Corn and Soybean Disease Update – August 12, 2011

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Corn
Gray leaf spot is increasing in irrigated corn but it is too late to affect yield. This late in the season we often see an increase in gray leaf spot as well as Northern corn leaf blight.

We have seen several samples of ears with the milk line half-way up the kernels with random discolored kernels. This is Fusarium ear rot caused by Fusarium moniliforme. The fungus travels down the silks and infects the individual kernels. Often white fungal growth can be seen as well. Hybrids vary in their susceptibility and infection is favored by hot, dry weather. When severe the whole ear can be whitish. Often most of the fungus growth is limited to the tips of the ears. If grain is to be stored it is important to dry it sufficiently to prevent growth of the fungus to prohibit growth of the mycotoxin, fumonisin.

Individual random kernels and some tip infection by Fusarium moniliforme.

Soybeans
Despite the hot, dry weather downy mildew is showing up in full season irrigated soybeans now. Varieties vary in their resistance, but this fungus disease has never been yield limiting or damaging here. The disease produces numerous small yellow spots on the upper leaf surface and a tuft of grayish fungal growth on the corresponding lower leaf surface.

Downy mildew on the upper leaf surface of soybean caused by Peronospora manshurica.

Grayish tufts of the downy mildew fungus on the lower leaf surface.

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