White powder on garden plants is the primary symptom of a fungal disease called powdery mildew, which is caused by a number of different plant-specific fungi. The severity of the infection varies depending on the type of plant.
The earliest symptom of powdery mildew is a patchy, grayish-white fungal growth on the surfaces of foliage, flowers or fruit. The patches grow until they cover the entire area. Small black fungal bodies appear in the white patches. Infected leaves may become distorted and curl or turn brown and drop from the plant. The powdery mildew fungus only grows on the surface of plants and rarely causes serious harm, but severe infections can interfere with photosynthesis and slow plant growth.
The small black patches inside the white fungal matter are called cleistothecia. They are winter-hardy dormant spores that mature when temperatures are between 60 and 80 degrees Fahrenheit, skies are overcast and humidity levels are high. The mature spores, called ascospores, shoot up into the air. The wind blows them onto new plants so that they can cause new infections. The fungus continues producing spores throughout the growing season.
Powdery mildew infections are temperature-specific and cannot function if the temperature drops below 60 degrees or rises above 80 degrees. Applying a preventative fungicide as soon as the first symptoms appear and spraying at timed intervals throughout the growing season will protect plants from becoming infected. Plants also stay healthier if they are grown in sunny locations and have adequate air circulation. Raking up and destroying leaf debris reduces the number of spores that can cause new infections the following spring.