Apple Chemical Thinning
Apple chemical bloom and postbloom thinning programs are intended to reduce the current season’s crop load in pursuit of three fundamental goals: 1) inhibit fruit set to minimize green fruitlet hand thinning; 2) improve size and quality of surviving fruit; and 3) promote return bloom to encourage annual cropping. Successful chemical thinning usually requires comprehensive programs employing multiple chemistries during the bloom and postbloom period. Bloom thinners (applied when flowers are open and viable) reduce fruit set by damaging flower parts and/or inducing plant stress. Most postbloom thinners (applied after petal fall) typically mimic the effect of plant hormones to elicit a specific physiological response (e.g. increased ethylene evolution, which triggers fruitlet abortion) to achieve reductions in crop load.
Fertilized flowers become more difficult to thin with each passing day, making early, aggressive thinning strategies more successful than those which rely primarily on chemical applications after 10 mm fruitlet size. Research indicates that early thinning results not only in more significant reductions in fruit set, but greater improvements fruit size, fruit quality, and return bloom. Even with more aggressive chemical rates, applications of postbloom chemical thinners after 15 mm fruitlet size are usually of marginal benefit in typical Washington conditions. Timings based on weather and crop developmental stage (i.e. mean fruitlet diameter) are generally more reliable and accurate than those based on the calendar (i.e. days after full bloom). Application timing for chemical bloom thinners may be improved with the guidance of pollen tube growth models available on WSU's AgWeatherNet system (weather.wsu.edu); these models can be used to predict when apple flowers are effectively fertilized, which can be helpful information when making chemical thinning decisions.
Chemical thinning efficacy is a function of many factors, including apple cultivar and strain, rootstock, tree condition, pollen strength and density, bee activity, weather, product chemistry, rate, application method, timing, and coverage. Therefore, thinning programs should be customized to individual blocks. Select materials, timings, and rates accordingly and observe label recommendations and restrictions. Spring frosts can induce significant fruitlet abortion in lower parts of the tree, but upper parts of the canopy may still be over-cropped; in these cases, thinning sprays targeted to tree tops are often advisable to keep the trees in balance and discourage alternate bearing.
Response to chemical thinners can vary relative to weather conditions before, during, and after application, especially in the case of postbloom materials. Caution should be exercised when applying thinning materials in temperatures above 80°F, especially during dark, cloudy conditions, as fruitlet abortion and/or phytotoxicity may become excessive in some cases. Thinner efficacy may be diminished below 60°F, but low temperatures can also temporarily mask the symptoms of a significant thinning response; growers dissatisfied with the performance of thinning sprays during cool conditions may be well advised to wait for a few days of warm temperatures to reassess fruit set before applying additional thinners. See individual product labels for additional guidance.
Effective chemical thinning is more difficult in some apple cultivars; Fuji, Golden Delicious, and Cameo generally require more aggressive tactics (i.e. more applications and/or higher rates) than do Red Delicious, Gala, Cripps Pink (Pink Lady®), Granny Smith, Honeycrisp, Jonagold, or Braeburn to achieve comparable results. Spur-type Red Delicious are often more difficult to thin than non-spur Red Delicious. Ineffective thinning can result in over-cropping and induce alternate (biennial) bearing in many apple cultivars, especially Fuji, Golden Delicious, Cameo, and Honeycrisp. Unfortunately, alternate bearing cycles are easy to establish and difficult to break and can dramatically hurt orchard profitability over time. Early, aggressive chemical thinning programs should be the first defense against over-cropping, but consistent annual bearing may also be promoted with effective use of bioregulators; please refer to the section “Other Programs for Apple – To Promote Bloom” for more information.