The photosynthesis inhibitor metamitron is an effective fruitlet thinner for ‘Gala’ apple in the warm climate of Israel

•Small fruit size is a limiting factor in marketing apples (Malus × domestica).•To improve the fruit size it is necessary to reduce crop load.•The photosynthesis inhibitor metamitron reduces assimilate fruitlets’ concentration.•180 g metamitron/ha at 6 mm stage reduces fruit number and increases fruit size.•High night temperature in Israel enables low dose of metamitron compare to Europe.

Small fruit size is a limiting factor in marketing apples (Malus × domestica). Several techniques have been used to improve this trait, among them blossom and fruit thinning with plant growth regulators such as auxins and cytokinins to reduce crop load. In the present study, carried out in three consecutive years (2011–2013), we evaluated the photosynthesis inhibitor metamitron (MM) as a thinner for ‘Gala’ apples. MM applied twice at 150 g/ha (in the form of 0.085% Brevis®) at the 6- and 10-mm diameter stages, or once at 180 g/ha (0.1% Brevis® at 1200 l/ha) at the 6-mm stage, resulted in significant thinning. Consequently, the average time spent on hand thinning was reduced from 40–80 day/ha to 10–20 day/ha. However, crop load was only slightly affected, depending on the year. In “ON” years, there was a considerable and significant shift to larger fruit size. The relatively low effective doses of MM, compared to those used in Europe and the USA, were likely due to the higher night temperatures for 3 weeks postapplication, which increased respiration and caused assimilation deficiencies during that critical period of fruit development. No negative effects were observed on the return bloom in the following year.