Concentration and time of application of Metamitron as an apple post-flowering thinning agent in apple trees

In years when climatic conditions increase flower buds formation, it is necessary to regulate yield and improve fruit size and quality by adopting chemical thinning. Due to the climatic conditions of southern Brazil, the fruit set from year to year is very variable. In these conditions, the adoption of thinning during bloom becomes a greater risk for the grower since it is only possible to assess the need and thinning intensity after fertilization. Recent works have demonstrated the effectiveness of metramitron (MM) alone or in combination with benzyladenine (BA) in post-bloom thinning of apples. The experiments were conducted in an experimental orchard located in Southern Brazil, with trees of 'Fuji Suprema' and 'Maxi Gala' grafted on 'M.9'. Compared with BA and hand thinning, three metamitron concentrations were tested at three application times (fruit with 8-10, 12-14, and 16-20 mm). The variables evaluated were: fruit drop (%), floral clusters with one, two, three, four, or more fruit (%), yield (fruit tree(-1) and kg tree(-1)), average fruit fresh mass (g), fruit classification, russeting incidence and estimated yield (t ha(-1)). Metramitron applications applied on fruit of 8 to 18 mm in diameter reduced yield to desirable levels in the cultivar 'Fuji Suprema' and 'Maxi Gala'. Applications on fruit with a smaller diameter (8-10 mm) in 'Maxi Gala' caused excessive thinning. Reducing yield in 'Maxi Gala' significantly increased the average fruit mass. All metamitron treatments reduced thinning labor.