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For a county to be colored red, at least one field has to be identified with populations over 200 eggs per cup soil.

SCN overcoming resistant soybeans

The soybean cyst nematode (SCN) continues to erode yields in Ohio, often largely unnoticed. Approximately every three years, a map is updated where SCN is found around the country using records from diagnostic clinics and data from field studies.

As researchers expected, SCN has expanded in Ohio — from the western to the eastern borders of the state. In addition, researchers are identifying more fields in the state with populations above the economic thresholds. SCN populations of 1,500 eggs per cup of soil lead to 25% to 50% yield loss without any above ground symptoms.

“We’re identifying fields where the counts are getting way out of line, up in the 18,000 to 25,000 eggs per cup of soils. The cautionary tale is that folks really need to keep track of the SCN populations and sample,” said Anne Dorrance, Ohio State University Extension plant pathologist. “SCN is an obnoxious little thing. You can’t even see the difference in the field and you are losing 15% to 20% of your yield. People think 40 or 45 bushels is not bad, but the yield potential for these varieties is 70 or 80 bushels. With SCN, you can get 10 extra bushels with just some simple changes in variety selection. You really are not going to know until you sample. Just sample the lowest yielding spots on your yield maps — a quart of soil from the same depth as a fertility sample.”

Along with spreading to the new fields, SCN is also finding ways to overcome resistant soybean varieties.

“We are also beginning to identify populations that can still reproduce on resistant soybeans,” Dorrance said. “That is another troubling sign.”

Researchers in the Ohio State University Department of Plant Pathology, including Chris Taylor, are working on assessing the magnitude of this challenge with the goal of developing management strategies.

“One of the things that has not been done in Ohio for a decade or two is evaluating the populations of SCN in Ohio. We don’t know their HG types, which used to be called ‘races.’ HG types are important because they tell us what type of resistant soybeans we should be growing,” Taylor said. “In screening populations through 27 counties in Ohio from the C. Wayne Ellett Plant and Pest Diagnostic Clinic or field samples, we get those populations, grow them and test them to see if they can grow on resistant soybean lines. I was shocked that we have not found a population we would classify as truly susceptible to resistant soybean varieties. Every population of SCN we have growing in the greenhouse right now can grow on at least one of the SCN resistant soybean varieties out there.”

There are 20 populations currently growing for testing with more on the way.

“We get a nice representation of samples from around the state and we are finding some differences in different areas,” he said. “The SCN we are dealing with are coming from the west and south and are slowly marching across the state. They are coming from areas where there were already resistant lines of soybeans being grown. They already have some of the genes that allow them to overcome resistant soybeans. Because of that, we don’t really stand a chance to as effectively utilize resistant soybean varieties. There are some differences we are finding with SCN’s ability to populate. SCN may be losing some ability to reproduce as quickly on resistant soybeans, but they can still reproduce. We need to learn more about this.”

At this stage of the soybean checkoff funded research, Taylor is cataloguing the populations to potentially identify genes that might be involved in overcoming the resistance of current soybean varieties.

“We are getting to the point where we can recommend a type of resistant soybean variety for the SCN populations in a field,” Taylor said. “There are four types of resistant soybeans available. So far, we have found SCN populations that can grow on three of the four resistant lines.”

Moving forward, the key will be cataloguing more SCN populations to get a better handle on how they are distributed around the state.

“By getting a better idea of what the HG types are according to area, we can make broader recommendations about the soybean varieties that should be used,” Taylor said. “I am excited that we are able to do these surveys. I suspect this will be a growing issue here in Ohio and we have to come up with new strategies in Ohio for nematode control — bio-control, crop rotation or the development of novel resistances in the soybean plant.”




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