 1,000 or more flowers may be found on a branch less than a foot long in space enough for no more than two fruit. He stated that less than one flower in 500 on a 'Fuerte' tree set fruit. If a tree produces a million flowers and there are 90 trees per acre, 90 million flowers should be produced. If one flower in 5,000 produces a fruit that weighs 12 ounces, the grower should harvest 18,000 fruits, or over 6 tons per acre. That this is seldom done is a good indication that only a small fraction of 1 percent of the flowers produce fruit.
FIGURE 48. - Longitudinal section of 'fuerte' avocado flower, x 18. A, Stage 1: stigma receptive, but stamens bent outward and anthers not dehisced; B, stage 2, the second day, with stigma no longer receptive, but stamens upright and anthers dehisced.
Pollination Requirements:
Peterson (1955b) showed that the pistillate stage, or stage 1, of the 'Rincon' cv. was open for 3 hours 40 minutes, the maximum time in which pollination of this cultivar could take place. He showed that the flower was incapable of selfing because first flowering began at 7:25 a.m. and ended by 11 a.m.; whereas the second stage of the 2-day-old flower did not begin until 11 a.m., by which time the current-day stigma had withered and was no longer receptive. In the 'Zutano' cv., stage 1 extended from 2:50 p.m. to 6:20 p.m., and stage 2 (the next morning) from 8:40 a.m. until after 11 a.m. Therefore, when the flowers of type A, for example, 'Rincon' cv., are receptive to pollination, the pollen is being shed by flowers of type B, for example,'Zutano' cv.. and when flowers of the 'Rincon' are shedding pollen, flowers of the 'Zutano' are receptive to pollination. This condition is considered by horticulturists to be highly fluid and influenced by the cultivars involved and various environmental conditions.
Peterson (1955a) showed that at least the 'Zutano' and the 'Haas' cvs. were capable of setting fruit when isolated from other cultivars if honey bees were present in abundance. He caged four individual trees, two of each cultivar with one tree of each group in a cage with honey bees during the flowering period. When flowering was over, the bees and cages were removed and the fruit counted. The results concerning the treatment and fruit produced were as follows:
Cultivar Bees in cage No bees in cage 'Zutano'..............................................4 120 ÔHaasÕ..................................................5 284
Whether the pollen was carried over on the bees from the normal time of anther opening until the time of stigma receptivity, whether the opening phases overlapped, or whether the bees forced open the anthers when the stigma was still receptive was not determined, but in any event the effect of the bees was striking.
The evidence is clear that avocados must be insect-pollinated, and that production is best when varieties are interplanted. Bees usually transfer avocado pollen no greater distance than two avocado rows (Bergh 1961). The varieties should intermesh in their blooming dates so that pollen is available on one cultivar when the stigmas on another are receptive, and vectors should be available to move the pollen to the receptive stigmas. Maximum set can only be achieved through adequate provision for cross-pollination - the interplanting of appropriate flowering types and the availability of adequate pollinating agents (Bergh 1969).
Pollinators:
Various pollinating agents visit the avocado flowers for nectar and pollen. These include the honey bee, various species of wild bees, wasps, flies, and hummingbirds (Chapman 1964*).
The consensus of various research workers who have studied the flowering and fruiting of the avocado is that only honey bees are sufficiently abundant on the blossoms at all times to set satisfactory crops of fruit (Clark 1923,1924; Clark and Clark 1926; Boyden 1930; Traub et al. 1941; Lemmerts 1942; Lesley and Bringhurst 1951; Winslow and Enderud 1955; Lecomte 1961; Popenoe 1963).
Many observers have noted that a bee tends to visit a single tree and thus fails to afford the cross-pollination desired. This can occur when the trees are separated by some distance, for example, when they are small or spaced too far apart (Bergh 1966). It also occurs when there is an insufficiency of bees in relation to the number of blooms available.
When the flowers per bee ratio is low, the bees are required to visit many flowers to obtain a load of food and their efficiency as cross-pollinating agents is increased. Ruehle (1958) stated that good crops are set consistently in groves a considerable distance from any bee hives hut that the presence of trees would increase production. Wolfe et al. (1942, 1946) stated that it is quite possible that a hive of bees per acre with sets of five in the middle of each 5-acre tract would materially increase production. Popenoe (1963) stated that honey bees are probably necessary for good pollination unless there is an abundance of wild bees in the area.
In an excellent survey of the reasons for low yield of avocados in California, Bergh (1967) unequivocally stated: "Practically every avocado fruit set means that a honey bee transferred pollen to that flower from some other flower. Gravity or wind may act, but they are so rare they can be ignored by the practical avocado grower." Further on, he stated, "At the present time the California avocado industry is dependent upon the honey bee. The greater the bee population, the more likely the bees are to travel from flower to flower and so make the best of such inter-flower overlap in male and female stages as may be present. This is probably the chief source of avocado set in California."
Pollination Recommendations and Practices:
Peterson (1955a) stated that there was no evidence that addition of bees to the "natural population of wild bees and other large insects" would increase fruit set. He gave no indication as to the population of wild bees honey bees, or other large insects present on the trees. Wolfenbarger (1954) showed that honey bees were more abundant within 375 feet of a 64-colony apiary than at more remote distances, and more avocados were harvested per tree within 250 feet of the apiary than at a distance of 1,000 feet. Wolfe et al. (1946) and Ruehle (1958) recommended that one colony of bees per acre be used with five colonies set in the middle of each 5-acre tract. Stout (1923) recommended providing "bees in abundance" and control of other plants in the area that might attract the bees. LeComte (1961) suggested one colony per acre. Stout (1933) went even further by stating that one hive per acre for other fruit is satisfactory, but the flowering habits of the avocado make it desirable to employ more than one hive per acre to supply the honey bees in abundance.
Bergh (1967) stated that the average California avocado grower would have better crops if he would use more honey bees. He recommended that growers use two to three strong colonies per acre, the colonies placed in groups no more than one-quarter mile apart with 0.1 mile being preferable.
Bergh (1967) made the following recommendations: (1) Place hives or have them placed by the beekeeper after the avocados begin blooming so the bees will "get the avocado habit" right away; (2) place hives in the grove if possible, at least avoid locations where the bees must fly past citrus or other attractive pasturage; (3) control other blooms, such as mustard; (4) avoid use of insecticides during the blooming season, (5) and for cross-pollination, interplant types A and B to increase production 50 to 150 percent.
Thus, after careful study of the research by these scientists, one must conclude that for commercial production of avocados bees are essential, that honey bees are the primary pollinators, and that two to three colonies per acre should be used, the colonies placed within or alongside the groves, and that steps should be taken to insure protection of the bees and discouragement of associated plants attractive to them.
The majority of avocado growers only passively encourage the keeping of bees in the area of their groves. Few if any actively contract for the bees or pay any type of pollination fee to insure the presence of adequate numbers. Many of them know that beekeepers usually move the colonies to the avocado growing areas to obtain nectar and pollen for buildup of the colonies. The bee population the beekeeper desires on the flowers for colony buildup, however, is far short of the population needed for maximum avocado pollination. Colonies vary enormously in strength and pollination effectiveness. Also, unless contracted for, the colonies may be transported to avocados when forest, range, or desert conditions are unfavorable for beekeeping, but may be placed elsewhere at avocado flowering time if the other flora is more favorable. For dependable pollination and maximum avocado fruit set, the grower should see that his trees are amply supplied with strong colonies of honey bees.
LITERATURE CITED:
ANONYMOUS.
1930. NEW AVOCADO HAS NO POLLEN. Off Rec. 9(43): 3BERGH, B. O.
1961. BREEDING AVOCADOS AT C.R.C. Calif. Avocado Soc. Yearbook 45: 67-74.______ 1966. AVOCADO TREE ARRANGEMENT AND THINNING IN RELATION TO CROSS-P0LLINATION. Calif. Avocado Soc. Yearbook 50: 52-61.
______ 1967. REASONS FOR LOW YIELDS OF AVOCADOS. Calif. Avocado Soc. Yearbook 51: 161-172.
______ 1968. CROSSP0LLINATION INCREASES AVOCADO SET. Calif. Citrog. 52(3): 97-100.
______ 1969. AVOCADO In Ferwerda, F. P., and Wit, F., eds. Outlines of Perennial Crop Breeding in the Tropics, pp. 23-51. H. Veenman and Zonen, N. V. Wageningen, The Netherlands.
______and GARBER, M. J. 1964. 1964 AVOCADO YIELDS INCREASED BY INTER- PLANTING DIFFERENT VARIETIES. Calif. Avocado Soc. Yearbook 48: 78-85. ______ and GUSTAFSON, C. D. 1958. FUERTE FRUIT SET AS INFLUENCED BY CROSS-POLLINATION. Calif. Avocado Soc. Yearbook 42: 64-66.
______GARBER, M. J., and GUSTAFSON C. D. 1966. THE EFFECT OF ADJACENT TREES OF OTHER AVOCADO VARIETIES ON FUERTE FRUIT-SET. Amer. Soc. Hort. Sci. Proc. 89: 167-174.
BOYDEN, A. L., CO.
1930. THE IMPORTANCE OF THE HONEYBEE TO AVOCADO CULTURE. A. L. Boyden Co., Alhambra, Calif. Leaflet, 4 pp.CLARK, O. I.
1923. AVOCADO POLLINATION AND BEES. Calif. Avocado Assoc. Ann. Rpt. 1922-1923: 57-62. 98CLARK, O. I.
1924. AVOCADO POLLINATION TESTS. Calif. Avocado Assoc. Ann. Rpt. 1923-1924: 16-22.______and CLARK, A. B.
1926. RESULTS OF POLLINATION AND OTHER EXPERIMENTS ON AVOCADOS AT THE ORCHARDS OF THE POINT LOMA HOMESTEAD. Calif. Avocado Assoc. Ann. Rpt. 1925-1926: 85-94.GOODALL, G. E.
1970. CAN YOU AFFORD TO GROW AVOCADOS? Calif. Avocado Soc. Yearbook 54: 43-45.______LITTLE, T. M., ROCK, R. C., and others.
1970. USEFUL LIFE OF AVOCADO TREES IN COMMERCIAL ORCHARDS IN CALIF. Calif. Avocado Soc. Yearbook 54: 33-36.GUSTAFSON, C. D., and BERGH, B. O.
1966. HISTORY AND REVIEW OF STUDIES ON CROSS-POLLINATION OF AVOCADOS. Calif. Avocado Soc. Yearbook 50: 39-49.HODGSON, R. W.
1930. CROSS-POLLINATION. Calif. Avocado Assoc. Yearbook 1930: 30-31.LECOMTE, J.
1961. [OBSERVATIONS ON POLLINATION OF THE AVOCADO IN THE FRENCH ANTILLES.] Fruits 16(8): 411-414. [In French]LEE, B. W., and BURNS, R. M.
1967. AVOCADOS IN VENTURA COUNTY. Calif. Citrog. 52(12): 520-522.LEMMERTS, W. E.
1942. PROGRESS REPORT ON AVOCADO BREEDING. Calif. Avocado Soc. Yearbook 1942: 36-41.LESLEY, J. W., and BRINGHURST, R. S.
1951. ENVIRONMENTAL CONDITIONS AFFECTING POLLINATION OF AVOCADOS. Calif. Avocado Soc. Yearbook 1951: 169-173.NIRODY, B. S.
1922. INVESTIGATIONS IN AVOCADO BREEDING. Calif. Avocado Assoc. Ann. Rpt. 1921-1922: 65-68.PELLETT, F. C.
1926. THE AVOCADO FOR BEES. Amer. Bee Jour. 66: 11.PETERSON, P. A.
1955a. AVOCADO FLOWER POLLINATION AND FRUIT SET. Calif. Avocado Soc. Yearbook 39: 163-169.______ 1955b. DUAL CYCLE OF AVOCADO FLOWERS: STUDY OF THE CONTINUOUS DUAL OPENING CYCLE OF THE AVOCADO FLOWER SHOWS NEED OF LARGE FLYING INSECTS FOR POLLINATION. Calif. Agr. 9(10): 6-7, 13.
______ 1956. FLOWERING TYPES IN THE AVOCADO WITH RELATION TO FRUIT PRODUCTION. Calif. Avocado Soc. Yearbook 40: 174-179.
POPENOE, J.
1963. THE RUEHLE AVOCADO. Fla. Agr. Expt. Sta. Cir. S-144, 4 pp.ROBINSON, T. R.
1930. SETTING OF FRUIT POLLINATION; SOME ABERRANT FORMS OF FLOWER MECHANISM IN THE AVOCADO. Calif. Avocado Assoc. Yearbook 1930: 107-111.______ 1933. POLLINATION AND OTHER FACTORS INFLUENCING THE PRODUCTION OF AVOCADOS. Fla. State Hort. Soc. Proc. 46: 109-114.
ROBINSON T. R., and SAVAGE, E. M.
1926. POLLINATION OF THE AVOCADO. U.S. Dept. Agr. Cir. 387, 16 pp.ROCK, R. C., and PLATT, R. G.
1968. ECONOMIC ASPECTS OF MARKETING CALIFORNIA AVOCADOS. Calif. Agr. Ext. Serv. AXT 279, 22 pp.ROWLAND, W. A.
1970. AVOCADOS. Fruit and Veg. Facts and Pointers, 12 pp.RUEHLE, G. D.
1958. THE FLORIDA AVOCADO INDUSTRY. Fla. Agr. Expt. Sta. Bul. 602, 100 pp.STOUT, A. B.
1923. A STUDY IN CROSS-POLLINATION OF AVOCADOS IN SOUTHERN CALIF. Calif. Avocado Assoc. Ann. Rpt. 1922-23: 29-45.______ 1932. SEX IN AVOCADOS AND POLLINATION. Calif. Avocado Assoc. Yearbook 1932: 172-173.
______ 1933. THE POLLINATION OF AVOCADOS. Fla. Agr. Expt. Sta. Bul. 257, 44 pp.
______and SAVAGE, E. M.
1925. THE FLOWER BEHAVIOR OF AVOCADOS WITH SPECIAL REFERENCE TO INTERPLANTING. Fla. State Hort. Soc. Proc. 38: 80-91.TRAUB, H. P., POMEROY, C. S., ROBINSON, T. R., and ALDRICH, W. W.
1941. AVOCADO PRODUCTION IN THE UNITED STATES. U.S. Dept. Agr. Cir. 620, 28 pp.VANSELL, C. H.
1931. NECTAR AND POLLEN PLANTS OF CALIFORNIA. Calif. Agr. Expt. Sta. Bul. 517, 60 pp.WARD W. F.
1933. PRACTICAL HINTS TO COMMERCIAL AVOCADO GROWERS. Fla. State Hort. Soc. Proc. 46: 139-142.WINSLOW, M. M., and ENDERUD J.
1955. FLOWERING BEHAVIOR AND YIELDS OF SOME AVOCADO VARIETIES AT RIVERSIDE. Calif. Avocado Soc. Yearbook 39: 133-135.WOLFE, H. S., TOY, L. R., and STAHL, A. L.
1942. AVOCADO PRODUCTION IN FLORIDA. Fla. Agr. Ext. Serv. Bul. 112, 111 pp.______TOY L. R., and STAHL, A. L
1946. AVOCADO PRODUCTION IN FLORIDA. Fla. Agr. Ext. Serv. Bul. 129, 107 pp.WOLFENBARGER, D. O.
1954. BIOLOGY AND CONTROL OF INSECTS AFFECTING SUB-TROPICAL FRUITS. Fla. Agr. Expt. Sta. Ann. Rpt., p. 290.
CACAO
Theobroma cacao L., family SterculiaceaeCocoa is the processed product derived from the beans of the cacao plant.
World production of cocoa exceeds a million tons, with Ghana producing 429,000 tons; Nigeria, 201,000 tons; Ivory Coast, 105,000 tons; Cameroon, 73,000 tons; Brazil, 94,000 tons; and Equador, 35,000 tons, with other countries of North and South America, Africa, Asia, and Oceania producing the balance. Of this amount, the United States consumes 25 percent; Germany, 13 percent; United Kingdom, 10 percent; and the Netherlands, 9 percent ( Purseglove 1968*). Europe, as a whole, takes over 50 percent and the American countries, about 40 percent of the entire crop.
Plant:
The evergreen cacao tree grows 15 to 25 feet primarily between latitudes 10 deg N to 10 deg S, usually below 1,000 feet in altitude, and in areas with a monthly average rainfall of about 4 inches. Various cultivars, propagated by seed, are grown. The oblong or oval fruit (fig. 58), commonly called a pod, is 4 to 12 inches long, and green when immature, but may be yellow, red, purple, or green when ripe. It contains afrom 20 to 60 reddish-brown beans 3/4 to 1/2 by 1/2 to 1 inch in size, usually arranged in five rows (fig. 59). Pods are produced throughout the year, but the main harvest usually begins at the end of the wet season and may extend for 3 months. From 7 to 14 pods will produce a pound of dry beans. Yeilds range from 200 to 3,000 pounds dry beans per acre, but 600 lb/acre is considdered a good yield (Purseglove 1968).
Inflorescence:
The cacao flowers arise in groups directly from old wood of the main stem or older branches at points which were originally leaf axils (fig. 60). Each flower has five prominent pink sepals, five smaller yellowish petals, each of which forms a pouch, an outer whorl of five staminodes, and an inner whorl of five double stamens, each stamen bearing up to four anthers. The staminodes are about as tall to twice as tall as the upright style and form a "fence" around the style. The stamens are curled so that the anthers develop inside the petal pouches. The ovary consists of five united carpels each having four to 12 locules, and one style that has several linear stigmatic lobes (van Hall 1932). According to Cheeseman (1932) and Urquhart (1961), the flower produces no nectar and has no discernible scent. However, Stejskal (1969) stated that there are two types of microscopic nectaries, ( 1 ) the cylindrical multicellular ones, 60 to 450 microns in size, on the pedicels, sepals, and ovaries, and (2) the conical unicellar ones 20 to 25 microns in size, located on the "guide lines" of the petals and on the staminodia. He showed that they secrete nectar, which has an odor that attracts male mosquitoes and lepidopterous insects.
The flower opens about dawn, and the anthers dehisce just before sunrise. The stigma is usually pollinated 2 to 3 hours later but is receptive from sunrise to sunset of the day of opening (Cheeseman 1932). The stigma is receptive to pollen along its whole length, and not merely at the apex as in most flowers. If the flower is not pollinated, it usually sheds the following day (Sumner 1962). Pollination before noon is best (Chats 1953).
[gfx]
FIGURE 58.- Maturing cacao fruit on the tree.
FIGURE 59.- Ripe cacao fruit opened to show the beans.
FIGURE 60.- Cacao flower cluster growing on the trunk of the tree, showing the open flower, a flower ready to open, and a small fruit.Pollination Requirements:
Although the full story of cacao pollination is not yet known, there seems little doubt that the flower is not self-pollinating, as flowers bagged to exclude insects invariably shed (Gnanaratnam 1954). Also, some plants are self-incompatible but set fruit well if pollinated with pollen from compatible trees (Chats 1953, Cope 1958, Knight and Rogers 1955). The method of the transfer of the pollen in nature is the somewhat questionable factor. The sticky pollen is not carried by the wind. Furthermore, it is produced and released in the petal pouches where wind is unlikely to disturb it (Cobley 1966*, Gnanaratnam 1954). Glendenning (1962) noted that pollen found on a stigma was usually from more than one flower, but the amount of foreign pollen depended on proximity to other plants. Little pollen seemed to move more than a couple of trees' distance.
Pollinators:
There is general belief that small insects are the primary pollinating agents of cacao, but no general agreement as to which insects are responsible. Numerous authorities credit midges, especially Forcipomyia quasiingrami Macfie and Lasiohela nana Macfie (Barroga 1964, Chatt 1953, Fontanilla-Barroga 1965, Macfie 1944, Saunders 1959, Toxopeus 1969). Others credit ants (Crematogaster spp.), aphids (Aphis gossypii Glover and Toxoptera spp.), thrips (Frankliniella parvula Hood), and unidentified wild bees (Billes 1941; Cope 1940; Harland 1925a, b; Hernandez 1966; Jones 1912; Muntzing 1947; Posnette 1942a, b, 1944, 1950; Posnette and Entwistle 1957; Urquhart 1961; Voelcker 1940).
Thrips and aphids move about but slightly from tree to tree, yet Glendenning (1958) reported, after a study of albino trees, that a considerable proportion of pollination takes place across two intervening trees, though less than over shorter distances. This would indicate an agent with considerable movement between trees.
The ants Wasmannia suropunctata (Roger) and Solenopsis geminata (F.) and the wild bee Trigona jaty Smith were occasional visitors. Glendenning (1958) concluded that the midges (Forcipomyia spp.) were the main pollinators, accounting for twice the pollination service performed by all of the other species combined. This was verified in various experiments with different numbers of insects per cage over cacao flowers. Hernandez (1965) reported pollination percentages ranging from 1 to 52 percent when he used midges, bees, thrips, and ants. However, Hernandez did not, indicate how pollination was accomplished.
Although midges seem to get the most credit as pollinators of cacao, there is clearly a lack of knowledge as to which insects are responsible in the different areas for the commercial set of fruit of this important crop.
Harland (1925a) found that of 5 percent of the flowers on trees not infested by ants and aphids, only 0.3 percent set fruit; whereas, on trees heavily infested by these insects, 35 percent of the flowers were pollinated and 2 percent set. At the same time, 5 percent of the hand pollinated flowers set fruit.
Little has been said about the adequacy of pollination of the individual flower or the minimum number of seed in relation to fruit set or shedding. However, at least as many pollen grains must fall upon the stigma as there are subsequently developed seeds. Thus, a minimum of 60 pollen grains is necessary to set the highest number of seed.
Many of the flowers are never pollinated (Harland 1925b), at least under Trinidad conditions. Apparently, wherever the crop is grown the lack of adequate pollination is a strongly limiting factor in production of the beans. Sumner (1962) stated that most of the pollination occurs 2 to 3 hours after dawn with a second much smaller peak in the afternoon, but only 2 to 5 percent of the flowers ever get pollinated, and these may not set if pollinated too late or with incompatible pollen. Urquhart (1961) stated that only about 5 percent of the stigmas ever get pollinated; Harland (1925b) found only 9 percent to be pollinated. Because some plants are self-incompatible - some are male sterile or sterile (Gnanaratnam 1954) - many of the flowers would appear to be doomed to shed. Knoke and Saunders (1966) tried a mist blower for mechanical transfer of pollen but achieved uneconomical success.
The use of honey bees under saturated pollination conditions has never been tried, probably because the blossom has no aroma and produces no nectar. Quite conceivably, however, honey bee colonies could be concentrated in numbers sufficient to exhaust the supply of pollen and nectar on competing plants and the bees induced to visit the flowers of this important crop for pollen and increase the percentage of cross- pollination and fruit set. A search for a selection of cacao pollen-loving honey bees might produce an acceptable and controllable pollinating agent. One or more of the various species of pollen-foraging wild bees might be found that could be controlled and used as a profitable pollinating agent of cacao.
Pollination Recommendations and Practices:
There are no recommendations on the use or manipulation of insect pollinators of cacao. According to Faegri and van der Pijl (1966*), the Forcipomyia spp in Africa breed mainly in decaying pods. If the pods are removed by too scrupulous cleaning of the plantations, these midges might also be removed. This would result in deficient pollination of the flowers. Otherwise, the presence or numbers of insect pollinators are left entirely to chance on this billion-dollar crop.
LITERATURE CITED:
BARROGA S. F.
1964. PROGRESS REPORT ON THE STUDY OF INSECTS, PARTICULARLY MIDGES ASSOCIATED WITH POLLINATION OF THEOBROMA CACAO, APRIL 1963. Philippine Jour. Plant Indus. 29(3/4): 123 - 133.BILLES, D. J.
1941. POLLINATION OF THEOBROMA CACAO L. IN TRINIDAD, B.W.L Trop. Agr. [Trinidad] 18: 151-156.CHATT, E. M.
1953. COCOA. 302 pp. Interscience Publishers Inc., New York.CHEESEMAN, E. E.
1932. THE ECONOMIC BOTANY OF CACAO. A CRITICAL SURVEY OF THE LITERATURE TO THE END OF 1930. Trop. Agr. [Trinidad] Sup., v. 9, June, 16 pp.COPE, F. W.
1940. AGENTS OF POLLINATION IN CACAO. St. Augustine, Trinidad, Imperial College of Tropical Agr. [Trinidad], Ninth Ann. Rpt. on Cacao Res. 1939: 13-19.______ 1958. INCOMPATIBILITY IN THEOBROMA CACAO. Nature 181: 279.
FONTANILLA-BARROGA, S.
1965. A PROGRESS REPORT ON THE STUDY OF INSECTS ASSOCIATED WITH POLLINATION OF THEOBROMA CACAO WITH SPECIAL EMPHASIS ON MIDGES. Philippine Jour. Agr. 27(3/4): 147-159.GLENDINNING, D. R.
1958. PLANT BREEDING AND SELECTION. Cocoa Res. Inst. Rpt. of West Africa, 1957-58, pp. 50-54.______ 1962. NATURAL POLLINATION OF COCOA. Nature 193(4822): 1305.
GNANARATNAM, J. K.
1954. POLLINATION MECHANISM OF THE CACAO FLOWER. Trop. Agr. [Ceylon] 110: 98 - 104.HALL, C. J. J. VAN.
1932. CACAO Ed. 2, 514 pp. Macmillan, London.HARLAND, S. C.
1925a. STUDIES IN CACAO. THE METHOD OF POLLINATION. Ninth West Indian Agr. Conf. Proc. Kingston, Jamaica, 1924: 61 - 69.______ 1925b. STUDIES IN CACAO. PART I. THE METHOD OF POLLINATION. Ann. Appl. Biol. 12: 403-409.
HERNANDEZ, B. J.
1965. INSECT POLLINATION OF CACAO (THEOBROMA CACAO L.) IN COSTA RICA. 173 pp. Ph.D. thesis and Diss. Abs. 28(1): 2B-3B, 1967, AA-257/71, Wis, Univ., Madison.JONES, G. A.
1912. THE STRUCTURE AND POLLINATION OF THE CACAO FLOWER. West Indian Bull 12: 347 - 350.KNIGHT, R., and ROGERS, H. H.
1955. INCOMPATIBILITY IN THEOBROMA CACAO. Heredity 9: 69 - 77.KNOKE J. K., and SAUNDERS, J. L.
1966. INDUCED FRUIT SET OF THEOBROMA CACAO BY MISTBLOWER APPLICATIONS OF INSECTICIDES. Jour. Econ. Ent. 59: 1427-1430.MACFIE, J. W. S.
1944. CERATOPOGONIDAE COLLECTED IN TRINIDAD FROM CACAO FLOWERS. Bul.:Ent. Res. [England] 35: 297 - 300.MUNTZING, A.
1947. SOME OBSERVATIONS ON POLLINATION AND FRUIT-SETTING IN ECUADORIAN CACAO. Hereditas 33: 397 - 404.POSNETTE, A. F.
1924a. NATURAL POLLINATION OF COCOA, THEOBROMA LEIOCARPA, ON THE GOLD COAST. Trop. Agr. [Trinidad] 19: 12-16.______ 1942b. NATURAL POLLINATION OF COCOA. THEOBROMA LEIOCARPA, BERN., ON THE GOLD COAST II. Trop. Agr. [Trinidad] 19(10): 188-191.
______ 1944. POLLINATION OF CACAO IN TRINIDAD. Trop. Agr. [Trinidad] 21(6): 115-118.
______ 1950. THE POLLINATION OF CACAO IN THE GOLD COAST. Jour. Hort. Sci. 25: 155 - 168.
______and ENTWISTLE, H. M. 1957. THE POLLINATION OF COCOA FLOWERS. Rpt. Cocoa Conf. Grosvenor House, London, Sept. 10-12, pp. 66-69. (Abs.) Plant Breeding 28(4): 4550. Oct. 1958.
SAUNDERS, L. G.
1959. METHODS FOR STUDYING FORCIPOMYIA MIDGES, WITH SPECIAL REFERENCE TO CACAO-POLLINATING SPECIES (DIPTERA, CERATOPOGONIDAE). Canad. Jour. Zool. 37: 33-51.STEJSKAL, M.
1969. [NECTAR AND AROMA OF THE CACAO FLOWER.] Oriente Agropecuario 1(2): 75-92. [In Spanish, English summary.]SUMNER, H. M.
1962. [COCOA] POLLINATION. In Wills, J. B., ed., Agriculture and Land Use in Ghana, pp. 260 - 261. Oxford University Press, London, Accra, New York.TOXOPEUS, H.
1969. CACAO. In Ferwerda, F. P., and Wit, F., eds., Outlines of Perennial Crop Breeding in the Tropics, pp. 79-109. H. Veenman and Zonen, N. V. Wageningen, The Netherlands.URQUHART. D. H.
1961. COCOA. Ed. 2, 293 pp. Longmans, Green & Co., Ltd., London.VOELCKER, O. J.
1940. THE DEGREE OF CROSS-POLLINATION IN CACAO IN NIGERIA. Trop. Agr. [Trinidad] 17: 184-186.
CASHEW
Anacardium occidentale L., family AnacardiaceaeThe cashew is a hardy drought-resistant tropical or subtropical tree. This limits its growth to the area of our continent from Mexico to Peru and Brazil, but includes Hawaii, Puerto Rico, and favored parts of the southern tip of Florida. Worldwide, India is the leading producer; other producing countries include Mozambique and Tanzania (Mutter and Bigger 1961, Purseglove 1968*).
Plant:
The cashew is a somewhat straggly evergreen tree, 12 to 15 m in height, seldom taller, with oblong 6- to 7-inch leathery green leaves and terminal, many flowered panicles. It is cultivated for its delicious 1- inch-long, kidney-shaped nut (fig. 65). The nut is inedible when raw and must be roasted to drive off the highly irritating volatile oil. The nut is produced on the end of a greatly enlarged fleshy pedicel disk and receptacle, called the cashew apple. The cashew apple is about 2 inches wide and 3 to 4 inches long (Kennarc and Winters 1960*), and when ripe it is shiny, red or yellow, soft, and juicy. It is used as a fresh fruit or in juices, jellies, or for making wine (Ochse et al. 1961*) The tree bark provides an indelible ink, and the shell provides an insect-repelling vesicant oil (Purseglove 1968 *).
The fruit ripens in 2 to 3 months and is harvested from the tree or picked up soon after falling. The nut is removed from the apple, dried or roasted in the shell, then hulled and vacuum packed.
Cashew plants are usually grown from seed and thinned to 30 by 30 feet. They begin bearing the second year, are in full production by the 10th year, and continue bearing for another 20 years. The yield varies from 1 to 100 pounds per tree (Purseglove 1968*, Haarer 1954).
[gfx]
FIGURE 65. - Cashew fruit. A, Cashew apple; B, cashew nut.Inflorescence:
The cashew inflorescence is a sweet-scented lax terminal, many- flowered panicle 4 to 8 inches long. Both male and hermaphrodite flowers occur on the same inflorescence (fig. 66). In Tanganyika, Bigger (1960) found as many as 767 panicles on a single tree, with 63 to 67 hermaphrodite and 250 to 400 male flowers per panicle. In Mangalore, Madhava Rao and Vazir Hassan (1957) counted 329 florets on a panicle, 316 of which were staminate and 13 hermaphrodite. Only about 5 percent of the hermaphrodite flowers produce fruit (Anonymous, 1964). In general, the fewer the hermaphrodite flowers the lower the percent set. Usually from one to less than half a dozen fruits mature per cluster (Ochse et al. 1961 *, Northwood 1966).
The five reflexed petals of the l/3 to l/2-inch flower are pale green with red stripes, later turning to solid red (Morton 1961). In the male flower, about nine stamens are 4 mm long and one stamen, 12 mm, not all of which may be functional. The hermaphrodite flower also has nine short stamens and one about 8 mm long. The one-ovule ovary contains a style that extends above its own anthers to the same height as the long anther of the male flowers. About six flowers open per day on an inflorescence (Northwood 1966).
The flower opens almost any time of the day, but the peak period of opening is 11 am. to 12:30 p.m. The stigma is receptive as soon as the flower opens, but the anthers do not dehisce until 5 hours later, giving opportunity for crossing. The stigma is receptive for only 1 day (Madhava Rao and Vazir Hassan. 1957). The flower produces an abundance of nectar, which is highly attractive to flies, bees, ants, and other insects (Morton 1961, Free 1970*).
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FIGURE 66.- Longitudinal section of cashew flower, x 7. A, Hermaphrodite flower with elongated style and short stamens; B, male flower with abortive pistil and elongated stamen.Pollination Requirements:
The hermaphrodite flowers are self-fertile but not self-pollinating as indicated by the fact that bagged flowers set no fruit, but when flowers were hand self-pollinated a set of about five fruits per inflorescence was obtained (Northwood 1966). Madhava Rao and Vazir Hassan (1957) obtained a set of 55.5 percent of self-pollinated flowers. Because only one ovule in one ovary exists per flower, there is no need for a large amount of pollen on the stigma.
Pollinators:
Madhava Rao and Vazir Hassan (1957) indicated that the cashew was wind pollinated, with insects being unimportant, and Bigger (1960) also concluded that the high percentage of male flowers suggested that wind was the pollinating agent. The study by Northwood (1966), however, leaves little doubt that fruit setting is the result of insect activity. He considered that flies and ants were the principal pollinators. Madhava Rao and Vazir Hassan (1957) stated that only black and red ants visited the flowers, but Wulfrath and Speck (no date) stated that the flowers are attractive to bees all day for their rich nectar. Smith (1960) stated that cashew can be added to the Iist of plants benefiting from insect pollination. Personal correspondence from bee specialists in Ghana indicates that when bees are moved to cashews the production is increased.
Pollination Recommendations and Practices:
There are no recommendations on the use of insects in the pollination of cashew. The evidence strongly indicates that concentration of honey bee colonies in cashew plantings during flowering would at least alleviate the problem of poor setting of fruit. Selection for clones with a higher percentage of hermaphrodite flowers would doubtless enhance fruit production.
LITERATURE CITED:
ANONYMOUS.
1964. ADMINISTRATION REPORT OF THE AGRICULTURE DEPARTMENT, GOVERNMENT OF KERALE, FOR THE YEAR 1962-1963. 325 PP. Plant Breed. Abs. 35: 3772 (1965).BIGGER, M.
1960. SELENOTHRIPS RUBROCINCTUS GIARD AND THE FLORAL BIOLOGY OF CASHEW IN TANGANYIKA. East Africa Agr. Jour. 25: 229-234.HAARER, A. E.
1954. THE CASHEW (ANACARDIUM OCCIDENTALE LTNN.) NUT. World Crops 6: 95-96, 98.MADHAVA RAO, V. N., and VAZIR HASSAN, M.
1957. PRELIMINARY STUDIES ON THE FLORAL BIOLOGY OF CASHEW (ANACARDIUM OCCIDENTALE LINN.). Indian Jour. Agr. Sci. 27: 277 - 288.MORTON, J.
1961. THE CASHEW'S BRIGHTER FUTURE. Econ. Bot. 15: 57 - 78.MUTTER, N. E. S., and BIGGER, M.
1961. CASHEW. Tanganyika Min. Agr. Bul. 11, 5 pp.NORTHWOOD, P. J.
1966. SOME OBSERVATIONS ON FLOWERING AND FRUIT-SETTING IN THE CASHEW, ANACARDIUM OCCIDENTALE L. Trop. Agr. [Trinidad] 43(1): 35-42.SMITH, F. G.
1960. BEEKEEPING IN THE TROPICS. 265 pp. Longmans, New York.WULFRATH, A., and SPECK, J. J.
[n.d.] [LA FLORA MELIFERA.] Enciclopedia Apicola, Folleto 28, ed. 2, 96 pp. Ediciones Mexicanas, Mexico, D. F. [In Spanish.]
CHERRY
Prunus spp., family RosaceaeCherries grown commercially in the United States include sweet cherries (P. avium L.); tart, sour, or pie cherries (P. cerasus L.); and Dukes (probably P. gondouinii (Poit. & Turp.) Rehder). The mahaleb (P. mahaleb L.) and the mazzard, a wild or seedling form of P. avium, are used as rootstock upon which the fruiting types are grafted. The mahaleb is used much more extensively than the mazzard (Howe 1926, USDA 1967).
In 1970, 121,650 tons of sweet cherries, including Dukes (usually grouped with the sweet cherries), were produced, primarily on the West Coast. Oregon produced 40,000 tons; Washington, 25,800; California, 25,400; and Michigan, 21,000 tons. There were 118,640 tons of tart cherries produced - 79,000 in Michigan and 18,200 tons in New York. Several other States produced smaller amounts of both kinds.
The value of the 1970 sweet cherry crop was $43.2 million, compared to $17.9 million for the tart cherries.
Plant:
The deciduous cherry tree does not thrive where summers are long and hot, yet the blossoms are susceptible to injury by cold spring weather (Cullinan 1937). For these reasons, the growing areas are limited to the more northerly States, except for some areas of high altitude and temperatures moderated by large bodies of water such as the oceans or the Great Lakes.
The trees are planted at various distances apart but most commonly 20 feet for tart cherries and 25 to 32 feet for sweet cherries. They are usually planted at equal distances apart, except when the contour or hedgerow systems are used (Griggs 1970*).
When hedgerow planting is used in California, the trees are placed 6 feet apart in the row and the rows are spaced 4 feet apart. The pollenizer trees are placed at every eleventh location in every other row, offset by five trees, about one pollenizer for each 20 recipient trees (Ryugo and Mikuckis 1969).
Inflorescence:
When in bloom the cherry tree displays white, faintly fragrant flowers in clusters of two to five on short lateral spurs on the many branches (fig. 71). The five petals of the flower are oval, white, and rather widely spread. There is a single upright pistil and about 30 loose stamens (fig. 72). The sweet cherry flower is about an inch across, the tart cherry slightly smaller. The flower remains open 7 to 8 days. When the flower opens the stigma is receptive, but the anthers are closed. Anthers begin opening shortly after flowers open and continue into the second day (Knuth 1908*, p. 703; Srivastava and Singh 1970). Nectar is secreted on the inner surface of the receptacle. Eaton (1959) stated that pollination on the first day after anthesis was much more effective than pollination on the second day, and he stressed the importance of the earliest possible pollination particularly in cultivars such as 'Schmidt'.
Both pollen and nectar are attractive to insects, particularly bees, throughout the day if weather permits. The sweet cherry nectar is much richer in sugar (55 percent sugar) than the tart cherry nectar (28 percent) (Vansell 1942*). Pellett (1947*) stated that in California the cherry is one of the best fruit trees for honey production. Because of the time of year that cherries bloom, colonies are frequently not sufficiently strong to store surplus amounts and cherry honey is practically unknown. There are usually few other floral visitors except honey bees, although Nevkryta (O.M.) (1957) reported that only 60 percent of the insects on flowering sweet cherries were honey bees.
[gfx]
FIGURE 71. - Fruiting branch of cherry, showing spurs and clusters of flowers.
FIGURE 72. - Longitudinal section of a 'Bing' cherry flower, x 7.Pollination Requirements:
The sweet cherry was shown by Gardner (1913), Anonymous (1926), Overholser and Overley (1931), Crane and Brown (1937), and Way (1968) to be self-sterile or self-unfruitful, and, furthermore, the most important cvs., 'Bing', 'Lambert', and 'Napoleon' ('Royal Ann'), were shown to be interincompatible. This interincompatibility continues to be a problem (Griggs 1970*), although Lapins (1971) reported that the 'Stella' cv. was a self-compatible sweet cherry, derived from a radiation-induced self- fertile selection obtained from England.
The attitude toward the pollination of tart cherries has changed over the years. Crane (1925), Dujardin (1921), Hooper (1924), and Schuster (1925) stated that the tart cherry was self-sterile or nearly so. Einset (1932) said that there was a continuous range from complete self- fruitfulness to complete self-unfruitfulness. Roberts (1922) and Marshall et al. (1929) said the blossoms were self-fertile and that insect pollinators were not needed. Murneek (1930) said they were self-fertile but benefited from insect pollination in unfavorable seasons. However, Hootman (1931, 1933) showed that only 4 percent of screened blooms (of 'Montmorency' cv.) produced fruit as compared to 49 percent that were hand pollinated. Lagasse (1928) and later Vansell and Griggs (1952*) stated that the commercially important tart cherry cultivars are self- fruitful if enough pollinizing insects are available, but better crops can be expected if the orchard contains more than one cultivar. The knowledge is now fairly well accepted that all of the important tart cherry cultivars will set fruit with their own pollen, but only after it is transferred by some outside agency from the anthers to the stigma.
The amount of fruit set expected on cherries has been mentioned by various research workers. All concede that set of every blossom is undesirable. Shoemaker (1928) reported a range of 13 to 60 percent with an average of 35 percent set of sweets, 21 to 42 percent with an average of 33 percent for tart cherries, and 10 to 53 percent with an average of 20 percent set for Dukes. As previously mentioned, Hootman (1931) obtained 49 percent set of hand-pollinated 'Montmorency' tart cherries. Gardner (1913) stated that 50 percent of the sweet cherry flowers should set. Griggs et al. (1952*) reported an overall average for several seasons of good crops at 21 to 32 percent set. Griggs (1970*) stated that self- fruitful cultivars of sweet cherries may be undesirable if they tend to set too heavily. Also, the fruit fails to develop adequate size without expensive thinning practices.
Luce and Morris (1928) stated that if the cherry blossom is not pollinated, the fruit develops to the size of a garden pea, then drops to the ground.
Tukey (1925), Free and Spencer-Booth (1964), and numerous others have reported decreasing production with increased distance from the pollenizer row of sweet cherries.
In summary, all cherries are basically incapable of automatic self- pollination. Tart cherries will set fruit if the pollen is transferred from anthers to stigma of the same flower but will set more fruit if other cultivars are interplanted in the orchard. Compatible cultivars can only be determined by tests (Griggs 1953*). Sweet cherries, with the exception of the 'Stella' cv. (Lapins 1970), will not set fruit with their own pollen, only with pollen of certain other cultivars.
Way and Gilmer (1963) showed that healthy trees are important in the set of cherries. When they used pollen from trees infected with tart cherry yellows disease, fruit set was only 25 to 90 percent of that with pollen from healthy trees. Such pollen would either decrease production or create a demand for more insect pollinators.
Pollinators:
Wind is not a factor in cherry pollination, as has been clearly and repeatedly established over the years (Roberts 1922, Burtner 1923, Murneek 1930, Claypool et al. 1931, and Brown 1968). Most researchers and growers give the primary credit for the pollination of cherries to honey bees. A heavy pollinator population is needed and flowering occurs too early in the year for other insects to be plentiful. Hendrickson (1922) stated that as early as 1894 a government report showed that a cherry crop near Vacaville, Calif., was greatly increased when several colonies of honey bees were placed in the orchard. Morrill (1899) also reported that bees increased cherry production. Gardner (1913) was the first to establish scientifically the need for pollination, and he stressed the importance of bees. This was supported with further research by various others, including Wellington (1923), Tuft and Philp (1925), Hooper (1930). Claypool et al. (1932), Weiss (1957), Skrebtsova and Iakovlev (1969), Eaton (1959), and Brown. 21
The fact that possibly only one pollen grain is needed to pollinate a cherry flower would indicate that repeated bee visits may be unnecessary, providing the pollen grain is compatible and successful fertilization of the ovule ensues. To play safe, the grower should insure the transfer of many pollen grains to the stigma. Tart cherry pollen may come from the same flower or the same tree, although greater benefit is usually derived if pollen comes from another cultivar. Sweet cherry pollen must come from another - and compatible - cultivar; therefore, a high degree of bee activity on the tree and between trees is required to adequately pollinate the crop.
The proper pollinator population is not easy to establish. Griggs et al. (1952*) counted 30 to 40 bees per sweet cherry tree that had been in production several years. The number of colonies per acre necessary to provide this population was not given. Skrebtsova and Iakovlev (1959) spoke of "saturation pollination" of cherries, but their data indicated that even with their maximum of 3.8 colonies per hectare (less than two colonies per acre) maximum set of all flowers was not achieved.
__________
21 Brown, K. BEES FOR SWEET CHERRY POLLINATION- UNDER WILLAMETTE VALLEY FONDITIONS. Polk County (Oregon) Agr. Ext. Serv. Agent, 2 pp. 1969. [Mimeographed.]Pollination Recommendations and Practices:
Schuster (1925) recommended one strong colony for each 1 to 2 acres "if the stands are strong." Tufts and Philp (1925) recommended at least one colony per acre. Marshall et al. (1929), Murneek (1930), Philp (1930, 1947), and Stephen (1961) concurred with the one-colony-per-acre recommendation. Hooper (1930) recommended that colonies be placed in the orchard during flowering. Brown (1968)22 recommended four to five colonies per acre for his area of Oregon, the colonies placed in groups on each 5 to 10 acres of the orchard. Eaton (1962) stated that strong colonies should be brought into the sweet cherry orchard on or before the day the first flowers open, because placement in the orchard even 1 day late could result in a reduced crop. Auchter and Knapp (1937*) recommended one colony containing 7 to 9 pounds of bees to every 3 to 4 acres but conceded that many growers use one colony for each acre or two. Coe (1934) and EIoffman (1965) urged the use of bees but did not designate the concentration. Nevkryta (A. N.) (1957) recommended four to five colonies per hectare (about two colonies per acre). Skrebtsova and Iakovlev (1959) recommended "saturation pollination" of the orchard, and showed that with 3.8 colonies per hectare, 15 percent of all flowers set fruit but with 2.8 colonies only 13 percent set. Luce and Morris (1928) recommended one colony per acre. Schuster (1925) also reported, "It is becoming the practice for cherry growers either to keep their own bees or to hire stands of bees during the blooming season." To take advantage of this needed cross-pollination between cultivars, various planting plans of trees in the orchard were recommended, ranging from one pollenizer and nine recipient trees to a 1:1 ratio. This recommended usage of bees barely seems to be accepted by the growers. Kelly (n. d.) reported that during 1959 - 63, tart cherry growers in Pennsylvania spent only 28 cents per acre for pollination fees; when colonies were rented, the fee was $4.50 per colony. Considering the pollination needs of this crop and the apparent lack of effort expended by these growers, one is not too surprised at his statement: "In the last decade sour cherry production and growers have both declined 31 percent." However, pollination is probably not the only reason for this decline. In a similar study made in Michigan on 37 tart cherry farms, Kelsey (1964) reported that growers paid an average of $1.33 per acre for bee pollination. The number of colonies of honey bees utilized, for which there was no remuneration, was not disclosed. In general, most cherry growers make some attempt to have bees present in their cherry orchards at flowering time. Frequently, if bees are rented and there are 2 or 3 days of good weather for bee flight, the tart cherry grower is ready for the bees to be removed. The number, strength, and placement of colonies necessary to provide 50-percent set of cherry flowers (Gardner 1913, Hootman 1931) is not known but should be determined. Also, the difference in the need of bee pollination between sweet and tart cherries should be determined. For highest production of cherriesÑthe setting of the maximum number of blooms for greatest production of sizeable fruitÑcross- compatible cultivars that flower at the proper time must be interplanted in sweet cherry orchards, and possibly also in tart cherry orchards, although large solid blocks are known to produce satisfactory crops. For highest production of either sweet or tart cherries as many as five strong colonies of honey bees per acre should be placed on each 5 to 10 acres just before flowering time. The colonies should contain 600 in2 or more of brood and 7 to 9 pounds of bees.
__________
22 Brown, K. POLLINATION OF ROYAL ANN (A-10) IN THE WILLAMETTE VALLEY. Polk County (Oregon) Agr. Ext. Serv. Agent, 4 pp. 1968. [ Mimeographed.]LITERATURE CITED:
ANONYMOUS.
1926. CROSS POLLINATION OF THE WINDSOR VARIETY. Amer. Fruit Grower Mag. 46(3): 26.BURTNER J. C.
1923. LATEST CHERRY POLLINATION STUDIES. Better Fruit 182: 5 - 6, 23 - 24.CLAYPOOL, L. L., OVERLEY, F. L., and OVERHOLSER, E. L.
1932. SWEET CHERRY POLLINATION IN WASHINGTON FOR 1931. Amer. Soc. Hort. Sci. Proc. 28: 67-70.____ OVERLEY, F. L., and OVERHOLSER, E. L.
1931. WASHINGTON SWEET CHERRY POLLINATION STUDIES IN 1931. 27th Ann. Mtg. Wash. State Hort. Assoc. Proc. December 1, 2, and 3 at Yakima, Wash., pp. 171-174.COE, F. M.
1934. CHERRY POLLINATION STUDIES IN UTAH. Utah Agr. Expt. Sta. Bul. 245, 53 pp.CRANE M. B.
1925. SELF-STERILITY AND CROSS INCOMPATIBILITY IN PLUMS AND CHERRIES. Jour. Genet. 15: 301, 322.____ and BROWN, A. G.
1937. INCOMPATIBILITY AND STERILITY IN THE SWEET CHERRY. Jour. Pomol. and Hort. Sci. 15: 86 - 116.CULLINAN, F. P.
1937. IMPROVEMENT OF STONE FRUITS. U.s. Dept. Agr. Yearbook 1937: 724 - 737.DUJARDIN F.
1921. [POLLINATION OF TREE FRUITS.] Rev. Hort. [Paris] 93: 300-302. [In French.]EATON, G. W.
1959. A STUDY OF THE MEGAGAMETOPHYTE IN PRUNUS AVIUM AND ITS RELATION TO FRUIT SETTING. Canad. Jour. Plant Sci. 39: 466-476.____ 1962. SWEET CHERRY POLLINATION, FRUITSET, AND VARIETIES. Mich State Hort. Soc. Ann. Rpt. 92: 102 - 104.
EINSET, O.
1932. EXPERIMENTS IN CHERRY POLLINATION. N.Y. Agr. Expt. Sta. (Geneva) Bul. 617, 13 pp.FREE, J. B. and SPENCER-BOOTH, Y.
1964. THE EFFECT OF DISTANCE FROM POLLENIZER VARIETIES ON THE FRUIT SET OF APPLE, PEAR AND SWEET-CHERRY TREES. Jour. Hort. Sci. 39: 54 - 60.GARDNER, V. R.
1913. A PRELIMINARY REPORT ON THE POLLINATION OF THE SWEET CHERRY. Oreg. Agr. Expt. Sta. Bul. 116, 37 pp.HENDRICKSON, A. H.
1922. WONDER WORK OF BEES. THEY MAKE MILLIONS FOR THE FRUIT GROWERS. BEES THAT RETURNED TO THE ORCHARDIST $100 PER COLONY. Gleanings Bee Cult. 50: 226-229.HOFFMAN, M. B.
1965. POLLINATION AND FRUIT DEVELOPMENT OF TREE FRUITS. N.Y. (Cornell) Agr. Ext. Serv. Bul. 1146, 8 pp.HOOPER, C. H.
1924. NOTES ON THE POLLINATION OF CHERRIES APPLIED TO COMMERCIAL CHERRY GROWING. Jour. Pomol. and Hort. Soc. 3: 185-190.____ 1930. THE STUDY OF POLLINATION IN RELATION TO CHERRY ORCHARDS. Gardners' Chron. 88(2293): 475 - 476.
HOOTMAN, H. D.
1930. RECENT DISCOVERIES IN POLLINATION METHODS AND PRACTICES AND THEIR INFLUENCE UPON GREATER YIELDS OF DESIRABLE FRUIT. Md. Agr. Soc. Farm Burl Fed. Rpt. 15,170-182; also in Md. State Hort. Soc. Proc. 33: 24-36.____ 1933. THE IMPORTANCE OF POLLINATION AND THE HONEYBEE IN FRUIT YIELDS. Mo. State Hort. Soc. Proc. 1930/1932: 59-67.
HOWE, G. H.
1926. RELATIVE MERITS OF MAZZARD AND MAHALEB ROOT-STOCKS FOR CHERRIES. Amer. Soc. Hort. Sci. Proc. 23d Ann. Mtg., pp. 53-55.KELLY, B. W.
[ n.d. ] FACTORS RELATED TO THE COST OF PRODUCING CHERRIES IN PENNSYLVANIA, 1959-1963. Farm Mangt. 20, (Pa. Agr. Ext. Serv.), 17 pp.KELSEY, M. P.
1964. THE COST OF GROWING TART CHERRIES IN THE VARIOUS AREAS OF MICHIGAN AND HOW THEY WERE DETERMINED. Mich. State Hort. Soc. 94th Ann. Rpt., pp. 90 - 94.
LAGASSE, F. S.
1928. PROPER POLLINATION OF FRUIT BLOSSOMS. Del. Agr. Expt. Sta. Bul. 15, 20 pp.LAPINS, K. O.
1971. 'STELLA', A SELF-FRUITFUL SWEET CHERRY. Canad. Jour. Plant Sci. 51: 252-253.LUCE, W. A., and MORRIS, O. M.
1928. POLLINATION OF DECIDUOUS FRUITS. Wash. Agr. Expt. Sta. Bul. 223, 22 pp.MARSHALL, R. E., JOHNSTON, S., HOOTMAN, H. D. and WELLS, H. M.
1929. POLLINATION OF ORCHARD FRUITS IN MICHIGAN. Mich. Agr. Expt. Sta. Spec. Bul. 188, 38 pp.MORRILL, F. L.
1899. BEES AND FRUIT. Gleanings Bee Cult. 27: 430 - 431.MURNEEK, A. E.
1930. FRUIT POLLINATION. Mo. Agr. Expt. Sta. Bul. 283, 12 pp.
NEVKRYTA, A. N.
1957. [DISTRIBUTION OF APIARIES FOR POLLINATING CHERRIES.] PchelovodstVo 34(4): 34-38. [In Russian.] AA-373l58.NEVKRYTA, O. M.
1957. [INSECT POLLINATORS OF WILD AND CULTIVATED CHERRY IN THE UKRAINE.] Zbirn. Prats Zool. Muz. 28: 49-61. [In Ukrainian, Russian summary.] AA-418/65.OVERHOLSER, E. L., and OVERLAY, F. L.
1931. CHERRY POLLINATION STUDIES IN WASHINGTON FOR 1930. Amer. Soc. Hort. Sci. Proc. 27: 400 - 403.PHILP, G. L.
1930. CHERRY CULTURE IN CALIFORNIA. Calif. Agr. Ext. Serv. Cir. 46, 43 pp.____ 1947. CHERRY CULTURE IN CALIFORNIA. Rev. Calif. Agr. Ext. Serv. Cir. 46, 51 pp.
ROBERTS, R. H.
1922. BETTER CHERRY YIELDS. Wis. Agr. Expt. Sta. Bul. 344, 30 pp.RYUGO, K., and MIKUCKIS, F.
1969. SWEET CHERRY HEDGEROW PLANTING. Calif. Agr. 23(11): 14 - 15.SCHUSTER, C. E.
1925. POLLINATION AND GROWING OF THE CHERRY. Oreg. Agr. Col. Expt. Sta. Bul. 212, 40 pp.SHOEMAKER, J. S.
1928. CHERRY POLLINATION. Ohio Agr. Expt. Sta. Bul. 422, 34 pp.SKREBTSOVA, N. D., and IAKOVLEV, A. S.
1959. [EFFECTIVENESS OF SATURATED POLLINATION OF CHERRIES BY BEES.] Pchelovodstvo 36(5): 25 - 26. [ In Russian. ] AA-154/61.SRIVASTAVA, R. P., and SINGH, I.
1970. FLORAL BIOLOGY, FRUIT-SET, FRUIT-DROP, AND PHYSICO- CHEMICAL CHARACTERS OF SWEET-CHERRY (PRUNUS AVIUM L.). Indian Jour. Agr. Sci. 40: 400-420.STEPHEN, W. P.
1961. BEES AND POLLINATION OF STONE FRUITS. Oreg. State Hort. Soc. Ann. Rpt. 53: 78 - 79.TUFTS, W. P., and PHILP, G. L.
1925. POLLINATION OF THE SWEET CHERRY. Calif. Agr. Expt. Sta. Bul. 385, 28 pp.TUKEY, H. B.
1925. AN EXPERIENCE WITH POLLENIZERS FOR CHERRIES. Amer. Soc. Hort. Sci. Proc. 21: 69-73.UNITED STATES DEPARTMENT OF AGRICULTURE.
1967. GROWING CHERRIES EAST OF THE ROCKY MOUNTAINS. U.S. Dept. Agr. Farmers' Bul. 2185, 30 pp.WAY, R. D.
1968. POLLEN INCOMPATIBILITY GROUPS OF SWEET CHERRY CLONES. Amer. Soc. Hort. Sci. Proc. 92: 119-123.____ and GILMER, R. M.
1963. REDUCTIONS IN FRUIT SETS ON CHERRY TREES POLLINATED WITH POLLEN FROM TREES WITH SOUR CHERRY YELLOWS. Phytopathology 53: 399-401.WEISS, K.
1957. [THE DEPENDENCE OF THE CHERRY HARVEST ON THE NUMBER OF COLONIES PRESENT.] Deut. Bienenw. 8(7): 124-126. [In German. ] AA-374/58.WELLINGTON, R. [A.]
1923. SELF-STERILITY AND SELF-FERTILITY OF FRUIT VARIETIES GROWN IN NEW YORK. N.Y. (Geneva) Agr. Expt. Sta. Cir. 71, 6 pp.
CHESTNUT
Castanea spp., family FagaceaeChestnut trees are cultivated for their nuts or as ornamentals. Probably the most notable species was the large and graceful ornamental American chestnut (C. dentata (Marsh.) Borkh.) (fig. 73), which extended from Maine southwest to Arkansas (Munns 1938). It has been almost completely destroyed by blight. The Japanese chestnut (C. crenata Sieb. and Zucc.) and the Chinese chestnut (C. mollisima Blume) are both cultivated for their nuts.
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FIGURE 73. - American chestnut tree. (Photograph taken in 1915.)Plant:
Chestnut is a deciduous tree or shrub, which is cultivated in a similar manner to other deciduous nut trees. It bears brown nuts, about an inch in diameter, which are usually consumed after they are roasted. From one to nine nuts are produced in a spiny involucre or burr (fig. 74).
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FIGURE 74.- Burrs and nuts of Chinese chestnuts.Inflorescence:
The fragrant inflorescence is about 12 inches long (fig. 75). It consists of a group of catkins 4 to 8 inches long. Catkins bearing only staminate florets make up the bulk of the inflorescence. Those produced near the base bear both staminate and pistillate florets. The latter, near the base of the catkin, are few in number. Usually three pistillate florets make up an involucre, each floret capable of producing three nuts.
Bees visit the staminate flowers for both nectar and pollen (Hazslinszky 1955, McKay 1939, Pellett 1947*). the degree of visitation depending upon competition from other flowers. The bees do not intentionally visit the pistillate flowers, but may accidentally come in contact with them while visiting the staminate flowers.
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FIGURE. - Chestnut inflorescence.Pollination Requirements:
Reed (1941) concluded that chestnut is self-sterile. He noted that isolated trees bear few nuts or even a fair crop, but best results are invariably obtained from trees in a mixed orchard where good pollen is available. McKay (1939) reported finding a C. crenata tree that was completely male-sterile. Its nectar production was normal, and it produced a normal crop of nuts. He also reported male sterility in C. sativa Mill. and C. sativa X C. dentata. Later, McKay (1942) reported that when flowers of C. mollissima were self-pollinated only 1.3 percent of the flowers set fruit, when they were cross-pollinated 34.9 percent set, but when they were open-pollinated 68.1 percent set. This showed the need for transfer of pollen between plants.
Crane et al. (1937) stated: "As a rule all chestnuts are more or less self-sterile and they bear better when interplanted with other cultivars."
Kawagoe (1955) stated that the stigmas may remain receptive as much as 45 days and that cross-pollination was best effected 8 to 22 days after stigma emergence.
Ohno et al. (1958), considering the effect of rain on the pollination of chestnuts, tested the effect of water on the pollen. They found that 17 to 19 percent of their pollen germinated even after soaking in water in the laboratory for 9 hours. In the field, this pollen caused 48 to 50 percent set of fruit if cross-pollinated but only 3 to 9 percent set if it came from the same plant.
Watanabe et al. (1964) reported much higher bur-set on adjoining rows to the pollenizer row than on the (decreasing) 3d to 10th rows. They recommended that pollenizer cultivars be set in the ratio of 1 to 1 or 1 to 2 of the main cultivar.
Pollinators:
Crane et al. (1937) and Clapper (1954) stated that chestnut pollen is produced in great abundance and is carried by wind. However, J. W. McKay (personal commun., 1972) questioned this. He indicated that honey bees, rose chafers, and wild bees are highly beneficial to chestnut in the transfer of pollen, and they frequently visit the staminate flowers in large numbers. He also considered that for highest production on younger trees, a high population of pollinators is especially needed. If production of newer cultivars and hybrids expands, the value of insects in cross- pollination for maximum set should be more fully explored.
Pollination Recommendations and Practices:
There are no recommendations on the use of pollinating insects on chestnut although evidence shows they are needed.
LITERATURE CITED:
CLAPPER, R. B.
1954. CHESTNUT BREEDING, TECHNIQUES AND RESULTS. II. INHERITANCE OF CHARACTERS, BREEDING FOR VIGOR AND MUTATIONS Jour. Hered. 45: 201-208.CRANE, H. L., REED, C. A., and WOOD, M. N.
1937. NUT BREEDING. U.S. Dept. Agr. Yearbook 1937: 827 - 889.HAZSLINSZKY, B.
1955. [THE IMPORTANCE OF THE CHESTNUT TREE FOR BEEKEEPING.] Meheszet 3(6): 109 - 110. [ In Hungarian.] AA-171/57.KAWAGOE, H.
1955. [STUDIES ON THE PERIOD DURING WHICH THE CAPACITY FOR FERTILIZATION OF THE CHESTNUT PERSISTS.] Okayama Nogaku Shikenjo Rinji Hokoku/Spec. Bul. Okayama Agr. Expt. stat 53: 141 - 154. [In Japanese.] Abs. in Plant Breed. 28(4): 826. 1958.McKAY, J. W.
1939. MALE STERILITY IN CASTANEA. Amer. soc. Hort. Sci. Proc. 37: 509-510.______ 1942. SELF-STERILITY IN THE CHINESE CHESTNUT (CASTANEA MELLISSIMA). Amer. Soc. Hort. Sci. Proc. 41: 156-160.
______ 1972. POLLINATION OF CHESTNUT BY HONEY BEES. North. Nut Growers' Assoc. Ann. Rpt. 63: 83-86.
MCKAY, J. W. and CRANE, H. L.
1953. CHINESE CHESTNUTS A PROMISING NEW ORCHARD CROP. Econ. Bot. 7(3): 228 - 242.MUNNS, E. N.
1938. THE DISTRIBUTION OF IMPORTANT FOREST TREES OF THE UNITED STATES. U. S. Dept. Agr. Misc. Pub. 287, 176 pp.OHNO M., SATO, s., and SAWABE, H.
1958. [THE STUDY OF CHESTNUT POLLINATION. 1. THE FRUIT SET OF CHESTNUTS WHICH WERE POLLINIZED BY THE WETTING POLLEN.l Chiba Univ. Faculty Hort. Tech. Bul. 6: 129-135. [In Japanese, English summary.]REED, C. A.
1941. THE PRESENT STATUS OF CHESTNUT GROWING IN THE UNITED STATES. Amer. soc. Hort. Sci. Proc. 39: 147-152.WATANABE, Y., ADACHI, M., and HIYAMA, H.
1964. [STUDIES ON THE POLLINATION IN CHESTNUT TREES. 1. INFLUENCE OF DISTANCE FROM THE POLLINIZER UPON BUR-SET IN VARIETY GINYOSE.] Ibaraki Hort. Expt. stat Bul. l: 7-12. [In Japanese, English summary.]
CITRUS
Citrus spp., family RutaceaeThe kind of citrus crop produced, its volume, area of profusion; and dollar value are shown in table 8. As this table shows, the bulk of the citrus crop is produced in Florida, and oranges and grapefruit account for more than 80 percent of all fruit produced.
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TABLE 8. - Estimated U.S. production of citrus by State, type, number of boxes, and total value in 1970-71Plant:
The cultivated citrus plants are mostly shrubs or small trees with dense foliage; sweet-smelling, whitish to purple flowers that are often produced in great profusion; and greenish to golden fruit. The trees may live for more than 100 years, but citrus groves more than 50 years old are rare. Depending on the kind involved, the fruit may mature from fall until summer of the year following flower development. Some fruits, for example certain mandarins, fall shortly after they mature. Others, such as the 'Valencia' orange or the grapefruit, will remain on the tree several months after maturity. Citrus has little cold resistance and is not grown in areas where the temperature is likely to fall below 20 deg F.
A high degree of cross-fertility exists between the species of Citrus as well as between the genera of Citrus, Fortunella, and Poncirus. This has permitted breeders to develop the various simple and multiple hybrids, some of which have become of considerable economic importance (Cameron and Soost 1969).
The common and scientific names of the more well-known cultivars are shown in table 9. The species frequently mentioned but of minor value or used as rootstock or in breeding work are as follows:
Common name Scientific name:
Calamondin..........................................................Citrus reticulata var. austera Swingle X Fortunella spp. Citrange................................................................C. sinensis X Poncirus trifoliata Citrangequat.......................................................P. Trifoliata X Citrus spp. X Fortunella spp. Citron....................................................................Citrus medica L.Common nameÑ(Con.) Scientific nameÑ(Con.) Cleopatra mandarin............................................................C. reticulata Blanco Kumquat.................................................................................. Fortunella margarita (Lour.) Swingle Meyer lemon...........................................................................Citrus limon x C. medica Pummelo (Shaddock)..........................................................C. grandis (L.) Osbeck Rough lemon...........................................................................C. Iimon (L.) Burm. Sour Orange............................................................................C. aurantium L. Trifoliate orange.................................................................Poncirus trifoliata (L.) Raf. TABLE 9.ÑCommon and scientific names and important cultivars of U.S. citrus crops __________________________________________________________ Common Scientific Important name name cultivars __________________________________________________________ Grapefruit Citrus paradisi ÔBurgundyÕ, DuncanÕ,ÔMarshÕ, Macf. ÔRedblushÕ, ÔThompsonÕ. Lemon C. limon (L.) ÔEurekaÕ, ÔLisbonÕ Burm. f. Lime C. aurantifolia ÔKeyÕ (Mexican or West (Christm.) Indian group), ÔBearssÕ Swingle (Tahiti or Persian group). Orange (sweet) C. sinensis (L.) ÔHamlinÕ, ÔMediterranean Osbeck SweetÕ, ÔParson BrownÕ, ÔPineappleÕ, ÔValenciaÕ, ÔWashington NavelÕ. Mandarin and ÔAlgerianÕ (ÔClementineÕ) mandarin-hybrid ÔDancyÕ, ÔKinnowÕ, ÔK- complex EarlyÕ, ÔMinneolaÕ, ÔMurcottÕ, ÔOrlandoÕ, ÔPageÕ, ÔRobinsonÕ, ÔTempleÕ, ÔWilkingÕ. __________________________________________________________
Inflorescence:
The outstanding characteristics of citrus flowers are the pleasant fragrance, the pleasing contrast between the whitish (to pink or purple in lemons) petals and the dark-green background of the leaves, and the attractiveness of the flowers to bees. Blossom size varies in grapefruit, lemon, lime, orange, and the mandarin and mandarin-hybrid complex, ranging from about three-quarters of an inch for the smaller flowers to 1 1/2 inches for the largest (fig. 78).
The flowers usually open in one great flush of bloom in the spring, although lemons and acid limes are particularly noted for their tendency to flower throughout much of the year. The flowers are mostly hermaphrodite, releasing pollen when the stigma is receptive; however, staminate flowers occur in the lime, lemon, and citron (Purseglove 1968*) and pistillate flowers occur in 'Satsumas' (Kihara 1951). The pollenless flowers of the 'Washington Navel' are well known for their ability to set parthenocarpic fruit (Webber et al. 1943).
The flowers are in small clusters in the leaf axil of a preceding growth flush but single in the axils of a just-completed growth flush (Coit 1916, Chandler 1958*, Reece 1945). The four to eight, but usually five, oblong, glossy, flared petals arise from the base of the sexual column. The staminate portion consists of 20 to 40 upright white filaments, sometimes united into several groups at the base, with yellow anthers on the tip.
The globose yellowish stigma terminates the style. At the base, the style unites with the greenish ovary, with its 9 to 13 locules, which stands well above the disk.
Nectar is secreted from the nectary or floral disk just within and above the point of attachment of the stamens. Vansell et al. (1942) stated that secretion of nectar continues at least 48 hours after flower opening. Also, a thick viscous stigmatic fluid is secreted from papillose hairs on the stigma. This material serves to catch and hold pollen grains and provides suitable media for their germination. A similar material can sometimes be seen inside the style, apparently providing a route and media by which the pollen tube may reach the ovary.
The flowers open primarily from 9 a.m. to 4 p.m. with the peak period about noon (Randhawa et al. 1961). They never close; the petals merely shed a few days later. The stigma becomes receptive just before the bud breaks open, but the stamens usually do not release pollen until several hours later, after the flower is fully open (Wright 1937).
To determine if bee visitation altered the period of time the flower is open, I kept records of development on 20 'Clementine' ('Algerian') tangerine flowers at Yuma, Ariz., in 1954 (previously unpublished data). Ten flowers were on a tree enclosed in a cage with a colony of honey bees and 10 on a tree in a cage that excluded bees. Shedding of the petals and stamens in the no-bee cage was slightly slower than in the cage with bees but only because they became stuck in the uncollected nectar. Anther dehiscence was completed by the end of the second day, and normal petal fall was completed on the third day. Whether this applies to all other citrus or even to the same cultivar in other areas is not known but probably it is similar.
The difference in the appearance of the stigmas in the cages was significant. Pollination apparently occurred shortly after flower opening in the bee cage, after which the stigma color changed to brown. In the no- bee cage, the stigmas remained cream-colored and apparently receptive at least 4 days. This might explain the observation by Climenko (1936) that stigmas are receptive for 6 to 8 days.
Citrus generally yields nectar copiously. Vansell et al. (1942) stated that some blossoms contained 1.5 bee-loads of nectar, averaging 20 microliters, compared to 0.8 to 2.4 microliters per blossom for an alfalfa flower, another important nectar source. Because of the large amount and superior quality of honey that citrus blossoms produce, many beekeepers place their colonies in or near most groves.
The value of citrus as a source of pollen is influenced by the kind involved. Hamakawa (1967) reported that less than 1 percent of the bees foraging on 'Satsuma' mandarin (C. unshiu Marc.) carried pollen loads as compared to 95 percent on 'Hassaku' orange (C. hassaku Hort. ex. Y. Tanaka). In general, citrus is not considered to be an excellent source of pollen by beekeepers. Only a small percentage of citrus flowers set and develop into mature fruit. For example, Reuther et al. (1968) showed that one 'Washington Navel' tree had 102,350 blooms but matured only 419 fruit, and a 'Valencia' tree with 47,112 blooms matured 708 fruit. Reed (1919) reported 4,440 buds on one lemon tree, 52 percent of which set, but only 6.6 percent (294 fruit) reached maturity.
Moss (1971) studied the relation of flowering and the tendency toward biennial bearing in the sweet orange. He recorded twice as many flowers on the trees in "on" years as in "off" years, but the percentage of flowers that set was the same. Although more flowers usually equal more fruit, if the grower can take steps to increase this percentage during the "off" years, he should do so.
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FIGURE 78. - Longitudinal section of citrus flowers, x 3. A, 'Red Blush' grapefruit; b, 'Meyer' lemon; C, 'Algerian ('Clementine') tangerine; D, 'Washington Navel' orange.Pollination Requirements:
In general, citrus has been considered as a crop with little or no need for insect pollination. However, that which was said about a crop years ago may not be true today for, as Webber et al. (1943) pointed out, no variety is likely to remain entirely static over long periods, even when propagated asexually. The likelihood that pollination requirements of citrus have changed in this way is minor. More likely, our increased knowledge, obtained through continued studies, has enlightened us as to the range of pollination needs.
Furthermore, economic conditions may require maximum production of a crop if a net profit is to be realized. Under such conditions, a slight benefit, derived from better pollination of the crop, can become highly significant economically. Considerable attention has been given to citrus pollination recently. Krezdorn (1970) stated that a growing number of citrus cultivars are known to be self-incompatible and, in some cases, cross incompatible. With such cultivars, an appropriate pollen supply and pollinating agents is needed.
The pollination requirements of the different kinds of citrus are quite diverse. In some there is almost complete self-sterility. Pollen must be transferred to these flowers from those of another compatible type for maximum fruit production. In others, the plant is benefited if pollen is moved from flower to flower within the cultivar or within the species. In still others, there is no known benefit from transfer of pollen to the stigma by external agents over production caused by the plant's own pollen coming into contact with the stigma without the aid of such insects. In addition, there are varying degrees of parthenocarpic development of the fruit. Because of such diversity, the more important kinds of citrus are discussed separately.
GRAPEFRUIT:
Authorities on citrus in the United States have consistently stated that cross-pollination is not required in grapefruit, and that grapefruit production presents no pollination problem (Coit 1915, Frost and Soost 1968, Krezdorn 1970, 1972, Soost 1963, Webber 1930). This does not necessarily mean that no benefit is derived from insect transfer of pollen within the cultivar.
Wright (1937) studied the effect of cross-pollination on seed development and fruit set of the 'Marsh' grapefruit. Although some of his data on unpollinated (emasculated and bagged) flowers are open to question, he reported that open pollinated flowers set about twice as many seeds, but more importantly four times as many fruit, as selfed flowers. The presence of seeds is generally undesired by the canners and other consumers, although the 'Duncan' grapefruit is preferred by canners in spite of its seeds. The difference in fruit set could be of considerable economic importance. Satisfactory crops of grapefruit are normally harvested from solid blocks of a single cultivar.
LEMONS:
Richter (1916) stated that without question (but also without showing data) all the blooms of the lemon could be protected from insect visitation without the slightest reduction in set of mature fruit. Webber (1930) also concluded that pollination by bees was probably a negligible factor in the production of citrus fruits, at least for the 'Eureka' and 'Lisbon' lemons, the 'Valencia' and 'Washington Navel' oranges, and the 'Marsh' grapefruit. However, Webber et al. (1943) stated that although self-pollination occurs rather commonly without insects, seedlessness sometimes results, and seedlessness is rather generally a handicap to setting of fruit. Frost and Soost (1968) and Soost (1963) concluded that supplying pollen of another variety does not appear necessary for most of the major types of citrus.
In Russia, however, where numerous tests have been conducted on caged citrus trees, Glukhov (1955) stated that lemon trees isolated from bees produced only one-fourth as much fruit as trees exposed to cross- pollination by bees. Burnaeva (1956) reported that lemons receiving supplemental pollen from other cultivars or citrus species, produced more than trees not exposed to cross-pollination. Zavrashvili (1964) reported that lemon trees caged without bees produced 42.5 percent less than open-pollinated trees, whereas the trees caged with bees produced only 10 percent less, indicating that bees contribute by distributing the self- pollen on the tree. Later, Zavrashvili (1967b) stated that the 'Novogrusinskii' requires cross-pollination by bees for fruit production. Randhawa et. al. (1961) obtained four mature 'Malta' lemon fruit from 25 cross-pollinated flowers but none from 50 selfed flowers.
LIMES:
There has been little research on the pollination requirements of limes. Krezdorn (1970) stated that the Tahiti lime is strongly parthenocarpic, and, although cross-pollination might increase the number of seed, the increase in production of fruit, if any, would be negligible. However, Motial (1964) reported that 80 to 100 percent of the open pollinated flowers he observed on sweet limes (C. limettoides Tan.) set fruit, but only 40 to 60 percent of the emasculated and hand pollinated flowers set. This indicates that strong pollinator activity might increase the set and total production of sweet limes. Motial concluded, however, that sweet lime is not self-incompatible but is merely a shy bearer because of the high percentage of staminate flowers the plant produces.
ORANGES:
A general statement about the pollination of oranges is difficult because of the variation among cultivars. Coit (1915) stated that certain oranges require pollination to set fruit, others will set fruit parthenocarpically without the stimulus of pollination, and some will not accept pollen from some other cultivars. Because of this difference, the 'Washington Navel' and 'Valencia' and other sweet oranges will be discussed separately.
'Washington Navel'. - The anthers of 'Washington Navel' blossoms produce no pollen and the embryo sac may degenerate before tubes of pollen from other cultivars can penetrate to it, yet fruit sets and develops if conditions are favorable. However, if the tree is stressed by desiccating winds or moisture shortage, drop of young fruit can be severe. Surr (1922) caged six 'Washington Navel' trees to increase the humidity around them, which also excluded pollinating insects. He found that by doing this the production was not increased but instead decreased as much as 86 percent. The cages may have influenced fruit set for reasons other than pollination. Krezdorn (1970) stated that cross-pollination in 'Washington Navels' does not increase the yield, yet he (1965) obtained the following results from hand-pollinating the flowers, which would indicate that cross-pollination might influence set:
[gfx] (fix table):
No. of flowers No. of Pollen Source pollinated fruit set 'Pineapple' orange 1,000 2 'Temple' orange 1,000 3 'Duncan' grapefruit 1,000 5 Self (None) 3,000 0
El-Tomi (1964, 1957) reported that cross-pollination of 'Washington Navels' significantly minimized the dropping of immature fruit.
An interesting report on pollination made by Zavrashvili (1967b) stated that 'Washington Navel' trees caged to exclude bees yielded fewer fruits than trees caged with bees or open plots. The flowers set the most fruit when crossed with the 'Grusinian' orange. He also reported that the transfer of stigmatic fluid between stigmas increased the percentage of set. No reason for this effect was given, and its significance has not been determined.
The effect of pollination on production of 'Washington Navel' oranges seemed to be summed up by Atkins (1963), who stated that there is a possibility that cross-pollination by bees may cause them to retain more fruit.
'Valencias.' - Richter (1916) stated, without showing data, that if all insects were kept off 'Valencia' flowers there would be no less production. Francke et al. (1969) also concluded that bees have no effect on production of 'Valencias', but Cameron et al. (1960) reported that fruit size of 'Valencias' was increased as the seed number increased and that 'Pearl' tangelo pollen may increase both seed number and fruit set on 'Valencias'. This would indicate that, with cross-pollination, fruit size and possibly number of fruit set might be increased.
Other sweet oranges. - Soost (1963) stated that commercial plantings show no obvious reduction of yield in the absence of other varieties, but this does not mean that cross-pollination is of no benefit. Khan and Chandhri (1964) concluded that five unidentified cultivars were self-pollinating. Oppenheimer (1935) (cited by Oppenheimer 1948) came to the conclusion that "citrus can be planted in large blocks with no admixtures of other varieties, without the least misgiving."
Conversely, Glukhov (1955) reported that orange trees (cultivar not given) pollinated by bees produced four times as much fruit as trees isolated from bees. Zavrashvili (1964) reported that the orange crop in cages without bees was 54.4 percent lower than that on trees in the open. The cultivar was not identified nor was there a measure of cage effect on the plant other than pollination effect. Wafa and Ibrahim (1960) obtained 31 percent increase in set of fruit on the 'Elfelaha' orange, 22 percent increase in fruit weight, 33 percent more juice, and 36 percent more seeds from fruits on trees visited by bees than on trees from which bees were excluded. Zacharia (1951) reported partial self-incompatibility in the 'Shamouti' orange.
Hassanein and Ibrahim (1959) reported a set of 2.6 percent of flowers of the 'Khalili' orange where insects were excluded, 10.4 percent set where honey bees were present, and 7.4 percent on control (open) blooms. Krezdorn (1967) showed that the 'Hamlin', 'Parson Brown', 'Pineapple', and 'Valencia' orange size increased linearly with fruit set.
Although the results of tests are meager, some beneficial effects of pollination on oranges are indicated.
PUMMELO:
Soost (1963, 1964), working with 11 different accessions and Nauriyal (1952) concluded that the pummelo, which is grown commercially only in the Orient, is self-incompatible.
Aala (1953) conducted pollination studies on the Siamese pummelo 'Siamese 3442' in the Philippines. It produces both complete and staminate flowers. Some of the flowers were left to visits by bees, some were selfed, and some were crossed with pollen of 'Sour', 'Siaver 14', and 'Valencia' orange, and 'Batanga' mandarin. He concluded that most pummelo trees were self-incompatible and should be inter-planted with other cultivars. He stated: "Bees or other insects are necessary for proper pollination and setting of fruits, whether a cultivar is self-fertile or self-sterile." He also noticed that a higher percent set of open-pollinated flowers was obtained during off seasons than regular seasons, which may indicate that an inadequate pollinator population existed at flowering time. Of course, it could also mean there was an interaction with unfavorable environmental or physiological factors.
MANDARIN AND MANDARIN-HYBRID COMPLEX:
More research has been conducted on the pollination requirements of this group than of all the other citrus species combined, because the pollination problem is more acute. The problem has been recognized since Lacarelle and Miedzyrzecki (1937) reported that fewer fruits of the 'Clementine' mandarin set on a tree enclosed for self-pollination without bees than on 30 others enclosed with bees, either with or without pollen of other cultivars. Oppenheimer ( 1948) also showed that production of the 'Clementine' tangerine was increased when it was cross-pollinated by bees with pollen from 'Dancy', 'Temple', 'Duncan', or some other seedy cultivars. He found that the 'Valencia', 'Eureka', 'Marsh Seedless', and 'Satsuma' were ineffective pollinators.
Van Horn and Todd (1954) caged 'Clementine' ('Algerian') tangerine trees with and without pollinating insects (honey bees) and with and without bouquets of other cultivars. They showed that trees having both bees and bouquets yielded 16 times as many tangerines as those with no bees, had double the yield of those provided with bees only, and had better fruit quality. Miwa (1951) showed that the 'Hyuganatsu' mandarin was self-sterile but cross-fertile. Lynch and Mustard (1955), Coste and Gagnard (1956), Soost (1956,1963), Mustard et al. (1957), and Barbier (1964) concluded that the 'Clementine' tangerine was self-incompatible. Minessy (1959) found that grapefruit pollen was highly effective in fertilizing 'Clementines'. Blondel and Barbier ( 1963) accepted the fact that pollination increased production but stated that it also increased the pips or seeds present. Hilgeman and Rodney (1961) and Krezdorn (1970, 1972) stated that yields of 'Clementine' can be improved with bee pollination.
Hearn et al. (1969) reported that the 'Lee', 'Page', 'Nova', and 'Robinson' were self-incompatible, but Reece and Register (1961) stated that the 'Osceola' was not completely so. Furr (1964) and Moffett and Rodney (1971b) reported that cross-pollination was necessary and should be provided for 'Fairchild'. Later (1973) they reported that bees increased the yield of 'Orlando' tangelo. Also, Moffett and Rodney (1973) showed that honey bee visits increased yields of 'Orlando' tangelo. Hearn et al. (