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