With temperatures statistically higher than past years, charcoal rot in soybeans is proving to be a serious threat. In fact, warmer weather this summer presented new dilemmas for farmers in the Midwest. Regions that typically wouldn't have to worry now are being forced to scout for newer diseases.
Soybean charcoal rot (Macrophomina phaseolina), commonly considered a southern disease, has made its presence known in recent years in the northern regions of the U.S. Constant high temperatures and little or no moisture has caused charcoal rot to spread and flourish.
A soybean plant can become infected from germinating sclerotia, the black spots that give the disease its name, at any point in its life cycle — from the seedling stage all the way through maturity.
“Although the disease is commonly associated with hot weather, actual infection can take place under relatively wet conditions,” says Doug Jardine, a plant pathologist from Kansas State University. “The disease stays latent in the plant as long as we see adequate moisture throughout the season. Plants will grow fine and you won't even know it's there.”
This destructive disease can create a 20-30% yield loss in years with prime weather conditions. However, fields last year near Ames, IA, reported up to 70% yield loss.
Farmers who find charcoal rot harvest the crop as usual. The actual bean itself is still usable, but the disease results in poor seed size and a lack of pod fill.
After harvest, the sclerotia are protected in the crop debris left in the field. They are slowly released into the soil, and can live there for up to four years.
“Research has shown that the single most significant management practice to deal with charcoal rot is to reduce plant populations,” says Jardine.
Over-populating a field puts stress on the plants, which greatly increases the chances of charcoal rot and other diseases. Less stress results in healthier plants. Other practices to prevent water stress should be used when possible. This includes irrigation and maintaining a field that is free of weeds and other moisture depriving factors.
Currently, there are no chemicals available that are specifically engineered to fight charcoal rot.
Chances of getting charcoal rot increase with the lack of crop rotation. But simply rotating with corn may not be the answer since corn is a host of charcoal rot as well. About 500 weed and crop species have been found to support the disease.
“Rotations could be somewhat beneficial, but in a normal corn/soybean rotation it's probably not enough,” says Dean Malvick, plant pathologist from the University of Minnesota.
Planting a high-quality soybean that isn't highly susceptible to diseases may also help reduce the risk of charcoal rot, experts say. Jardine also recommends planting later maturing varieties.
“Stresses including other diseases and pests can increase the possibility of charcoal rot, especially under warm, dry conditions,” says Malvick.
Genetic relief may be on the horizon. Scientists in Stoneville, MS, at the Agricultural Research Service, Crop Genetics and Production Research Unit, have recently developed a soybean line that is genetically resistant to charcoal rot. It's significant since there's a lack of effective and economical fungicides for charcoal rot.
Although it's a long way from commercialization, Malvick hopes they can start breeding with the genetic line and incorporate it into commercial varieties. “It could be a real breakthrough for management of this disease,” he says.
The seasonal drought outlook and other meteorological assessments can be found at www.drought.noaa.gov.