No abstract

In laboratory tests, germination of conidia on glass slides of Sphaerotheca fuliginea was greatest at 25°C in high humidity, although the germination rate was generally low. No germination was observed below 15°C or above 30°C, or at relative humidity below 94%. Field studies of the development of powdery mildew on squash plants showed that symptoms first appeared on old leaves. Symptoms did not appear on first leaves until 7-8 weeks after emergence, but appeared progressively earlier on late formed leaves (2 weeks). Only a small proportion of the conidia germinated when placed on young leaves (2 weeks), but many germinated on older leaves (6 weeks). A field experiment indicated that the first symptoms of powdery mildew appeared approximately 1 week after a prolonged period of continuous leaf wetness (about 12 h) and high humidity (about 95%) in the summer when temperatures frequently rose above 22°C. The disease begins in isolated patches on leaves in dense canopies more than on exposed leaves.

No abstract