Issue No. 18, Article 4/July 22, 2005
Charcoal Rot Appears Early in Illinois Soybeans
Charcoal rot, also known as summer wilt or dry weather wilt, has recently been reported to be in Illinois soybean fields. Although the recent rains in much of the state may have significantly alleviated the potential damage from this disease, a return to dry and hot conditions may favor more development of charcoal rot.
Charcoal rot is typically more common across southern than northern and central Illinois, but it can occur anywhere in the state where plants are stressed, especially because of hot and dry weather. The most seriously affected areas are often in the drier parts of fields.
Look for several characteristics for diagnosis. Leaves of severely infected plants turn yellow and brown, wilt, and stay attached to the plant. The initial yellowing is usually more uniform across the leaf than that caused by sudden death syndrome or brown stem rot. A more clear diagnosis is based on the appearance of the root and lower stem. Scrape off the surface of both with a fingernail or knife, and look on and under the epidermis. Plants infected with charcoal rot will have tiny gray-black specks called microsclerotia, which appear similar to scattered bits of charcoal dust. They are difficult to see without the help of a hand lens. If the root and lower stem are cut, there are often wavy gray to black streaks inside the root and lower stem.
Some reports have suggested that the pathogen that causes charcoal rot (Macrophomina phaseolina) infects early in the season and causes most of its damage when dry, hot weather (soil temperature >84°F) puts stress on soybeans. The fungus that causes charcoal rot is a pathogen that some reports suggest is weak by itself and may cause minimal damage when plants are not stressed, but it can severely damage soybeans when plants have been subjected to other stresses. This pathogen can survive for years in soil and has a wide host range that includes corn and grain sorghum. The pathogen can be transmitted via infected seed at levels reported from 1.5% to 8%.
Charcoal rot is a difficult disease to manage. Some cultivars have been reported to be less affected by charcoal rot than others, but none appears to be highly resistant. Crop rotation, especially with cereal grains for 1 to 2 years, is recommended where this disease has been a severe problem. Lower than normal seeding rates and optimal fertility may help reduce charcoal rot.
Additional basic information about charcoal rot may be found at the following Web sites: http://cropdisease.cropsci.uiuc.edu/soybeans/index.html (look under root and stem diseases) and http://www.planthealth.info/charcoal_basics.htm.-- Dean Malvick