RESPONSE OF TWO APPLE CULTIVARS TO CHEMICAL FRUIT THINNING

The effects of two types of NAA thinners (Rhodofix and Agrostim Forte) on fruit set, fruit quality, yield and return bloom of ‘Golden Delicious’/M.26 and ‘Jonagold’/M.9 apples were studied. Experiments were conducted in a commercial orchard in 2004 and 2005. The following variants were used: applications of Agrostim Forte at a rate of 5 ml•L^-1, applications of Rhodofix at a rate of 3 g•L^-1 (both applied at 4-11 mm size of king fruit diameter), hand thinning (after June drop) and untreated control. In 2004, the application of Agrostim Forte to the cultivar ‘Golden Delicious’ at all three stages of development proved to be effective. The treatment at the first stage (Ag1-fruit diameter 4.3 mm) gave the best results. Regardless of the stage of development at application, the agent Agrostim Forte favorably influenced flower set of treated trees in the following year. The cultivar ‘Jonagold’ responded to treatment with Agrostim Forte similarly to ‘Golden Delicious’. The treated trees had larger fruits, but no influence on blossom rate (return bloom) was observed. In 2005, the best fruit quality of ‘Golden Delicious’ was recorded with hand thinning. The chemical thinning with this cultivar had a positive effect only after use of Agrostim Forte. The agent Rhodofix had no significant effects on fruit quality in comparison to the control. Both Agrostim Forte and Rhodofix did not affect the flower set in the following year. ‘Jonagold’ responded better to the chemical thinning than ‘Golden Delicious’. Both treatments, Agrostim Forte and Rhodofix, increased the fruit size and the proportion of fruits >65 mm in diameter compared to the control. The time of application was not important. Both chemicals influenced positively flower set of this cultivar in the following year.