Charcoal rot has been one of the most talked-about soybean diseases in Illinois in the latter part of this growing season. Other diseases, including SDS, BSR, Phytophthora rot, white mold, stem canker, and others, have also been important in some areas, but charcoal rot seems to have captured the attention of many this year.
Charcoal rot is also known as summer wilt or dry weather wilt. The disease is typically more common across southern than northern and central Illinois but was also common across central and parts of northern Illinois this season. It occurred in many fields, some with scattered, infected plants in small patches or parts of rows and others with patches 30 feet or more in diameter. The most seriously affected areas are often in the drier parts of fields.
For diagnosis, look for several characteristics after midseason. 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 SDS or BSR. A more solid diagnosis is based on the appearance of the root and lower stem. Scrape off the surface of the root and lower stem 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 that appear similar to scattered bits of charcoal dust. They are difficult to see without the help of a hand lens. Cutting the root and lower stem often reveals distinct wavy, gray to black streaks inside the root and lower stem. Many plants with charcoal rot in central Illinois this year also had rotted roots and dead Rhizobia nodules, but it was not clear whether this was caused by charcoal rot or another disease that had also infected the plants.
Microsclerotia of the charcoal rot fungus.
Charcoal rot of soybean.
Charcoal rot occurs primarily when dry, hot weather (soil temperature >84°F) puts stress on soybeans. The fungus (Macrophomina phaseolina) 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 that can severely damage soybeans when plants have been subjected to other stresses. That is, in many cases charcoal rot appears to be a secondary disease that takes advantage of weak plants. The 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 re-ported 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. Use lower- than-normal seeding rates, and follow a sound fertility program.
Additional basic information about charcoal rot may be found on the Crop Disease website (look under root and stem diseases). More information along with photos is available on the Plant Health website.--Dean Malvick