Soybean yield losses can run 15-20%

Stem canker, a disease capable of killing full-grown soybean plants, is reappearing in the central U.S.

"During the early 1950s, northern stem canker was prevalent in the north-central U.S. It caused losses up to 50%," says Glen Hartman, a USDA-ARS plant pathologist at the University of Illinois. "In 1999 and 2000, there has been a resurgence of the disease in Illinois and neighboring states."

In random monitoring of Illinois soybean fields this past summer, Hartman and colleagues found stem canker in one of every 20 fields they checked.

"Every time we went out, we saw the disease in some fields," he reports. "We also were called out to specific fields to confirm stem canker. In one of those fields, one of every 10 plants was affected."

Yield reductions today can run 15-20% in severe cases, but are generally less than 1%, Hartman notes.

He says many seed company representatives are talking about stem canker, indicating that it's an increasing problem. But it's not in the same class as soybean cyst nematode, sudden death syndrome or white mold.

Pioneer Hi-Bred agronomist Mark Hartelt, who observed a few cases of stem canker in his central Illinois area last summer, says today's soybean varieties have been holding up relatively well against the disease.

"Some of the older varieties were quite susceptible, but they no longer are in the breeding programs," he says. "Possibly we are seeing some reappearance of the disease due to earlier planting and more crop residue."

Stem canker occurs when conditions are cool and moist early in the season, Hartelt explains.

"There is a reddish-brown to black lesion near a lower node, although it doesn't show its effects until pod filling," he says. "At that point you see individual plants or small groups of plants with premature yellow leaves.

"Stem canker is similar in appearance to sudden death syndrome, phytophthora root rot, brown stem rot and charcoal rot, but is distinguished by the lesion. The plant dies out above the lesion yet is healthy below it."

How can a grower defend against stem canker?

"There is not a lot we can recommend, management-wise, that would be of significant help," says plant pathologist Hartman. "In the South there are varieties with resistance to southern stem canker. In the North there are varietal differences in susceptibility to northern stem canker, but those differences have not been well-categorized."

Hartelt suggests that growers continue to select the best all-around varieties for their areas. "Most current varieties have a respectable level of tolerance to stem canker," he notes. "We generally don't advise that growers select varieties too heavily on the basis of disease resistance unless a given field has a history for a particular disease.

"It may help against stem canker to plant later when conditions are drier."