Return Bloom 



One of the primary reasons for thinning is to 

 assure adequate return bloom. BA appears to 

 be quite effective at stimulating flower bud 

 formation, and therefore, BA compares favor- 

 ably with NAA and carbaryl at stimvdating 

 return bloom. In some years, BA will enhance 

 flower bud formation beyond that which would 

 be promoted by the level of frmt thinning that it 

 causes. 



Time of Application 



BA can thin over a three- week period. It will 

 thin modestly when applied at full bloom to 

 petal fall, but fruit are most susceptible to BA 

 and it is most effective when it is applied at the 

 8- to 10-mm stage of fruit development (14 to 18 

 days after fuU bloom). Once iniit reach about 20 

 mm and trees experience several days of sunny 

 weather in the 80's, no thinner, including BA, 

 will thin. 



Spray Coverage 



Good and uniform spray coverage is impor- 

 tant. Translocation and redistribution of 

 foliarly appHed BA is limited. Further, research 

 has shown that BA must come in direct contact 

 with the spvu" leaves for fruit in that cluster to be 

 thinned. BA application directly to the young 

 fruit wiU increase fruit size and flesh firmness 

 at harvest but wiU not influence fruit abscis- 

 sion. 



Fruit Effects 



Perhaps the biggest advantage that BA has 

 over other chemical thinners is its effects on 

 fruit. 



Fruit Size 



Generally, chemical thinners increase fruit 

 size by lowering fruit numbers, thus reducing 

 competition for metabolites among the remain- 

 ing fruits. Although BA enhances size by reduc- 

 ing competition, it also causes increased fruit 

 size independent of and in addition to this effect. 

 This effect on fruit size independent of thinning 



is unique to BA. BA is especially effective at 

 increasing fruit size on Mclntosh-tjrpe cultivars 

 such as Mcintosh and Empire. 



Flesh Firmness and Sugars 



It is rare for chemical thinners to increase 

 flesh firmness because they usually increase 

 finiit size, and there is an inverse relationship 

 between fruit size and flesh firmness. BA, 

 however, increases flesh firmness approxi- 

 mately half of the time, even though it also 

 increases finiit size. Because BA is a cytokinin 

 (a group of plant hormones) it likely increases 

 flesh firmness by increasing the number of cells 

 in an apple. Also, BA increases the sugar 

 content of fruit about half the time. Thinners 

 can increase sugar because they increase the 

 leaf-to-fruit ratio. 



Red Color and Fruit Asymmetry 



If used at high concentrations, BA can re- 

 duce red color and increase finiit asymmetry. 

 Given the label restrictions per acre per applica- 

 tion, we do not believe that either one of these 

 situations is likely to occur. 



Cultivars 



BA is not equally effective on all cultivars. 

 BA is especially effective on Empire and Mcin- 

 tosh and extremely useful on Jonamac, Rome, 

 Idared, and Golden Delicious. 



Recommendations for 

 the Use of Accel* 



Accel is the first step by Abbott Laboratories 

 to make BA available as a thinner on apples. It 

 is not a perfect product, but it is a start. It is an 

 altered Promalin formulation so GA, , is in- 



4+7 



eluded, but it is present only at 1/10 the level 

 found in the original Promalin formulation. 

 Also, on the present label is a limit of 35.6 fluid 

 ounces of Accel (20 g active ingredient, ai) per 

 acre p>er application, and this level may limit its 

 effectiveness when used on large trees that 

 have a high tree-row-volume requirement. 



* Please see the end of this article for a discussion of a 

 pending label change. 



Fruit Notes, Spring, 1994 



19 



