Downy Mildew

Guy Padgett  |  8/19/2011 11:31:32 PM

Downy mildew.

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Downy Mildew (Peronospora manshurica)
Daniel V. Phillips
University of Georgia

Downy mildew, one of the most common foliar diseases of soybean, occurs worldwide, but is seldom a serious problem. This disease reduces seed quality and seed size. If extensive defoliation occurs, yields can also be reduced.

Disease development is favored by high humidity and cool temperatures. Symptoms are found on young plants, but the disease does not become widespread in a field until the late vegetative or early reproductive stages. The increased resistance of older leaves and higher temperatures at mid-season stops disease development before extensive damage occurs.

Lesions start as small light green to yellow indefinite spots on the upper leaf surface. As spots enlarge they become yellow-brown to dark brown. A grayish downy tuft of sporangiophores develops on the lower leaf surface during periods of moist weather. Severely infected leaves drop prematurely.

Cultivars with some resistance are available and should be used. However, many races of the pathogen have been identified, and cultivars which are resistant to all known races have not yet been developed.

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