Towards apple bearing stability in strictly alternating orchard

Abstract

Investigation of the effect of rootstock on apple bearing stability was performed at the Institute of Horticulture, Lithuanian Research Centre for Agriculture and Forestry in 2014-2016. Apple cultivars ‘Auksis’ and ‘Ligol’ on M.26 semi-dwarf rootstock; M.9, B.396 and P 67 dwarf rootstocks; and P 22 super-dwarf rootstock were included in the trial. The orchard was planted in spring 2005. Fruit thinning was not performed up to 2014, in order to maintain the apparent effect of the rootstock. During this period the alternate bearing index (ABI) of ‘Ligol’ varied from 0.58 on B.396 rootstock up to 0.80 on M.9; and ABI of ‘Auksis’ varied from 0.70 on B.396 rootstock up to 0.78 on P 67. Starting from 2014 blossom thinning was initiated in order to stabilize tree bearing. Three crop levels were established expecting apple yield from 20 to 54 t ha(-1). More intensive accumulation of phytohormones at lower crop load levels was recorded. At the same time more intensive flowering inhibiting phytohormones along increasing crop load was established for both cultivars on all rootstocks. Positive effect of crop load adjustment was less obvious in the first year after the treatment but more expressed in the second year. Return bloom depended both on crop load in previous year and on rootstock. Experiment revealed differences between rootstock impact on apple bearing stability. Stable average yield of ‘Auksis’ is possible to maintain at 30 t ha(-1) level with all tested rootstocks. Stable average yield of ‘Ligol’ is possible to maintain at 35 t ha(-1) level on M.9 and B.396 rootstocks, but at more than 50 t ha(-1) level on P 67 rootstock only.