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Stem canker rises in South Dakota

Something didn’t look right in the soybean field on the north side of the road. It was the wilted look of those leaves turned bottom up to the early September sun that made South Dakota State University plant pathologist Thomas Chase pull over and stop.

Though it had the look of some more familiar late-season soybean diseases, it reminded Chase of a disease he’s seen increasingly since the late 1990s: stem canker.

“When we first started finding stem canker in South Dakota, it was kind of a surprise. I didn’t even have it on my radar screen. I didn’t even teach it in my class, hardly. Then in about 1998, we started getting fields that were just hammered with stem canker,” Chase recalls.

“We had these growers calling us up and saying, ‘I think my crop’s maturing early. I’m not sure what’s going on.’ We went out there and found that 50% of the plants were hit with stem canker. Yes, they were maturing, because they were getting killed — they had a pathogen that was causing a disease.”

On this early September day in northeastern South Dakota, Chase’s windshield hunch made him suspect the same problem. He found dead plants scattered through one corner of the field in only a few minutes of scouting. And then the telltale sign he was looking for — a plant that showed an area of reddish-brown to black girdling the stem. It’s that zone of dead tissue that is called a canker — essentially a roadblock for water and nutrients trying to zip from the root of the plant to the leaves.

Causes

Stem canker is caused by a fungus. Northern stem canker is caused by D. phaseolorum var. caulivora (DPC) and southern stem canker is caused by D. phaseolorum var. meridionalis (DPM). At this point, only the northern stem canker has been found in South Dakota.

Northern stem canker caused severe losses in the Midwestern and north-central regions of the United States and Ontario in the late 1940s and early 1950s. Once growers removed extremely susceptible varieties from production, it became insignificant as a problem.

Southern stem canker arose in the southern United States in the 1970s and continues to be significant problem for southern soybean farmers.

The resurgence of stem canker in the past few years in northern states may be related to the deployment of highly susceptible varieties, changes in farm practices or changes in the pathogen population. It may also have to do with seed sources, Chase says.

Scientists don’t fully understand the circumstances that allow northern stem canker to thrive. Figuring out how host, pathogen and environmental factors contribute to stem canker epidemics in South Dakota and the surrounding region is one of the main research topics for the SDSU Row Crops Pathology Project.

At least so far, northern stem canker’s impact has been scattered and intermittent. The loss to the grower depends on how many stems get infected and how quickly it kills them.

“It’s been so intermittent and dispersed that we have to have the growers let us know where it’s happening,” Chase says. “But I think we may be building up to year that’s going to be just perfect, and we’ll have a more widespread epidemic.”

At least on a field-by-field basis, the disease can take a heavy toll. Chase’s hunch on his early September scouting trip was that the field he found was in trouble, and he was right. When he checked the field in mid-September, he found 90% of the plants had stem canker in a field that had been seeded to a mixture of alfalfa and intermediate wheatgrass for more than a decade.

“In other words, the field hadn’t seen soybeans for 11 years,” Chase says. “We will continue to do research on the field, including doing soil assays for the pathogen as well as assaying old alfalfa and wheatgrass residues and testing alfalfa for susceptibility to the pathogen.”

Nixon is a writer for SDSU AgBio Communications.

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Appearance: Stem canker is characterized by an area of reddish- brown to black girdling the stem. That canker is a zone of dead or diseased tissue that can block water and nutrients trying to get from the root of the plant to the leaves.

This article published in the February, 2010 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.