Soybean Disease Update – August 21, 2009

Bob Mulrooney, Extension Plant Pathologist;

Powdery Mildew on Soybeans
We received a report of powdery mildew on soybeans from Sussex County this week. The weather has been very favorable for powdery mildew on many crops, as well as ornamental plants of all kinds. Powdery mildew on soybean is a rare occurrence these days. It is caused by the fungus Microsphaera difusa and produces the white talcum-like growth on the leaf surfaces which can infect all the plant parts eventually. Symptoms can vary from one cultivar to another. Rusty spots, chlorosis, green islands, defoliation or combinations of these symptoms may occur. Most cultivars have the dominant gene for resistance to this disease. Fungicides are not recommended for this disease since it is not known to reduce yields.

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Soybean Rust Report
On August 19, soybean rust was reported on soybeans in Bolivar, Issaquena, Sharkey and Warren Counties in Mississippi. On August 18, soybean rust was reported in commercial soybean fields in West Baton Rouge, Pointe Coupee and East Carroll Parishes in Louisiana. On August 15, soybean rust was reported in nine new counties in Mississippi, all in commercial soybean fields. The positive counties include Carroll, Grenada, Humphreys, Leflore, Montgomery, Sunflower, Yalobusha, Yazoo and Washington.

In spite of the rash of SBR finds in Mississippi most of these are at low levels and most soybeans are at R5 and later, so the threat from soybean rust for these growers is low. Earlier soybeans at R3/R4 would be at risk in these areas. Rust is heating up a bit in the South and may become more of a threat if spores are spread in the developing cold front that is forecast to bring rain to the region during the next several days. Spore deposition is forecast from the Gulf Coast through Arkansas and spreading as far north as Kentucky.

Soybean sampling for soybean rust continues here in Delaware. The only diseases that we are seeing in the sentinel plots are low levels of Septoria brown spot, downy mildew and Phyllosticta leafspot. Visit for more information.