Metamitron application could replace carbaryl treatment for chemical thinning in 'Red Delicious' apple

Abstract

Carbaryl is Argentina’s most used thinning compound for ‘Red Delicious’ apple. Due to environmental concerns about its application, there is a need to find alternatives for chemical thinning. The objective of this work was to evaluate the thinning efficacy of diverse strategies of Brevis® (15% metamitron) applications in the apple ‘Red Delicious’. Experiments were conducted in a commercial apple orchard of ‘Red Delicious’ located in Rio Negro (Argentina) during four seasons (2014-2018). One or two applications with metamitron (MM) were made when central fruit in the clusters were 8-10 mm and 12-14 mm in diameter. First, MM was evaluated at three concentrations (165, 248, and 330 mg L‑1). Secondly, single applications (165 and 248 mg L‑1) and double applications (82.5 and 124 mg L‑1) were tested. All treatments were compared with untreated control and carbaryl application at 1200 mg L‑1. Spraying volume was 2000 L ha‑1 in all treatments. Fruit set, yield, fruit size, maturity indexes, phytotoxicity symptoms, and return bloom were assessed. Under trial conditions, all MM treatments significantly reduced crop load, fruit set, and the number of fruits per tree. In some seasons, the highest MM concentrations (248 and 330 mg L‑1) generated an excessive thinning effect. On the other hand, carbaryl treatment was sometimes ineffective. 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. Therefore, a single treatment with MM at 165 mg L‑1 (±Brevis® 2.2 kg ha‑1) with 8-10 mm fruit diameter is recommended for chemical thinning of ‘ Red Delicious’ and can be as effective as carbaryl application.