Comparison of metamitron efficiency for postbloom thinning of young ‘Gala’ and ‘Golden Delicious’ apple trees

The aim of this study was to evaluate the thinning efficacy of metamitron applied either alone (single or double application) or in combination with 6-benzyladenine (BA) or naphthalene acetic acid (NAA) in young 'Gala' and 'Golden Delicious' apple trees. The experiment was carried out during the first three crop seasons, 2014, 2015, and 2016, at the commercial orchard located on the mountain Fruska Gora, Serbia. Application of chemical thinners was carried out at two fruit diameter stages (6-9 and 12-15 mm). The number of fruit and yield per tree, fruit size distribution, parameters of fruit ripeness, and return bloom were assessed. Single metamitron treatments were effective when the weather conditions were favourable on the day of application and 3 days after (minimum night temperature >10 °C, maximum daily temperature >20 °C) regardless of fruit diameter stages. A reduction in fruit number per tree was followed by an increase in mean fruit weight. Double metamitron application at lower concentration caused relatively stable thinning activity during the whole experimental period in both cultivars tested. Metamitron combined with either BA or NAA at standard rates significantly reduced the negative influence of low temperature on metamitron thinning activity in the first experimental year. These treatments slightly influenced overthinning in 'Gala' trees if applied when the air temperature was too high (minimum night temperature >15 °C, maximum daily temperature >25 °C). Repeated metamitron treatment and treatments in which metamitron was combined with BA and NAA resulted in an increase in fruit size and share of large fruit in the total yield during the whole experimental period in both cultivars tested. A reduction in fruit number per tree affected by these treatments was followed by increasing flowering intensity in the subsequent year, with the exception of double metamitron application to 'Golden Delicious' trees in the last experimental year. Fruit maturity parameters were slightly different under the influence of metamitron treatments.