Soybean Disease Update – July 24, 2009

Bob Mulrooney, Extension Plant Pathologist;

Soybean Rust Update
On July 21, soybean rust was reported on kudzu from Escambia County in far west Florida. This site has been positive in previous years. On July 20, soybean rust was reported from Avoyelles Parish, Louisiana on soybean. On July 10, soybean rust was reported on kudzu in Columbia County, Florida. Soybean rust scouting continues in the U.S. and Mexico. Soybean rust scouting in DE will begin in August.


Downy Mildew
Downy mildew in soybeans is now being seen. Soybean downy mildew is caused by the fungus, Peronospora manshurica which only infects soybeans. The fungus causes irregular yellow spots on the upper leaf surface and a tuft of gray fungus growth on the corresponding lower leaf surface. Varieties vary in their level of resistance to this fungus. As best we know here downy mildew rarely, if ever, affects yield. In heavy infections seeds can become infected and have a coating of the reproductive structures of the fungus (oospores) over them. Fungicides are not recommended.

soybeandownymildewDowny mildew on soybean

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