Components of spore production in apple powdery mildew (Podosphaera leucotricha)

Abstract

The morphology of powdery mildew ( Podosphaera leucotricha) colonies of known age was studied on six apple cultivars inoculated in a glasshouse. Colonies were observed first on the older leaves. Mean disease incidence ranged from about 20% for cv. Laxton's Superb to about 45% for cv. Crispin. Conidiophore density (number of conidiophores/mm 2 colony) was higher on younger than on older leaves, higher on the upper than on the lower leaf surface, and decreased with the age of the colony. The number of hyphal interceptions on a line transect was higher on the upper surface but the ratio of conidiophore density to hyphal interceptions was greater on the lower surface and did not differ with colony age or cultivar. The number of conidia/conidiophore was higher on the younger leaves and on the upper surfaces and was at a maximum 7‐12 days after the first sign of the colony. The number of conidia/mm 2 colony was higher on lower surfaces, especially of the younger leaves. There was generally an associated variation between cultivars in components of spore production.