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.
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