Upper Midwest soybean farmers grow one of the world's most important crops. But the ag producers must deal with new-to-them soybean diseases, some with colorful names, that are pushing northward.

"These diseases will continue to challenge the goal of attaining high and consistent soybean yields. So the challenge is trying to figure out which of these new diseases we have to worry about, which ones we have to think about, which ones we have to manage," said Dean Malvick, a plant pathology extension specialist with the University of Minnesota.

Malvick spoke during the recent Prairie Grains farm show. The annual event, held online this year and open to the news media, focused in part on issues facing northwest Minnesota and northeast North Dakota — an area where several soybean diseases have popped up or are expected to soon.

The incoming diseases include sudden death syndrome, brown stem rot and frogeye leaf spot, as well as new developments with white mold, a longstanding crop disease. Here's a closer look at each of them.

Sudden death syndrome

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Sudden death syndrome is essentially a root disease, a fungus that attacks the roots of soybean plants and produces a toxin in the root that moves up into the leaves.

SDS, as it's commonly called, now is found in most of southern Minnesota and clearly is moving north. Once the pathogen is found in a field, "it doesn't go away readily" (and) "probably is there for a long time," Malvick said.

The crop disease should be managed by scouting fields to determine where the risk is greatest, he said, noting that resistant varieties are available and that there are specific seed treatments for SDS.

A complicating factor is that leaves on soybean plants stricken with SDS can look very much like leaves on plants hit with brown stem rot.

Brown stem rot

The risk of brown stem rot increases the more often soybeans are planted in a field and the more common beans are in a particular area, Malvick said.

Brown stem rot is fostered by cool temperatures during flowering and pod fill, and also can be boosted by the presence of soybean cyst nematode, a microscopic roundworm that can devastate soybean fields.

BSR, as it's commonly called, can cause yield losses of more than 30%, though losses in the 10-20% range are more common. Symptoms often aren't visible on leaves, and plant stems must be split to diagnose the disease, Malvick said.

The disease can be managed with crop rotation and resistant varieties.

Frogeye leaf spot

Once rate in Minnesota and North Dakota, frogeye leaf spot has become more common in the area over the past three years. Symptom include brown/tan spots surrounded by a brown, purplish ring on leaves. The spots can grow together and kill large areas of leaves, Malvick said.

Frogeye leaf spot does best in warm, humid weather, and can be managed with tillage, crop rotation and fungicides.

White mold

White mold is a relatively common disease that attacks a number of crops. Among new developments that soybean farmers should be aware of:

There's an insect, the gall midge, that feeds on white mold — and indicates presence of white mold — but isn't known to hurt soybeans directly, Malvick said. Also, some new herbicide-resistant soybean varieties have low resistance to white mold.

Soybean farmers should consider applying fungicide when rows are filling and risk of white mold is high. Though yield loss can occur even when fungicide is used, 2020 research can help to reduce the chemical's efficiency. Learn more at https://blog-nwcrops.extension.umn.edu/2020/11/a-new-way-of-managing-white-mold-in.html.