Any one of seven closely-related fungi can cause powdery mildew. These fungi usually first attack leaves that are crowded and close to the ground. The spores are carried by air currents and germinate on the leaf surface. Liquid water on leaves inhibits spore germination. From the time a spore germinates to the time new spores form may require only 48 hours.
High humidity favors spore formation while low humidity favors spore dispersal.
Maintaining conditions that favor rapid drying of foliage will help reduce disease incidence. Locate susceptible plants in open areas where they will not be crowded. Plants in shade are more prone to mildew than are those growing in sunlight. Powdery mildews are generally most severe on young. A balanced fertilization program is advisable.