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Powdery mildew on Poinsettia
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 Golovinomyces cichoracearum formerly 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.
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.
Powdery mildew on most deciduous trees does little damage and does not require fungicides.
Powdery mildew asexual spore formation.
Sexual fruiting structure (cleistothecia).
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