SHADING AS AN EFFECTIVE MEANS FOR CROP LOAD MANAGEMENT AND FRUIT QUALITY ENHANCEMENT IN APPLE TREES

Since carbon starvation due to photosynthesis inhibition may induce fruit abscission, four-year-old apple trees ‘Golden Delicious’, ‘Elstar’ and ‘Gala’ on M9 at Bonn were shaded to achieve desired crop load viz., thin. Whole trees were covered with shade cloth (80% shade/PAR reduction, 90% UV reduction) for either 3, 6 or 9 days either at the end of bloom (EB) or 23 days after full bloom (DAFB); uncovered adjacent trees served as control. While shading for 3 days (23 DAFB) was optimal for fruit quality (fruit size, colour and sugar content) with only a slight decrease in yield, prolonged shading (for >6 days) also led to fruit quality improvement, but also to a considerable decrease in yield, caused by a higher rate of June drop; fruit trees shaded earlier at the end of bloom shоwed weaker June drop and more fruit set with insufficient thinning efficacy. Later and prolonged shading (more then 6 days) increased fruit sugar content (SSC) by 1.0-2.3% (and taste) by improving tree source: sink relationships, and fruit mass by 41% in ‘Gala’ and 13% in ‘Elstar’, with better (75-100% red surface) colouration of 85-96% than ca. 65% in the un-shaded control; the same shading treatment also induced the desired stronger return bloom viz., less alternate bearing, with the least flowers in the un-shaded control. Thus, this study has shown that shading for 3-6 days at 23 DAFB due to its effects on source:sink relationships and tree carbon starvation may be an environmentally-friendly technology to obtain fruit of the desired quality in terms of firmness, size, colour and sugar content and prevent biennial bearing of fruit trees.