Can hail nets reduce alternate bearing in 'Elstar' apple?

R. Wehrle, P. Steinberger, A. Kunz, L. Damerow, M.M. Blanke
The objective of the present study was to identify the effect of hail nets on alternate bearing, using 2,086 apple trees of the alternate bearing sensitive ‘Elstar’ in three years. The control without hail nets comprised 245 ‘Elstar’ apple trees with considerable alternate bearing; trees grew 51.5 flower clusters in 2009, but doubled this value to 100 flower clusters tree-1 in the following year. Trees under hail nets from flowering to harvest developed an averaged 41.5 flower clusters tree-1 for 2009, which increased ca. 1.5-fold to 71.7 flower clusters tree-1 in 2010. This fruit set of the control trees without hail net was significantly larger than that under hail nets. The apple yield of control trees without hail nets of 14.6 kg tree-1 in 2009 declined in the following year to 6.3 kg tree-1 in line with the number of flower clusters. Under hail net, however, apple trees showed significantly lower yield compared with the cultivation without hail nets. Under hail nets, alternate bearing was also pronounced with 11.5 kg tree-1 in 2009, but 4.1 kg tree-1 in 2010, respectively. The lesser degree of alternate bearing under hail nets, observed with the alternate bearing sensitive 2,086 ‘Elstar’ apple trees at Klein-Altendorf, may be due to a combination of a) reduced light availability under hail nets, b) subsequently less flower induction with a low need for thinning and vice versa higher efficacy of thinning (chemical or mechanical) actions and c) lower yields. Alternate bearing is discussed with respect to a combination of a) flower-promoting cytokinins derived from the root and b) flower inhibiting basipetal GA7 transport from the young developing apple seeds.
Wehrle, R., Steinberger, P., Kunz, A., Damerow, L. and Blanke, M.M. (2018). Can hail nets reduce alternate bearing in 'Elstar' apple?. Acta Hortic. 1229, 151-156
DOI: 10.17660/ActaHortic.2018.1229.23
alternate bearing, biennial bearing, climate change, flower induction, hail net, Malus domestica Borkh., sustainability

Acta Horticulturae