- Several different fungi cause powdery mildew, according to Cornell University. Gardeners see the disease spread during dry warm temperatures. Powdery mildew does not need water to spread or infect plants. However, humid weather causes fungal spores to germinate. Humidity higher than 90 percent increases the chances of powdery mildew infection, according to Colorado State University Extension. Infection is more likely in new stem growth than older stems.
- Gardeners may see a white powdery substance develop over the surface of stems or leaves. Patches of powdery mildew may look white or gray. Left untreated, leaves become distorted and drop and budding may not occur. The injury it causes depends on the plant species, according to Cornell University. Begonias may succumb to plant injury, while lilacs infected late in their growing season may experience little damage.
- Use fungicides containing triforine or potassium bicarbonate may on ornamental plants, according to Colorado State University Extension. Apply horticultural oils, such as neem oil, vegetable plants when the first signs of powdery mildew appear on stems and leaves, according to the University of California. Apply the fungicide every seven to 14 days according to the directions. Avoid applying a fungicide to any plant that looks water stressed or exhibits leaf wilting or discoloration from lack of water.
- Refrain from applying a fertilizer high in nitrogen late in the growing season or in late summer. Because late summer weather is warm and dry, encouraging new growth raises the risk of your plant contracting the disease. Find disease resistant varieties of vegetable plants at your local nursery if powdery mildew is an annual problem. Clean the garden bed area. Removing infected stems, leaves, blossoms from the soil prevents the disease continually infecting your plants.