Apple Thinning

It has been estimated that 90% of apple growers in the United States use some form of chemical thinning to improve fruit size and to promote annual cropping. However, no single thinning program applies to all cultivars. Each cultivar requires a slightly different thinning program, and the thinning response varies depending on climate and cultural conditions. The past history of the orchard, the grower's experiences with particular blocks, the cultivar, and the weather conditions should be used to determine the best time of application and the rates of the individual or combinations of thinners.

Factors influencing chemical thinning
Temperature
Temperatures during a thinner application and temperatures after the application are critical to successful thinning. All thinners have a greater response as the application temperatures increase because in order for thinners to work, they have to be absorbed, and absorption increases as the temperatures increase. Also, high temperatures increase the waxy cuticle of the foliage, thus decreasing the absorption of the thinner. In a study conducted in Michigan (Black et al., 1995), the effect of temperature on the uptake of NAA  by Redchief 'Delicious' apple leaves demonstrated that there were differences in the uptake of NAA by the two surfaces, adaxial (top) and abaxial (lower),  of the leaves and this uptake was greatly influenced by temperature.  Uptake after 24 hours from application by the adaxial surface was ~4 and 8% of the applied NAA at 15C and 35C (59F and 95F) respectively.  Uptake by the abaxial layer was 24 and 48% of the applied NAA at 15C and 35C respectively.  This experiment might explain why there is often overthinning when application of NAA is followed by higher than average temperatures.  In addition, temperatures at time of application are very important.  When application of thinners is done at temperatures below  65F, absorption is greatly reduced and poor or no thinning occurs. 

Light
Several theories on the mode of action of thinners indicate that a reduction of carbohydrates in the affected areas results in fruit abscission.  Thus, any factors that decrease photosynthesis such as cloud cover or within tree shade increase the effectiveness of thinners. Conversely, sunny weather, which increases photosynthesis, decreases the sensitivity of the fruitlets to the thinners.   

Drying conditions
Poor drying conditions such as high humidity increase the amount of time the thinning chemical stays on the leaf surface, thus increasing the amount of chemical that is absorbed.

Fruit size
Fruit size is very important in determining the efficacy of thinners. Most chemical thinners are most effective at fruit diameter's of 4 to 14 mm.  At about 50 days after bloom, fruits begin to accumulate starch, and for approximately 6 weeks,   the fruit can not be induced to drop by the use of chemical thinners.
In spur Delicious, fruit diameter and temperature interact to determine the efficacy of the thinner. For example, when the fruit was less than 8 mm in diameter, thinning results with carbaryl were similar regardless of the temperature. However, for larger fruit thinning (14 to 16 mm) was better when the temperature was at 70 or 80 than at 50 to 60.
In a study conducted in England (Knight and Spencer, 1987), it was demonstrated that the response of two cultivars, 'Bramley',  and 'Spartan' was relatively uniform over a wide range of application times (from 80% petal fall to 18 mm) of carbaryl.  'Discovery' responded in a similar manner, however, the response to carbaryl decreased with the later applications.  For the cultivar 'Cox', the timing of applications was greatly influenced the re response to carbaryl.  The thinning response was greatest when applied at 11 mm than at 14 mm.  Clearly, this study emphasizes the importance of the interaction between fruit size and cultivar

Cultivar
Cultivars vary in their thinning response. Table 1 includes common cultivars and their thinning response to NAA