Department of Crop Sciences
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The soybean cyst nematode (SCN) does more than reduce soybean yields by hijacking soybean physiology. SCN is also one of the partners involved in the development and spread of sudden death syndrome (SDS), and recent research suggests that SCN also is involved in brown stem rot (BSR) of soybean. That’s bad news for Illinois farmers, considering that SCN occurs in more than 80 percent of Illinois soybean fields.
SDS has rapidly emerged in the past few years as a major threat to soybean production in Illinois. SDS has few equals in its ability to rapidly and completely devastate a soybean field. The fungus that causes SDS (Fusarium solani f.sp. glycines) is fully capable of acting on its own, but research in the past decade has shown that SCN hastens the development of SDS symptoms and increases their severity, leading to greater yield loss. The fungus, on the other hand, can infect SCN eggs!
BSR, caused by the fungus Phialophora gregata, is an “old” threat to soybean production, but recent observations suggest that it’s becoming more widespread and severe than we’ve seen in the past. An interaction between the fungus and SCN that affects BSR development was not considered likely until very recently, when researchers at Iowa State University found that infection by SCN can actually break resistance to BSR. At the moment, no one knows what effect the fungus has on SCN.
Figure 1. Can you tell BSR from SDS?
In addition, BSR is moving farther south, just as SDS is moving farther north. Our old rules of thumb—that BSR is only a northern problem and SDS is only a southern problem—are showing signs of breaking down. (By the way, can you tell BSR and SDS apart? Which is which in Figure 1? Be wary of diagnosing these diseases unless you’re an expert!)
Collaborative efforts among many researchers and Extension Educators in Illinois may help address the problems posed by these soybean disease interactions. In the laboratory, greenhouse, and the field at a number of locations, we are studying the interactions between SCN and the fungi involved in SDS and BSR. Some of the questions we want to answer are:
What is the effect of SCN infection on the development of foliar symptoms of SDS and BSR? How and why does this interaction occur?
Does the interaction depend on the number of nematodes, genotype of the fungus, pedigree of the soybean cultivar, or other factors that haven’t been identified yet?
What effect do the fungi have on SCN?
How do disease interactions affect soybean management practices? If you have to make a choice, is it more important to choose a cultivar resistant to the nematode or the fungus?
Worst of all, is it possible for all three pathogens to interact in the same field?
So, what should you do if you have both SCN and SDS or BSR (or—heaven forbid!—all three) in the same field? Most people would recommend that you take care of the SCN problem first and grow an SCN-resistant variety. Why? Because SCN is always present reducing your yields, regardless of the environment, while SDS and BSR don’t develop every year. (Answer to the question about Figure 1: BSR is on the left, SDS on the right.)
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