Chemicals, Timing, and Environmental Factors Involved in Thinner Efficacy

Abstract

Chemical thinning of apples may be done over a 3- to 4-week period, starting at bloom and extending through the time fruit are 20 mm or larger in diameter. Potentially useful thinners at bloom include ammonium thiosulfate, Wilthin, endothall, and pelargonic acid. These thinners reduce set by preventing pollination, inhibiting pollen germination, or slowing pollen tube growth. Ethephon can be an effective thinner at bloom, but it is erratic. Petal fall application of thinners has become widely adopted by the industry, primarily because it reduces risks associated with one time thinner application, routinely done in the past. Carbaryl is often the thinner of choice at this time, but where more aggressive thinning is desired, NAA is used either alone or in combinations with carbaryl. Naphthale-neacetamide is effective when used at bloom but it less frequently used. Apple fruitlets are very vulnerable to the stress imposed by chemical thinners at the 8- to 12-mm stage of fruit development. NAA and Accel are particularly effective at this time, but carbaryl is frequently used because of its mild thinning action and its ability to reduce clustered spurs to one fruit. Unless weather conditions after bloom have been cool, effective thinning after the 18-mm stage of fruit development is less effective. Although erratic, ethephon is the most effective thinner on large fruit. Warm temperature during and following thinner application favor thinner uptake and response. Longer spray drying times favor thinner uptake into the plant. Environmental influences and interactions influencing thinning will be discussed.