2018 Crop Protection Guide for Tree Fruits in Washington

Bioregulator Sprays

Monday, March 25, 2019

Apple Postbloom Thinning

NAA (K-Salt Fruit Fix 200, K-Salt Fruit Fix 800, Fruitone, PoMaxa, Refine). Naphthaleneacetic acid (NAA) is an auxin analog which can be applied from petal fall to 30 days past full bloom, with best results occurring from applications between 5 – 15 mm. The thinning effects of NAA increase dramatically when apple trees are under photosynthetic stress, so excercise caution if applying shortly before or during hot and/or cloudy conditions. Use higher concentrations of NAA if applying without a surfactant; see product label for more guidance. To increase thinning, tank mix NAA with other thinning agents such as carbaryl or benzyladenine (see sections). For other uses of NAA, please refer to the section “Other Programs for Apple".

NAD (Amid-Thin W). Naphthaleneacetamide (NAD) is chemically similar to NAA and can be used on varieties other than Red Delicious, where it may cause abnormally small (pygmy) fruit. Apply from petal fall to 14 days past full bloom. When the weather is cool after bloom, delay applications until the largest fruit are 2-3 mm in diameter and forecasted temperatures after spraying are above 50°F, and preferably above 65°F. Use higher concentrations of NAD if applied without a surfactant; see product label for more guidance. To increase thinning, tank mix NAD with carbaryl (see section on carbaryl).

Carbaryl (Sevin 4F). Carbaryl is a carbamate-class insecticide that also mimics the action of auxins and can be applied to apple as a chemical thinner any time from 80% petal fall to 16 mm fruit size. Results from carbaryl depend on temperature, chemical rates, variety, and pollination. Other factors to consider include potential toxicity to bees in or near the orchard and possible impacts on mite management. Please check the product label for additional guidance. Carbaryl may be applied with NAA or NAD; their use in combination is more effective than when used separately. For optimal response, apply the combination of carbaryl + NAD or NAA at 3-10 mm fruit diameter. Excessive thinning may occur if daytime temperatures are above 80°F.

Caution: Many formulations of carbaryl are highly toxic to bees; use 4F or 4L formulations, which are less hazardous. If open bloom is present, apply when bees are not foraging. Before using any carbaryl formulation, it is advisable to eliminate flowers in the cover crop (i.e. by mowing) to minimize bee kill. 

Carbaryl can also be highly toxic to predatory mites and the rust mites on which they feed. The hazard is greatest in orchards where carbaryl has not been used extensively and little resistance has developed. Reduce the hazard of injury to mites by directing sprays towards tree tops, applying early in the season, and limiting the total number of applications.

Ethephon (Motivate). Ethephon is a synthetic precursor of ethylene and may be applied with carbaryl, NAA, and/or NAD to increase fruit thinning and promote return bloom. Applications for thinning are most effective 10-20 days after full bloom.

Caution: high rates of ethephon may reduce fruit size; Red Delicious fruit shape may also be affected by inhibiting calyx-end development when applied earlier than 3 weeks after bloom. Please refer to the product label for more guidance and section “Other Programs for Apple” for more information on other uses of ethephon.

6-BA, BA (MaxCel, Exilis Plus, Exilis 9.5SC): 6-benzyladenine (BA) is a cytokinin analog which has the ability to thin fruitlets as well as enhancing cell division in developing fruit, ultimately resulting in larger fruit size.

For thinning, use one to two applications of BA when king-bloom fruit are 5–10 mm fruit diameter according to specific recommendations of the product label. Best results are obtained when BA is combined with carbaryl or NAA and temperatures greater than 65°F occur during and for a period of several days following application.  If increased fruit size is desired without reducing fruit set, BA may be used by itself at lower concentrations than the maximum allowed by the label (see section on promotion of fruit size).

Use a well-calibrated sprayer for BA applications to ensure uniform and complete coverage. Spray volumes of 100 to 200 gallons per acre should be adequate for most orchard spacings and tree row volumes.

Tree Fruit Research & Extension Center, 1100 N Western Ave, Washington State University, Wenatchee WA 98801, 509-663-8181, Contact Us