Powdery mildew occurs on many different flowers, woody ornamentals and trees including roses, snapdragons, African violets, kalanche, English ivy, zinnias, photinia, oak, lilac, and begonias. Several different genera of fungi cause powdery mildew. Although usually one genus specifically attacks one or two different plants, some species of powdery mildew (such as Erysiphe cichoracearum) attack a wide range of plants. All the powdery mildew fungi are obligate parasites, requiring live tissue to grow and reproduce. In greenhouses, the fungus survives by spreading from the diseased plants to the new plants of that same crop. If that crop is not grown for several weeks, the fungus dies out and diseased plants must be brought into the greenhouse to establish the fungus again. Outdoors, fungal structures form on leaves and twigs that allow the fungus to survive winter conditions.
White powdery fungus grows on the upper leaf surface of the lower leaves and flower parts.
Leaves may be twisted, distorted, then wilt and die.
On some plants such as kalanchoe, infected leaves have dry, corky, scab-like spots and fungal growth is not obvious.
Conditions favoring powdery mildew:
High relative humidity at night
Low relative humidity during day
70-80 F (22-27 C) temperatures (These conditions prevail in spring and fall)
The spores are carried by air currents and germinate on the leaf surface. Liquid water on leaves inhibits spore germination. The fungus grows on the leaf surface but sends fine threads (haustoria) into the cells to obtain nutrients. From the time a spore germinates to the time new spores form may require only 48 hr. High humidity favors spore formation while low humidity favors spore dispersal.
Some powdery mildew are inhibited by free moisture on leaves while others are favored by wetness on leaf surfaces.
Managing powdery mildew in greenhouses:
When conditions are favorable for 3-6 consecutive days, heat and ventilate in late afternoon to reduce night humidity.
Antitranspirant materials such as Vapor Gard* or Wilt Pruf* applied to coat the leaf can prevent infection. In the landscape, application remains effective up to 30 days. Treat plants such as lilac on June 15, July 15, and August 15 (*Trade name).
Apply a fungicide on a regular schedule until conditions change. Be certain the crop is on the label.
Don't use any fungicide that's not approved for use on something that is consumed by humans. Many of the fungicides that treat powdery mildew are only approved for use on ornamentals.