17 

 RESULTS OF 1986 CHEMICAL THINNING TRIALS ON MCINTOSH 



Duane W. Greene and Wesley R. Autio 



Department of Plant and Soil Sciences 



University of Massachusetts 



Chemical thinning is one of the most critical activities undertaken by 

 apple growers each year. Effective thinning can mean the difference between 

 profit and loss not only the year of application but also the following year. 



There have been no new chemical thinning agents registered in 

 Massachusetts in more than 20 years. However, during this period of time there 

 has been a steadily increasing demand for larger apples. We are continually 

 looking for new and better thinning agents, but until these are found and 

 registered it will be necessary to use more effectively those thinning agents 

 that are presently available. 



Experiments were initiated at the Horticultural Research Center in 

 Belchertown in 1986 with two goals in mind: 1) evaluate the effectiveness of 

 several thinning treatments and combinations in an attempt to identify 

 promising treatments for future recommendations, and 2) determine the 

 importance of bloom intensity on final fruit set following a chemical thinning 

 treatment . 



Experiment One 



Mature Mcintosh trees (M.7 rootstock) with a heavy bloom were selected for 

 Experiment One. Thinning treatments were applied 16 days after full bloom on 

 May 26, 1986, when fruit diameter was approximately 10 mm. Treatments used 

 were: naphthaleneacetic acid (NAA) at 5 or 7.5 ppm and benzyladenine (BA) at 

 50 ppm. These were applied alone as a dilute spray or in combination with 1 

 lb. carbaryl (50 % WP) per 100 gal. One group of trees received no chemical 

 thinning spray and one received only carbaryl. After June drop all fruit on 

 previously tagged limbs were counted and fruit set was calculated. A 30-apple 

 sample was taken at harvest, weighed, and evaluated for percent color, flesh 

 firmness, and soluble solids (sugars) content. 



Generally, a fruit density of about 5.5 to 6.0 fruit/cm limb circumference 

 on Mcintosh is considered to be ideal, and all treatments thinned when applied 

 alone, although only BA at 50 ppm and NAA at 7.5 ppm thinned adequately (Table 

 1). When carbaryl was combined with NAA or BA additional thinning occurred. 

 NAA at 7.5 ppm, BA, and carbaryl alone increased fruit size. Size was 

 increased further when carbaryl was added. No chemical thinning spray 

 influenced flesh firmness, soluble solids, red color, or seed number. 



Because of its detrimental effects on mite predators the use of high rates 

 of carbaryl is discouraged. Attempts are being made to minimize the amounts 

 used. Although the effectiveness of carbaryl is somewhat concentration 

 independent, rates of 1/4 lb per 100 gal. or below may be insufficient to thin 

 adequately by itself. NAA is a very effective thinning agent but it is also 

 the compound most likely to over-thin. Lower rates of NAA are frequently 

 chosen to reduce the chance of overthinning. Therefore, the most satisfactory 



