Influence of Temperature and Darkness on Apple Fruit Abscission and Chemical Thinning

Abstract Stayman/M.27 (Malus Xdomestica Borkh.) placed in the dark for 60 hours at 21 days after full bloom lost more fruit if the dark period was at 21.1°C than if it was at 4.4°C. Spraying these trees with carbaryl or NAA prior to the dark treatment caused no additional fruit loss. Ethephon caused defruiting at all temperatures of darkness. Spraying ‘Braeburn’/M.27 trees with a combination of carbaryl + Accel caused increased pygmy fruit development when trees were placed in the dark for 63 hours, 32 days after full bloom at temperatures from 4.4–21.1°C. In a field experiment, applications of carbaryl at 2-hour intervals from 6:00 AM to 8:00 PM to ‘Empire’/Mark trees caused similar thinning even though application temperatures ranged from 17.8–35.6°C. In another field experiment, spraying ‘Starkrimson Delicious’/Mark trees with ethephon, Accel, or carbaryl at 6:00 AM, 2:00 PM, or 8:00 PM caused similar thinning even though temperatures were 13.9°C, 33.3°C, or 19.8°C, respectively. Regression analysis of 20 years of thinning trials on five spur ‘Delicious’ strains using 50 different spray treatments indicated that higher night temperatures were more highly related to chemical thinning (accounting for about 20% of the variation) than daytime temperatures or the average of the day and night time temperatures. Degree of thinning was not significantly related to days after full bloom or fruit diameter; however, a ranking of spray treatments by chemical combination chosen for degree of expected thinning accounted for 70% of the variation. In addition, a ranking of the strains of ‘Delicious’ by ease of thinning was highly significant.