Towards grower-friendly apple crop thinning by tree shading.

Light management with shading nets, which reduce sunlight by 74%, might be an alternative to chemicals commonly used for thinning on apple trees. To study the effect of shading on crop load and fruit quality, trials were conducted in field experiments with the cultivars Golden Delicious and Elstar in 2006. Trees were either covered 25 days after full bloom (DAFB) with a net during three days, or until the peak of fruit fall, observed after seven days shading. Ideal time length for optimal crop yield was seven days shading for Elstar and three days shading for Golden Delicious. Alternate bearing could be decreased as flower initiation counts the following year showed. In both experiments, inner quality of fruit such as sugar and firmness showed good values at optimal shading duration compared with chemical + hand thinning. In 2007, a second field trial was conducted with cultivars Golden Delicious and Topaz to study the time period for shading in further detail. Shading was done for three days at 19, 26 and 33 DAFB using two net types (three- and two-meter-net width, covering the trees entirely or only down to 50 cm above ground). For Golden Delicious, shading after 19 and 26 days reduced fruits per 100 flower cluster to the same extent as with chemical + hand thinning. There was no difference between the two net types. For Topaz, shading after 19 days showed the best results. Regarding inner quality of both cultivars, only sugar content for Golden Delicious could be significantly improved after 19 and 26 days shading. Further analyses are still under way (e.g. for acidity). This study is part of an effort for increasing European consumption with fruit from sustainable production systems, the ISAFRUIT-EU-project.