This common plant disease is caused by a number of closely related fungi that thrive in both humid and dry weather. The powdery patches consist of fungal strands and spores. The spores are spread by the wind to healthy plants. The fungus saps the plant nutrients, causing the leaves to turn yellow and sometimes to die. A severe infection may kill the plant. Since some powdery mildews attack many different kinds of plants, the fungus from a diseased plant may infect other types of plants. (For a list of powdery mildews and the plants they attack, see Powdery Mildews and Some of the Plants They Infect.) Under favorable conditions, powdery mildew can spread through a closely spaced planting in a matter of days. In the late summer and fall, the fungus forms small, black, spore-producing bodies, which are dormant during the winter, but which can infect more plants the following spring. Powdery mildew is generally most severe in the late summer and under humid conditions.
Look up your plant in the index to determine which of the different fungicides should be used. Spray at regular intervals of 10 to 12 days, or as often as necessary to protect new growth. Remove and destroy severely infected plants. Where practical, pick off diseased leaves. Clean up and destroy plant debris. Plant in sites that have good air movement and receive early morning sun.
