Alan
McNab
Professor of Plant Pathology |
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Downy Mildew is caused
by the fungus Peronospora parasitica. Leaf symptoms appear as
small yellowish areas that later turn brown with bluish-black
lace-like markings. In moist weather, a whitish downy mold develops
on the underside of leaf spots. Roots also may be affected. Inner
root tissue is discolored, sometimes with net-like brown or black
areas. In advanced stages the skin is roughened by small cracks
and the root may split. In the absence of leaf symptoms, downy
mildew could be confused with black root. The mildew fungus overwinters
in roots from diseased plants. Disease development is promoted
by cool weather and prolonged moist conditions associated with
dense fog, drizzle, and persistent dew in dense and shated plantings. |
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Blue
Mold (Downy Mildew), caused by the fungus Peronospora effusa,
affects only spinach. Initial symptoms are indefinite yellowish
areas on upper leaf surfaces. On the corresponding lower surface,
a gray to violet gray mold appears. Affected areas turn black
and die. The disease may affect entire leaves and plants. In
severe attacks, all plants in a field may be destroyed within
a few days. The blue mold fungus overwinters in soil, in seed,
and in spinach plantings. Fungus reproductive spores are spread
from plant to plant by wind. Disease develops best in cool moist
weather and is most prevalent in coastal areas. |
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