Suggestions for Use of the New 

 Postbloom Thinner Accel® 



Duane W. Greene and Wesley R. Autio 



Department of Plant and Soil Sciences^ University of Massachusetts 



Chemical thinning of apples continues to be 

 one of the most important management activi- 

 ties. It is reqviired nearly every year to assure 

 adequate fruit size at harvest and to encourage 

 repeat bloom the following year. Carbaryl and 

 NAA are the two most commonly used thinners. 

 Both have their faults. Orchardists frequently 

 are reluctant to use carbaryl because of the 

 potential detrimental effect that it can have on 

 mite predators, and it is a relatively weak thin- 

 ner. When used alone, often it is not potent 

 enough to thin adequately. NAA is stronger, 

 but it also has several detrimental effects. 

 Overthinning is possible if either cloudy or hot 

 weather immediately follows apphcation. It 

 can retard finiit growth, even when used accord- 

 ing to label directions. Under these conditions, 

 NAA may not increase fruit size, even when it 

 causes significant thinning. This lack of size 

 increase is emerging as a major problem associ- 

 ated with NAA. Finally, NAA cannot be used on 

 some cultivars because it causes pygmy finait. 



In the 1980's benzyladenine (BA) was found 

 to have chemical thinning capabiUties. Since 

 then, researchers have demonstrated repeat- 

 edly that BA is a consistent and effective thin- 

 ner with some unique properties that may make 

 it the postbloom thinner of choice. Accel® re- 

 cently was approved as a chemical thinner on 

 apples. Accel is an altered Promalin™ formula- 

 tion, but the primary active thinning compo- 

 nent is B A. The purpose this article is to explain 

 some of the characteristics of BA that make it a 

 unique thinner, and to make suggestions for 

 successful use of BA when applied in the Accel 

 formulation. 



General Effects of Accel 



Thinning Activity 



BA can thin over a wide range of concentra- 

 tions, starting fi*om as low as 25 ppm. Undesir- 

 able side effects may be noted if it is appUed 

 above 150 ppm; however, label restrictions on 

 the active ingredient per acre make it unlikely 

 that too high a concentration will be applied. 

 BA has been applied in heavy set years and in 

 light set years, and the thinning response to 

 varying concentrations is linear. Although con- 

 centrations as low as 25 ppm can be effective, 50 

 to 100 ppm generally are required to do an 

 effective job. 



Comparison with other 

 Chemical Thinners 



BA has been compared with NAA and car- 

 baryl in several thinning trials in Massachu- 

 setts. It thins as consistently, if not more 

 consistently, than either NAA or carbaryl when 

 applied at the proper time and at an appropriate 

 temperature. The activity of chemical thinners 

 differs fix)m year to year, depending on weather 

 and other factors; however, when applied at the 

 appropriate tree row volume, 75 ppm BA thins 

 Mcintosh comparably to 1 Ib/100 gal carbaryl 

 (50% WP Sevin) and 6 ppm NAA. BA has been 

 shown to have no detrimental effects on mite 

 predators, a problem frequently associated with 

 the use of carbaryl. When applied alone, BA 

 does not have the negative effects of NAA, such 

 as leaf epinasty, reduced fruit size, or pygmy 

 fruit. 



18 



Fruit Notes, Spring, 1994 



