There are many different powdery mildew fungi. The fungus spores overwinter on fruit trees. In spring, the fungus begins to grow, and spores are released to travel on the wind to young leaves. Powdery mildew thrives where cool nights follow warm days. Insufficient sunlight and poor air circulation favor the development.
Bee balm, begonia, clematis, dahlia, euonymus, hydrangea, lilac, pansy, phlox, rhododendron, rose, sunflower, zinnia, bean, grape, pea, potato, pumpkin, squash, strawberry, tomato, apple, dogwood, peach, crepe myrtle, maple, oak, and sycamore trees.
A white or gray powdery fungus appears on foliage and flowers. Round, white spots on upper leaf surface expand and merge, covering both sides of leaves. Infected leaves turn yellowish-green to brown. New growth may be stunned, curled, and distorted. Infected blossoms may not set fruit; fruit may develop a rough skin or be covered with the powdery fungus. Fruit drops early or is dwarfed.
Some weeds get easily infected with powdery mildew, such as henbit. Plant resistant selections. Give plants sufficient light and air circulation. Water plants from underneath rather than above to keep leaves dry.
Reduce nitrogen fertilizer. Pick off and destroy infected leaves and flowers. Spray ornamentals with horticultural oil, triforine (funginex), thiophanate-methyl (Thiomyl, Domain), azadirachtin (Neem), or triadimefon (Bayleton). Spray fruits and vegetables with wettable sulfur or horticultural oil. Discard infected flowers of annuals and leftover produce in fall.