Early Misdiagnosed Stem Canker Poses Risk for Soybean Growers

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. At least in some locations, it will again be a problem in 2010.

“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 recalled.

“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 percent 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 that 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 shows 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 — or more accurately, two different pathogenic varieties of the fungus Diaporthe phaseolorum. The disease is divided into Northern stem canker and Southern stem canker types. 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, Canada 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, changes in the pathogen population. It may also have to do with seed sources, Chase said.

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 said. “But I think we may be building up to a 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 had found was in trouble, and he was right. When he returned to check the field in mid-September, he found 90 percent of the plants had stem canker in a field that had been seeded to a mixture of alfalfa and intermediate wheat grass for more than a decade.

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

Chase learned afterward from the farmer working that field that his average yield for the plot in 2009 was 15 bushels an acre, whereas in a normal year he would have expected at least 40 bushels an acre.

Management tips
Here is Chase’s advice to growers during the 2010 season:
Start serious scouting for stem canker in early to mid-August and look for symptoms that include cankers or discolored areas centered on lower leaf nodes, dead and dying petioles, rapid wilting resulting in shriveled gray to brown colored leaves. Canker appearance is quite variable. On susceptible cultivars, cankers “run” and are diffuse and not well defined. If cankers girdle stems, the plants die. In some cases cankers are more limited, meaning there is less early wilting and yield loss.

Two soybean diseases that may be confused with stem canker are white mold (Sclerotinia stem rot) and Phytophthora root and stem rot (PRR). Absence of sclerotia on and in stems and absence of white moldy growth, shredded stems and bleached/tan lesions would rule out white mold.

PRR can be ruled out by location of stem lesions, as PRR lesions are found on the base of the stem and are continuous with the soil line. Developing stem canker lesions are found low on the main stem, usually the V2 to V5 nodes. In addition, the distinctive appearance of rapidly wilted leaves and widespread but even distribution within fields — sometimes an entire field or large sections of a field — are more typical of stem canker than of PRR.

Management advice is scanty for growers once Northern stem canker is confirmed in a field.

“We would probably suggest rotating out of soybean for several years in fields that have a history of NSC. Growers should also probably try to obtain a variety with a higher level of resistance if they can and consider using fungicide-treated seed,” Chase said. “At this stage it’s important for growers to understand that they may have a different disease problem than white mold or PRR, and they don't want to apply a management strategy based on a misdiagnosis.”

For more information, read Chase’s article about northern stem canker at the North Central Soybean Research Program’s Plant Health Initiative at this Web link: http://www.planthealth.info/stem_canker_basics.htm.

Find additional images to help in scouting for stem canker by referring to Chase’s row crops pathology Web page, http://plantsci.sdstate.edu/rowcropspath/.

Chase’s work is partially funded through what was formerly the Cooperative State Research, Education, and Extension Service, or CSREES, now the National Institute of Food and Agriculture; and by the South Dakota Agricultural Experiment Station. Additional funding is provided by grants from the South Dakota Soybean Research & Promotion Council.