Abstract
Epidemics of apple powdery mildew were monitored on leaves of vegetative extension shoots in a mixed‐cultivar orchard in 1979. 1980 and 1981. Cultivars Cox's Orange Pippin. Golden Delicious. Ida Red and Suntan were very susceptible but little mildew developed on cv. Discovery in any year. Results with cvs Crispin and Spartan were less consistent due, in part, to differing patterns of vegetative growth. The rankings of cultivars for disease incidence (proportion of leaves with mildew), number of colonies and mildewed area were similar, and were also consistent on fungicide‐sprayed and unsprayed trees in 1981. Less disease occurred on fungicide‐treated Golden Delicious than on Cox's Orange Pippin. Most infections occurred on the rolled leaves at the shoot tip. Spore production was greatest on leaves at positions ‐1 to ‐4 where leaf 0 was the youngest and leaf ‐ 1 the adjacent and older unrolled leaf.
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