Apple fruit growth responses to varying thinning methods and timing

During several years of hand and chemical apple thinning trials, individual fruit diameter was monitored on many apple fruits to examine the immediate and season-long effects on fruit growth and abscission as affected by different methods (hand versus several chemicals) and timing of fruit thinning at different times after bloom. A general response was found between fruit abscission and reduced fruit growth rates (FGR) after shade and chemical treatments that cause fruit abscission. Among chemical thinners the most severe reductions in growth rates of the fruits ultimately retained to harvest occurred with Naphthaleneacetic acid (NAA) treatments applied at 15 days after bloom (DAB) with fruits about 10-15 mm diameter. Increases in final fruit weights in relation to crop reduction were greatest for hand thinning up to 20 DAB and for bloom or 5 DAB. NAA, benzyladenine and carbaryl at 15 DAB all inhibited fruit growth too much to allow maximum response to crop reduction. Fruit diameters did not recover to equal that of the non-thinned fruits for about 30 days. Benzyladenine and carbaryl also caused reductions in FGR of retained fruits but the reductions were less severe than with NAA and fruits from these treatments recovered to equal that of the non-thinned fruits by about 20 days.