March 14, 2002

Increased Soybean Acres Raise Disease Concerns

A dramatic increase in soybean acres in North Dakota is bringing a corresponding increase in soybean diseases, according to a North Dakota State University plant pathologist.

More than two million acres were planted last year and that number is expected to go higher this year. "Low grain prices and diseases associated with small grains, especially during the 1990's, forced producers to look for alternative crops and soybeans became that crop," according to Berlin Nelson, NDSU plant pathologist. "Soybeans were raised mainly in the southern end of the Red River Valley, but now soybeans are moving north and into the central part of the state."

There is growing concern, however, over various soybean root rots such as Pythium, Phytophthora, Rhizoctonia and Fusarium Nelson says. Root rot is associated with wet soil conditions which have occurred on numerous occasions in the past decade. Adding to the problem, in some cases, is the close rotation of soybeans by some growers. Some growers plant soybeans on the same plot every year or every two years. "It’s difficult for producers to rotate with other crops that don’t make any money or don’t make as much, but soybeans are a fairly good crop and they are growing it a lot," he says.

Phytophthora root rot is one of the biggest problems notes Nelson. It has caused considerable yield loss through the years. Resistant varieties have been developed, however the fungus changes over time to attack some of the resistant varieties. Ted Helms, NDSU's soybean breeder and Nelson are working to develop new varieties of soybeans that are root rot resistant. "Right now we’re in good shape, but root rot pathogens, over time, slowly evolve so we’re always trying to stay one step ahead," Nelson says. "Not every field is the same and some growers can go for years without problems, but we have seen a build-up of root rot pathogens."

Fungicides can protect seeds and seedlings against various diseases according to Nelson. Although fungicides can improve the soybean stand, they do not always guarantee a yield increase. Fungicides are also not effective against all species of fungi. Fungicide combinations are often used to control a complex of fugal pathogens.

Good seedbed preparation is also important in the control of soybean root rot.

Also of concern, Nelson says, is the spread of the soybean cyst nematode (SCN). That pest is currently the most important soybean disease in the country although no known cases of SCN have been discovered in North Dakota. "The nematode has spread throughout the soybean growing areas of the Midwest, southeast, eastern seaboard and into Canada," Nelson says. "It was first reported in Minnesota in 1978 and in South Dakota in 1995. While we haven’t had confirmed reports in North Dakota, its introduction into the state in the near future is likely."

It may take many years from initial infestation to reach damaging levels which may be a reason SCN has not been reported in the state according to Nelson. The damage caused by the soybean cyst nematode varies so it can be confused with other causes such as iron deficiency, soil compaction, chemical injury or root rots.

Growers moving equipment from South Dakota or Minnesota should power wash their machines before use in North Dakota. "Because we have sugarbeets and potatoes, we move a lot of soil. We think those activities could spread the organism very quickly throughout the area," Nelson says. "We also raise dry beans in the state which are susceptible to SCN and serve as another host."

There are some soybean varieties that are resistant to SCN. Nelson and Helms are also developing SCN resistant cultivars for future release by NDSU. The two scientists are working with the North Dakota Soybean Council which is funding some of their research.

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Source: Berlin Nelson, (701) 231-7057, berlin.nelson@ndsu.nodak.edu
Editor: Rich Mattern, (701) 231-6136, richard.mattern@ndsu.nodak.edu

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