Multi-site Thinning Comparisons with `Empire' and `McIntosh' Apples

Many chemical, environmental, and physiological factors have been reported to be important to apple chemical thinning, so we have been developing a multi-site and multi-year database of chemical thinning results and potentially important factors. For 3 years, we have conducted replicated thinning trials in `Empire' and `McIntosh' apple orchards at six or seven sites around New York state in different climatic regions. Different concentrations of NAA and Accel (primarily benzyladenine), NAA/carbaryl and Accel/carbaryl combinations and unthinned controls were tested with treatments applied at the 10-mm king fruit stage by airblast sprayers. Flower cluster counts, set counts, yields, fruit sizes, and other factors thought important to thinning response (orchard condition/history, weather, application conditions, etc.) were measured or estimated in each trial. Analysis of factor importance is continuing, but some general results have come from the thinning trials so far. Thinning effectiveness varied among years from poor to adequate. There have not been consistent thinner concentration responses. Commercial NAA and Accel concentrations have not thinned adequately. NAA/carbaryl and Accel/carbaryl have thinned the most. For the same crop load, trees thinned with Accel or the carbaryl combination have had better fruit size than when thinned with NAA.