Chemical Thinning--Apple
Chemical thinning uses caustic materials or hormonal-type growth regulators to reduce the amount of fruit, and to overcome alternate year or biennial bearing. This may require the use of several different chemicals and combinations during the bloom and postbloom period.
The degree of thinning and the effect on return bloom the following spring depends on a number of factors: variety and strain, tree condition, fruit set, proximity to pollinizers, weather, the chemical, and application method.
In general, higher rates and different applications are required with Golden Delicious and spur-type Delicious trees in high vigor or with good pollinating weather. The rates for some of the chemicals in the tables are conservative and may be adjusted up or down by 50%, depending on conditions and the amount of thinning desired. Do not use rates higher than those on the label.
NOTE: The effectiveness of NAD (Amid-Thin W), NAA, and carbaryl (Sevin) is related to spring temperatures. During cool springs, when growth of fruit is slow, fruit size is a better guide for timing sprays than days after full bloom. Neither NAD nor NAA is effective below 50ƒ F. Avoid use of carbaryl at temperatures below 70ƒ F. Use can result in seed abortion, no thinning, and small-sized fruit. Delay applications until the predicted maximum daytime temperature is likely to reach these limits within five days after application.
To avoid overthinning, use caution in making applications following spring frosts. Do not spray self-unfruitful varieties more than two tree spaces away from a pollinizer. Avoid spraying (or spray lightly) lower and inside limbs since they are lower in vigor and more easily thinning. Direct most of the spray to the top two-thirds of the canopy.
Bloom applications of chemical thinners
Post-bloom application of chemical thinners
Thin-it: an expert system for chemical thinning of apples
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May 2, 2000