Efficacy of metamitron as a postbloom thinner - the American Northeast experience

Apple fruitlet growth after pollination is dependent on carbohydrates, which are produced through photosynthesis. When photosynthesis is limited, the carbohydrate supply is reduced and so the competition for carbohydrates among fruits and vegetative sinks increases. Metamitron is a photosynthesis inhibitor which has been used to cause fruit thinning in apple. It likely acts by reducing carbohydrate supply to young fruitlets. In 2016 and 2017 we investigated the effect of metamitron on whole tree photosynthesis and on fruit set of mature ‘Crimson Gala’/M.9 trees at two timings in the apple thinning window – petal fall and at 10-12 mm fruit size, at Geneva, NY, USA. Three different metamitron rates were imposed at each timing: 200, 350, and 500 mg L^‑1. Untreated control trees were also included for comparison. In 2016, the higher the rate of metamitron the greater was the reduction in whole-tree photosynthesis, with the maximum reduction at three days after application. Whereas, in 2017 the photosynthesis reduction due to metamitron reached its lowest level around 7-8 days after application. We associated the rapid decline in 2016 with warm temperatures and a significant carbohydrate deficit while in 2017 the temperatures were lower especially at night. The maximum reduction in photosynthesis due to metamitron was around 40% in both years. Metamitron reduced fruit set and fruit load in a concentration-dependent manner. There was no interaction of year or timing with treatment; however, the thinning effect was greatest when metamitron was applied at 10-12 mm fruit size. Final fruit size was greatly increased when the chemical thinner was applied due to the reduction in crop load; though no significant differences were found between 350 and 500 mg L^‑1. Metamitron has great potential as a tool to optimize chemical fruit thinning. Further investigation is needed to better understand the response of metamitron in years when photosynthesis rates are low.