Charcoal rot has surpassed soybean cyst nematode as the No. 1 disease in a region stretching from southern Illinois to Delaware to Texas, reports John Russin, Southern Illinois University plant pathologist.

The charcoal rot fungus survives drought, winter weather and crop rotation, and attacks more than 500 plant species, including soybeans, Russin says.

Infected soybeans may turn yellow and drop their leaves early, but the main problem is yield loss.

"If you pick up the pod and shake it, you can sometimes hear a rattle like a maraca," says Salliana Stetina, head of the university's charcoal rot research.

The fungus, which attacks the roots and lower stems, takes its name from the fact that infected tissues, when split open, look as if they have been dipped in charcoal dust.

Varietal resistance may be the first line of defense. Southern Illinois researchers are screening 100 varieties of Maturity Group III, IV and V soybeans, hoping to develop a range of susceptibility and resistance.