Charcoal rot, unlike most soybean diseases, thrives in tinderbox growing conditions. And although it is most prevalent in the South, it can strike anyplace where those conditions exist.
"The disease also needs a high level of inoculum and plants that are under stress," explains Scott Abney, USDA-ARS plant pathologist at Purdue University. "The hot, dry weather produces much of the stress."
Yield losses from charcoal rot can run 20-30%.
Abney points out that the disease organism is unique in that it's most active when the soil temperature ranges between 80 and 95 degrees. "Growth of most fungi that cause soybean diseases taper off at a soil temperature of 80 degrees," he notes.
Soybeans are most susceptible to charcoal rot in the reproductive stage of growth, Abney says. But it's possible to have damage in the seedling stage if conditions are right.
Charcoal rot is characterized by premature yellowing of the top leaves and premature leaf drop.
"This is sometimes mistaken for normal maturity," points out Abney. "However, it's brought on by the stresses of heat, dryness and the charcoal rot organism."
Unfilled upper pods accompany symptoms of the disease.
Scraping of the lower stem and taproot reveal tiny black structures (microsclerotia of the fungus) that resemble charcoal dust.
The best way to combat the disease is to rotate out of soybeans for two years, Abney says. This is advisable if a field has a history of severe infestation.
It helps if wheat or cotton can be planted during the two-year interval between soybeans, since the charcoal rot organism does not survive well in those crops.
For growers in a typical corn-soybean rotation who don't normally plant other crops, Abney recommends selecting soybean varieties that are not highly susceptible to charcoal rot. Check with your seed supplier.
Another strategy is to plant the fullest-season varieties that are practical. The longer-season varieties tend to flower later and are in a vegetative growth stage during the early part of the hottest, driest portion of the growing season.
Some agronomists suggest that no-till-drilled soybeans may have an advantage because their faster ground shading tends to hold in moisture and maintains a cooler soil temperature.
"That sounds logical," Abney says. "However, research data that confirm the effectiveness of drilled soybeans as a way to prevent the disease are extremely limited.
"Any cultural practices that minimize plant stress will reduce disease potential. That includes moderate plant populations, which is a way to promote rapid and vigorous plant growth."
Optimum fertility levels, especially phosphorus, can help minimize the disease. Adding phosphorus won't control charcoal rot but can reduce the disease's severity.
Plowing or other tillage does not readily destroy the charcoal rot organism. "The fungal pathogen produces many tiny microsclerotia that can survive for long periods in the soil," Abney says.