Department of Plant Pathology |
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Black Root, caused by Aphanomyces raphani, affects only radishes. The first symptoms appear where side roots emerge. The skin turns purple to black in an area that finally girdles the root. The girdled area becomes constricted and may crack as unaffected tissue continues to grow. The black discolored area extends inward in radial streaks and generally remains firm. The black root fungus can persist in soil for long periods. Disease development is favored by warm weather. |
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Downy Mildew, 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 shaded plantings. |
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