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Issue No. 12, Article 10/June 15, 2007

Soybean Disease Update

Root rot and damping-off. In some parts of the state, observations have been made of soybean plants with reddish-brown sunken lesions on the roots and hypocotyls. In some cases, postemergent damping-off has also been observed. These observations are all symptoms of Rhizoctonia root rot, caused by the soilborne fungus Rhizoctonia solani. At this point in the growing season, there are no control options for the disease. No soybean varieties have complete resistance to Rhizoctonia root rot, but there are subtle differences in susceptibility among varieties. Fungicide seed treatments may reduce seedling blight caused by Rhizoctonia, but they do not provide season-long control. Some of the newer strobilurin fungicide seed treatment active ingredients that are now available for soybean, such as azoxystrobin and trifloxystrobin, may provide added protection against Rhizoctonia. Rhizoctonia root rot may be more severe when additional plant stresses are present (e.g., herbicide injury), so practices that promote good seedling health are recommended.

Reddish-brown sunken lesions on the roots and hypocotyls of soybean plants from Urbana, Illinois, caused by Rhizoctonia solani.

Postemergent damping-off of soybean plants from Urbana, Illinois, caused by Rhizoctonia solani.

Leaf spots. Small brown "spots" on lower leaves (especially unifoliolates) are appearing in some areas of Illinois. These spots are caused by the fungus Septoria glycines, and the disease is known as brown spot. Under the current dry conditions in most of the state, brown spot progression to other leaves will be halted. In general, brown spot does not cause economic losses to soybean fields in Illinois. Because the Septoria fungus overwinters on soybean debris, rotation with a nonlegume crop for at least one year will reduce the amount of Septoria present in a field.

Brown spots on a soybean unifoliolate leaf caused by the fungus Septoria glycines.

Soybean rust. Some recent soybean rust activity has been reported in Texas and Louisiana. On June 5, soybean rust was detected in kudzu in St. Mary's Parish in Louisiana. Earlier this year (May 8), rust was detected in Iberia Parish. In Texas, soybean rust was reported in kudzu in Liberty County on June 2; it is the only known active rust site in the state. As part of a national monitoring effort for soybean rust, Illinois has 40 sentinel plots located throughout the state. Leaves from the sentinel plots are collected regularly and sent to the University of Illinois Plant Clinic to be checked for soybean rust. Sentinel plot data from all states are available at the USDA's soybean rust Web site (www.sbrusa.net). This site also contains the latest soybean rust fungicide decision guidelines (found under "Tactics"), which were recently revised.

Distribution as of June 13, 2007, of soybean rust in the United States (map courtesy of the USDA soybean rust Web site: www.sbrusa.net).

--Carl A. Bradley

Carl A. Bradley

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