Pre-selection of apple seedlings for partial powdery mildew (Podosphaera leucotricha Ell. et Ev. /Salm./) resistance

Abstract

Incidences of powdery mildew were repeatedly evaluated for two years on 1 420 young seedlings of 20 progenies (of different levels of mildew susceptibility) in a green house, and then for 10 years on 642 seedlings in an orchard. Part of the seedlings in the orchard were pre-selected for the characteristic and others not. Except for the first scoring done in the first year, there was no correlation between mildew incidence on individual seedlings in the green house and their mean performance in the orchard. The seedlings with scores above 6 (resistant or tolerant) at the first stage of evaluation in the green house, however, yielded four times more desirable seedlings after final selection in the orchard than the mean of the total. The progenies that had a better healthy state as a whole yielded more partially resistant genotypes than those with low mean scores. Therefore, the progenies that most rapidly develop infestation on the whole lot should be discarded, whereas those that retain a healthy state longer should be subjected to individual selection according to the previous item.