Powdery mildew is a common disease that affects many plants, from azaleas to zucchini. Other types of mildew exist, but powdery mildew is the disease you're most likely to see in your garden. It spreads by spores and occurs in warm, humid weather. The cool evening fogs in places such as the San Francisco Bay Area can also create the conditions mildew loves. With some cultural methods and spraying certain substances on your plants, you should be able to keep mildew away from your prized plants.
Cut off any leaves or other plant parts if you begin to see any evidence of a fuzzy white or gray substance. Dispose of affected plants and parts of plants in your municipal greenwaste program or your regular garbage rather than sending them to your compost pile.
Prune plants to allow air to circulate between and around their leaves. Also allow plenty of space between plants to promote good air circulation.
Trim overhanging trees in order to introduce more sunlight---mildew does not survive as well in direct sun as in shadier areas.
Spray your plants with half a cup of milk mixed with 1 gallon of water---the lactobacillus in the milk will grow on the plants, thereby making it impossible for mildew spores to gain a foothold.
Rake fallen leaves around your plants, especially if they have signs of mildew.
Treat your affected plant(s) with a chemical antifungal product, which are available at nurseries, if the situation is serious and other methods have not been effective. Sulfur products are the least toxic. Mix the antifungal product according to label instructions and then spray the affected plant parts, top and bottom.