Rate and timing of metamitron affect thinning efficacy of ‘Gala’ apple trees under American northeast conditions

Knowing the effect of timing and application rate of a spray product is a crucial element for all thinning programs. The objective of the current study was to evaluate the thinning efficacy of different rates of metamitron applied at two timings in the northeast of the United States. Trials were conducted over four seasons from 2015 to 2018 in apple orchards in Geneva (New York State). The rates tested were between 200 and 500 mg L^‑1 applied at petal fall or 12 mm fruit size on ‘Crimson Gala’/M.9 trees. Metamitron showed variation in chemical thinning efficacy from year to year and within years. Generally, the application of metamitron reduced number of fruits tree^‑1, fruit set, crop load, yield efficiency and yield in a dose dependent manner. When metamitron showed high thinning efficacy, there was an improvement in fruit weight, coloration and diameter. Additionally, metamitron had no negative influence on fruit quality or fruit characteristics.