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Powdery mildew fungi are obligate parasites and survive on living host tissue. The fungi grow primarily on leaf surfaces and aerial plant parts. They are not systemic and they do not infect roots. The name powdery mildew accurately describes the appearance of the fungus on host tissue. Nutrients are obtained from host tissue by means of microscopic, rootlike organs called haustoria that penetrate the plant tissue.
Infected plants typically have a whitish, powdery covering infected leaves and stems. Infected leaves are often curled or twisted. Severe leaf infection may cause leaf yellowing, reduced leaf size and defoliation. Climate, host location (shade effects), host and the causal species of mildew influence the severity of the symptoms.
Spores are carried by the wind to host tissue where they germinate and infect the plants. They germinate on dry leaf surfaces even when humidity and free moisture are low. Germination and penetration of tissue usually occurs in 6 hours or less so overnight infection is possible. The disease can produce a new generation of spores in less than a week under favorable conditions.
Powdery mildews are controlled by the use of resistant varieties and also by a number of fungicides, including different formulations of sulfur. These chemicals are used as protective sprays, and it is very important to apply them at the first sign of disease since powdery mildew is extremely difficult to control once established. Powdery mildew is not usually worth controlling on bluegrass lawns. It is also not worth controlling when it occurs late in the summer or in the fall on squash plants, lilacs or many garden flowers. If these plants are damaged, clean up and destroy all of the affected foliage because the spores overwinter on infected tissues.