Chemical Fruit Thinning Response of Spur ‘Delicious’ Apple as Influenced by Light Intensity and Soil Moisture

Abstract

Abstract Percent fruit set and crop density of mature ‘Starkrimson Delicious’ apple trees (Malus domestica Borkh.) were reduced by a whole tree airblast spray of 450 ppm ethephon plus 1200 ppm carbaryl applied 16 days after petal fall (DAPF). Irrigation regimes consisting of half-normal, normal, and twice normal rainfall had no effect on nonchemically or chemically induced fruit drop. Individual limbs within each tree received 30% shade from 0 to 16 DAPF, 73% shade from 0 to 16 DAPF, 73% shade from 17 to 35 DAPF, or no shade. At 16 DAPF, both early shade treatments reduced fruit size and set, but neither influenced nonchemically induced fruit drop occurring from 16 to 35 DAPF. Only heavy shade imposed from 0 to 16 DAPF decreased chemically induced fruit drop. Heavy shading from 17 to 35 DAPF without chemical thinner application doubled fruit drop. Excessive fruit drop occurred when heavy shading followed chemical thinner application, but thinner efficacy was not different from the unshaded, chemically thinned control. Chemical names used: (2-chloro-ethyl)phosphonic acid (ethephon); 1-naphthalenyl methylcarbamate (carbaryl).