Effects of Blossom Thinners on Peaches 



Duane W. Greene, James Krupa, and Karen I. Hauschild 

 Department of Plant and Soil Sciences, University^ of Massachusetts 



Peaches are thinned to increase fruit size, improve 

 fruit quality, and reduce limb breakage. A number of 

 physical methods have been devised to thin peaches 

 including use of shakers, spraying of trees with a high- 

 pressure stream of water, hitting limbs with rubber 

 hoses or foam-covered sticks, and running ropes on a 

 tractor-mounted frame through trees. No method of 

 physically reducing crop load has been widely 

 accepted due to variable or unsatisfactory responses. 

 The majority of fruit thinning on apples is done 

 with chemicals that are applied after bloom. They 

 cause thinning by either affecting hormone content or 

 influencing carbohydrate distribution within rapidly 

 developing fruit. Blossom thinner application may 

 precede postbloom thinners so that less aggressive 

 postbloom thinning is required. Unfortunately, all 

 postbloom hormone-type thinners are ineffective on 

 peaches. In recent years, several compounds have 

 been report to reduce crop load on peaches when 

 applied at or slightly before bloom. Among those 

 chemicals most frequently evaluated are: endothall, 

 pelargonic acid, sulfcarbamide, ammonium thiosul- 

 fate, and hydrogen cyanamide. There has not been 

 universal acceptance of blossom thinners for use on 

 peaches for several reasons. Some thinners have not 

 been registered for use on fruit crops, results have been 

 erratic and inconsistent, and there is a reluctance by 

 growers to apply chemicals designed specifically to 

 reduce fruit set before a crop has been set and initial 

 crop load can be assessed. 



Apples have been the primary crop grown by 

 orchardists in New England, but the focus is changing 

 due to global competition and low price. Increasingly, 

 growers are decreasing their dependence on apples, 

 reducing total acreage and diversifying into other 

 crops, including peaches. Peaches can be a very 

 lucrative crop, but only if large sized fruit are 

 produced. Further, peaches are a more labor intensive 

 crop, and labor requirements for hand thinning of 

 peaches frequently coincides with cultural demands of 

 apples. Therefore, there is intense grower interest in 

 using blossom thinners to increase fruit size and to 



reduce the amount of time required to hand thin. 



The purpose of this investigation was to evaluate 

 the effects of the most promising blossom thinners on 

 peaches. We also hoped to identify appropriate 

 concentrations to use and to evaluate consistency of 

 response. 



Methods & Methods 



Mature Garnet Beauty and Redhaven trees 

 growing at the University of Massachusetts Horticul- 

 tural Research Center in Belchertown were used in this 

 investigation. Tree spacing was 17' x 24', giving a 

 density of 107 trees per acre. Endothall, Wilthin, and 

 ammonium thiosulfate (ATS) were evaluated in 1997, 

 1998, and 1999 on the same block of peach trees. 

 Based upon phytotoxicity and thinner efficacy, thinner 

 concentrations were adjusted yearly. Here, we are 

 presenting 1999 data only, since we feel that the 

 chemical concentration and timing of application are 

 close to that which may ultimately be adopted for 

 commercial application. 



In each year, 18 Redhaven trees and 24 Garnet 

 Beauty trees were blocked into three groups 

 (replications) and four groups (replications), respec- 

 tively, of six trees each. Within each replication trees 

 were randomly assigned one of six treatments; control, 

 two rates of Wilthin, two rates of ATS, and one rate of 

 endothall. 



In 1999, prior to the application of blossom 

 thinners, three limbs on each tree, 10 to 12 cm in 

 diameter, were tagged and measured. At the time of 

 application, bloom on Garnet Beauty was estimated to 

 be 60% open while that on Redhaven was judged to be 

 80% open. Treatments were applied on May 2 using a 

 rear mounted airblast sprayer delivering 1 00 gallons of 

 water per acre. Wilthin was applied at rates of 6 and 8 

 quarts per acre with 1 pint Regulaid per 1 00 gallons of 

 spray. ATS was applied at 4 and 6 gallons per acre, and 

 the endothall rate was 1.5 pints per 100 gallons. One 

 tree per block was not sprayed and served as the 

 control. Temperature at the time of application was 



Fruit Notes, Volume 64 (Number 4), Fall, 1999 



