Metamitron, a photosynthesis inhibitor, is an effective thinner for ‘Bartlett’ pears

Abstract

'Bartlett' pear is the main cultivar in Argentina and for export fruit size is a limiting factor. Chemical thinning adjusts fruit load to a desirable level and reduces the need for hand thinning. The aim of this study was to evaluate the efficacy of diverse strategies of metamitron (MM) applications. Experiments were conducted during four seasons in two orchards located in Rio Negro, Argentine. First, MM was evaluated at three concentrations (165, 248 and 330 mg L‑1) at 8 to 10 mm fruit diameter stage. Secondly, single applications (165 and 248 mg L‑1) at 8-10 mm and double applications (82.5 and 124 mg L‑1) at 8-10 and 12-14 mm were tested. All treatments were compared with an untreated control and 6-benzyladenine application (6-BA) at 120-160 mg L‑1 and 8-10 mm fruit diameter. Fruit set, yield, fruit size, maturity indices, phytotoxicity symptoms and return bloom were assessed. All MM concentrations applied at the 8-10 mm fruit diameter stage resulted in a significant thinning compared to the control and also in some seasons compared to 6-BA. The reduction in fruit number by MM treatments was more evident in small fruits (<60 mm). Double applications increased the thinning efficiency but did not always promote fruit size at harvest. MM treatments only generated slight chlorosis of the leaves and did not negatively affect fruit size and bloom return. Thinning treatments did not affect maturity indices. MM shows great potential as a chemical thinner for 'Bartlett' pears, since it consistently reduced the number of fruits tree‑1.