Components of resistance of apple to powdery mildew (Podosphaera leucotricha)

Abstract

Components of resistance of apple to Podosphaera leucotricha were assessed in the glasshouse by spraying shoot tips of cultivars with conidia. Disease incidence (proportion of leaf surfaces with mildew) and colony numbers were higher and incubation periods correspondingly shorter on the older leaves. unrolled at the time of inoculation. Numbers of colonies were higher on the lower surface. Spore production per colony was higher on the younger leaves. In general, a short incubation period on a cultivar was associated with a high disease incidence, many colonies and high spore production. For some cultivars the association between these components permitted resistance to be classified as very high (cv. Discovery), very low (cv. Golden Delicious), or intermediate(cvs Bramley's Seedling. Suntan). The effects of inoculum concentration and post‐inoculation environment (glasshouse or outdoors) on components of resistance were assessed in a further study. No interactions between cultivar and inoculum concentration were detected. There was generally an associated variation in components between cultivars within an environment, but inconsistencies in cultivar rankings were noted between the two environments and the two studies. Mean incubation period was shorter and colony area larger in the glasshouse than outdoors. There were significant differences in conidia/cm2 colony between cultivars only in the glasshouse. Components conferring high resistance in cv. Discovery and low resistance in cv. Golden Delicious were confirmed in both environments and in both studies.