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CHBMICAL THINNING OF APPLES IN 1979 



F. W. Southwick 

 Department of Plant and Soil Sciences 

 University of Massachusetts, Amherst 



When weather conditions during bloom are favorable for bee 

 activity, many apple varieties will overset if they have an abun- 

 dance of blossoms. In such instances, chemical thinning with 

 naphthaleneacetic acid (NAA) , naphthaleneacetamide (NAAm) or car- 

 baryl (Sevin) avoids a tendency toward biennial bearing and also 

 helps increase fruit size and color. It involves some risk since 

 the exact degree of thinning cannot be accurately predicted in 

 advance. Furthermore, it is realized that attempts to determine 

 the time of application of the chemical thinning sprays on the 

 basis of days after petal fall (PF) is not entirely satisfactory. 

 Prevailing temperatures play a primary role in the rate of young 

 foliage and fruit development. If the temperatures are cooler than 

 usual after PF, the time of application should be delayed beyond 

 the suggested treatment period or vice versa if warmer than average 

 temperatures prevail. 



Weather conditions before and when applying NAA and NAAm 

 are important. If weather conditions are cool and cloudy or rainy 

 a week or two before spraying, the leaves developing during this 

 time will have a very thin cuticle. Under these conditions, NAA 

 or NAAm would penetrate into the leaf more easily and overthinning 

 may occur. On the other hand, warm, dry, sunny conditions prior 

 to spraying would result in a leaf having a thick cuticle that would 

 impede the movement of NAA or NAAm into the leaf. In this case, 

 the concentration used may have to be increased to obtain adequate 

 thinning. Weather conditions before or after application may not 

 greatly affect the thinning action of carbaryl since the fruit is 

 its primary absorption site rather than the leaves as in the case 

 of NAA or NAAm. Light frost which may not injure flowers or young 

 fruits may injure the foliage and the use of NAA or NAAm at this 

 time may cause overthinning and increased foliage injury. There- 

 fore, delay treatment for several days after such occurrences and 

 reduce the spray concentration and gallonage per tree is thinning 

 still seems necessary. 



In 1978, Mcintosh blossomed quite heavily in Massachusetts and 

 in many cases were not thinned sufficiently to produce as many 

 fruits of good marketable size as desired. Consequently, it would 

 not be surprising if the bloom on such trees in 1979 was only light 

 to moderate and not require much chemical thinning. A grower should 

 carefully observe the fruit set in his Mcintosh blocks 7-14 days 

 after petal- fall (PF) and be reasonably certain that chemical thin- 

 ning of Mcintosh is necessary on the lighter blooming older trees. 

 It should be remembered, however, that trees with a light to 

 moderate bloom may occasionally overset and be more difficult to 

 chemically thin than trees which blossom and set heavily. 



