Multi-year investigation on the rate, timing, and use of surfactant for thinning apples with post-bloom applications of metamitron

Several experiments were conducted in Simcoe, Ontario, to evaluate the efficacy of metamitron (MET) as a post-bloom thinner for Ambrosia, Gala, and Honeycrisp apple trees. Trees were treated with rates of MET ranging from 165-480 mg.L-1, as well as different timings ranging from 5-22 mm fruit diameter. The effect of including a non-ionic surfactant on thinning efficacy with MET was also evaluated. Treatments were compared with untreated trees and industry standard sprays of carbaryl, 1-naphthalene acetic acid (NAA), 6-benzyladenine (6-BA), or combinations thereof. Response to MET varied by cultivar and season. In six of the seven experiments MET reduced fruit set, but only in four experiments did MET reduce the number of fruit per tree or crop load compared with the untreated control trees. Petal fall (5-7 mm) applications of MET were less effective than later timings. Thinning response increased with higher rates of MET in four of the seven studies. For Honeycrisp and Ambrosia, 175 mg.L-1 MET was effective in reducing fruit set and crop load, while rates at or above 263 mg.L-1 MET were required to thin Gala. MET improved fruit size distribution into larger categories and caused minimal leaf phytotoxicity with or without a non-ionic surfactant. Environmental factors such as nighttime temperature and solar radiation largely could not account for the seasonal or application timings in thinning response to MET. Greater understanding of the carbon balance and interplay of solar radiation, nighttime temperature, cultivar and fruitlet size on thinning response is required to improve the predictive thinning response of apple to MET.