Thinning apples via shading: an appraisal under field conditions

Abstract

SummaryFruit development and the fate of fruits was followed on ‘Imperial Gala’ apple (Malus × domestica Borkh.) trees thinned chemically or using shading. The percentage fruit drop, fruit growth rates using callipers and electronic gauges, and whole tree gas exchange rates were measured before, during, and after covering with a shading cloth that blocked 90% of solar radiation, applied for 1 week starting 30 d after full bloom (DAFB). While fruits were thinned to similar crop loads and all reached a similar size at harvest, their growth patterns reflected the treatments applied, with fruit growth slowing down and fruit dropping sooner in chemically-thinned trees which were treated earlier, than in the shading treatment, which took place later. Daily fruit growth patterns changed and were greatly reduced under shading, as were the net carbon exchange rates (NCER), both in total per tree and per unit of light intercepted by the trees. Regression analysis of the NCER per unit of light intercepted revealed no difference between the two treatments before and after shading, but a significantly lower relationship during shading. This work supports the hypothesis that C-starvation may induce fruit abscission at approx. 30 DAFB. Fruit growth patterns, and their changes during shading, were consistent with this hypothesis. This method of thinning, may be of interest to reduce the use of chemicals, and in organic fruit growing. However, before it can be adopted, a method to estimate the length of the period of shading is required.