B. Rosie Lerner, Consumer Horticulture Specialist, Purdue University
If you're getting calls from panicked home fruit growers saying all the little fruits are dropping from their trees, chances are good that the plants are simply shedding excess fruit. Most trees set many more flowers than needed for a full crop, especially following a relatively mild winter.
Only 1 bloom in 20 is needed for a good crop on a full-blossoming apple tree. Although the mass dropping of small fruit in June can be alarming, its just nature's way of thinning the crop so the remaining fruit can reach full size.
Most fruit trees have at least two waves of fruit drop. The first occurs shortly after bloom. Lack of or incomplete pollination usually causes this drop.
The second drop occurs 3-4 weeks later. The second drop is usually bigger and more dramatic because the fruits have developed to a larger size, usually 1/2 to 1 inch in diameter. This second drop is called "June drop" because it usually occurs in early June. Competition among the fruits for water and nutrients is thought to be the cause of June drop. Fruits that contain the fewest or weakest seeds are usually the first to drop.
Although June drop may appear to be devastating, many trees do not shed enough fruit naturally for good production of the remaining fruit. For best quality, some hand thinning is recommended before the fruit is halfway to maturity.
Appropriate final spacing varies with the type of fruit. Apples should be thinned to about 6-8 inches between each fruit. Peaches, plums, and nectarines should be thinned to about 4-5 inches between fruit. Apricots need only 2-4 inches between fruit, while cherries rarely require thinning.
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Plant and Pest Digital Library and Digitally Assisted Diagnosis, Purdue University, West Lafayette, Indiana.