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.
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