Thinning Stone Fruit
Unfortunately, PGR formulations are not available for post-bloom thinning of stone fruits such as peaches and cherries. Application of some mildly caustic materials during full to late bloom has been used successfully in the past to reduce fruit set in both apples and stone fruits. The mode of action is to allow pollination to occur on early blossoms and then damage the later blossoms with the caustic material, preventing further pollination. Blossom thinning with these caustic materials in cold and frost-prone areas of the Intermountain West is extremely risky. Recent trials in Utah indicated that application of caustic bloom thinners to peaches, followed by cold but non-freezing temperatures, resulted in complete crop loss.
Application of gibberellins (GA3, ProGibb) can be used in stone fruits to reduce the number of flower buds formed for the following season. This has been used successfully to prevent over-cropping of weak tart cherry trees, to delay fruiting in young tart cherry orchards, and to “thin” processing peaches where hand thinning and detailed pruning are not justified by the value of the crop.
As older tart cherry trees begin to decline, the natural tendency is for these trees to produce too many flower buds. If fruit are produced at lower nodes on one-year-old wood (which happens often in Montmorency), blind wood results because there are no vegetative buds to produce spurs or branches. Reducing the number of flower buds relative to vegetative buds allows for spur formation and greater long-term productivity. To reduce flower bud formation, apply GA3 at 2 to 4 weeks after bloom, at a rate of 4 to 18 grams a.i. per acre, depending on tree age and vigor. Older trees typically need higher rates than younger trees. Optimum timing is when 3 to 5 terminal leaves are fully expanded, or when 1 to 3 inches of terminal shoot extension has occurred. Similar applications can be used to prevent or reduce flowering in young non-bearing tart cherries.