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

Abstract

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, NAA, 6-BA, or combinations thereof. Response to MET varied by cultivar and season. In six of the seven experiments, MET reduced fruit set, but in only four 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.

Metamitron Thinning Efficacy of Apple Fruitlets Is Affected by Different Rates, Timings and Weather Factors in New York State

“…Our results with metamitron indicate that its effectiveness as a thinning agent is similarly related to the diameter of developing fruitlets, application rate, and weather conditions, as shown by Byers, et al [24]. Other studies have previously shown that the thinning efficacy of metamitron depends on the application rate [3,8,13,18,[25][26][27]. Although we observed a linear rate effect of metamitron in all our trials, there were large differences between the slopes of the regression between the trials.…”

“…However, this result differs from that of Brunner [25], who obtained maximum efficacy at 12 mm; Reginato et al [29], who obtained maximum efficacy at 16 mm; and Gonzalez et al [17], who reported maximum efficacy at 11.5-14 mm. Many authors have reported metamitron efficacy results at different fruit stages between petal fall and 20 mm [13][14][15][16][17][18][25][26][27]30,31]. All of these reports, together with results from the present study, show that metamitron thinning efficacy varies significantly with phenological stage from petal fall to 18.5 mm in 'Gala' apples.…”