Thinning is done for two reasons. First, a certain portion of the fruit is removed so that the remainder will develop adequate size and quality. Second, the thinning process serves to increase the plant's ability to form flower buds for the next year--provided the thinning is done early enough. Thinning also reduces the total load on the branches and reduces breakage. Thinning is necessary for apples, nectarines, pears, plums, and peaches.
Hand thinning is the easiest and safest way to remove excess fruit. Hand thin fruit when the fruits are the size of the end of your little finger--about 1/2 inch in diameter. Simply start at one end of a branch and systematically remove fruit, leaving one fruit every 6 to 10 inches. It is best to cut the fruit off rather than pulling the fruit. Cutting the fruit will lessen the chance of damaging the spur. Be sure to leave only one fruit at a given site. Where doubles or triples are left, insects and disease will be difficult to control. Keep in mind that only 7 or 8 percent of the tree's flowers are needed to set a full crop of fruit.
An alternative to hand thinning is foliar application of the insecticide Sevin (carbaryl). This method might be necessary on large trees that are difficult to reach. Disadvantages of chemical thinning include the inability to selectively position fruit, the danger of removing too many or too few fruit, and the potential to increase mite populations.
Sevin as a 50WP should be mixed at a rate of 2 to 4 tablespoons per gallon of water and applied in a foliar spray to thoroughly wet the leaves. Apply 10 to 14 days after bloom. Wait for 7 days, then assess the number of remaining fruit. If too many fruit remain to be hand thinned easily, then a second application of Sevin can be applied at the same rate.