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Pamela Smith 6/19 4:50 AM

When it comes to soybean adversaries, there is no threat bigger than soybean cyst nematode (SCN). The cheeky interloper hitchhikes into fields and steals yield -- all while unscrupulously multiplying and without giving an aboveground sign of its freeloading ways.

Those of us who follow agronomy topics write about SCN regularly. I can safely say that I have devoted more words to this pest than any other. Still, it remains stubbornly persistent and underestimated as a pest.

Last fall DTN decided to double down on our coverage. We asked two farmers to soil sample a field for SCN and share their results as part of a project sponsored by The SCN Coalition, a public/private/checkoff partnership formed to encourage growers to actively manage this pest, which is estimated to cost farmers $1.5 billion annually.

For the last few days, you've been reading about all about that and more about SCN as part of a series called: Stomp on SCN Yield Losses. The last of the series runs on June 20 on DTN; at the bottom of this blog you can link to the other stories we've already run. You can also see the story package in your Summer issue of Progressive Farmer magazine.

There's a lot to know about SCN, so I asked University of Illinois nematologist Nathan Schroeder to answer some basic questions about the pest. The following is our Q&A.

**

Q: Soybean cyst nematode was first discovered in the United States in 1954 and it's been stealing soybean yields for decades. Yet, farmers still seem reluctant to test for it. Why?

A: I think it is a host of reasons. It's not fun to do soil sampling. It's extra work and time. There are now several states that subsidize the cost of testing, but getting it done is a chore that sometimes just doesn't get done.

Also, SCN doesn't cause obvious above-ground symptoms. So, you may have a problem and not realize it -- so some may think why spend the effort and energy in testing?

Resistant varieties have for years done a good job. Farmers have relied on that and may still think they are getting good utility because the label says the variety is protected.

Q: Is there some "I just really don't want to know" psychology involved?

A: I've heard when SCN wasn't so widely distributed that there was a stigma attached to having it in your field. Today, it is so widespread that I don't believe that to be true.

Perhaps a farmer tested years ago and didn't have much of a problem then and hasn't investigated it since. Mostly, I think many growers think they are successfully managing it and just don't see obvious above-ground symptoms.

Or, they may be having decent yields and figure they aren't really losing that much. Here in central Illinois, for example, the soil is so good and can be so productive. So maybe you get 80-plus bushels per acre (bpa) and know that's 20 bpa above the Illinois average. So, they figure they are doing good and aren't thinking about what they are losing.

Q: How does SCN move?

A: Basically, anything that can move soil can move the eggs. Implements, shoes, water. There is some research that implies migratory birds can move it.

As a grad student, I tracked a bunch of mud into the lab one day on my shoes. I swept it up, did an extraction and found a bunch of plant parasitic nematodes in there.

Q: Can you prevent SCN spread?

A: In theory, yes. But the problem is now so universally geographically distributed in our region that it isn't always practical today.

In states and areas where SCN is not as prevalent, there is still concern about moving it into fields where it doesn't exist.

In regions where SCN is more established, the concern is the possibility that you could be moving a population that is more highly adapted to PI 88788 into your field.

Equipment moving between fields, used equipment and custom applications are all potential transport mechanisms for the pest. Prevention would involve washing equipment between fields.

Q: Is there a way to get a clue whether you have a problem without soil testing?

A: Head to the field in July or early August and do some digging. Don't pull the soybean plants but dig them up and carefully move the soil away. You should be able to see the females in various developmental stages. They are small little dots like grains of salt or sugar, so a handheld magnifying glass might be helpful.

If you see the females feeding on roots, that's a good indication that you need to do further testing. As the season progresses, and the females die, they transition into a yellowish color and then rusty brown -- which is almost impossible to see, and they begin to fall off the roots.

A: Is it evenly distributed in fields?

Q: No. That's another problem. It can have very patchy distribution within a field. The entrances to fields or areas that flood might test very differently.

Q: How does the female SCN find the soybean?

A: That's something we don't know, precisely. We know that there are probably some chemical cues coming off the roots. Plant leaves and roots are secreting stuff all the time. Nematodes are animals. They've got a nervous system and can sense. We just don't know what those cues are or exactly how they are sensing. We know they can find roots even though they don't have eyes, but it's dark down there anyway!

Q: I've never thought of them as animals.

A: They are more closely related to insects than any other pathogen.

Q: Can you eliminate SCN from the field once it is there?

A: No. The only way to do that would be to never plant soybeans again and maintain a weed-free environment. Even then, there are some weeds that can serve as alternative hosts.

Q: Are all nematodes bad?

A: No. Most nematodes are what we call free living. That means they hang out in the soil and feed on bacteria and fungi. They probably play key roles in nutrient cycling -- so we certainly like those. There are insecticidal, or "entomopathogenic," nematodes that live in the soil and seek out particular soil-dwelling insect species, like corn rootworm. When the nematodes find a rootworm larva, they enter its body and release bacteria that kill the rootworm. Then the nematodes feed on the rootworm's carcass, getting the nutrition needed to produce the next generation of nematodes.

SCN gets much of the press in soybeans, but there are other plant-parasitic nematode species that dine on soybeans. There are also many other nematodes that are very problematic in crops such as corn, cotton and potato.

Q: Why is SCN so much worse than other nematodes in soybeans?

A: They sort of trick the plant into feeding them. They get into the root and set up feeding sites and the metabolites and sugars go to the nematode instead of to the plant being produced.

Lesion nematodes, by contrast, can be very damaging, but tend to feed more like grazers. They feed on individual root cells and those cells die. If you get enough of them, you've got a problem. With SCN, you've got this added component where they're kind of like freeloaders.

Q: So, they are the obnoxious, bad actor relative that eats all the food and never goes home?

A: Exactly.

Q: What are some of the other hosts for SCN?

A: They like soybeans best. But they will feed on weeds such as henbit and purple deadnettle.

Pennycress is another and we are studying it because it is being suggested as an alternative cover crop. Like insects, nematodes are very responsive to temperatures. They are not very active in soil temperatures below 15 degrees (Celsius; 59 degrees Fahrenheit). So, there may be ways to terminate it so they don't increase population or perhaps use it as a trap crop. There's more work that needs to be done. Crimson clover is another that has shown that it wasn't a great host, but nematodes could reproduce on it.

Q: What question do you get most from farmers?

A: What I always hear from my farming relatives is: When are you going to solve the problem? The reality is, SCN is a really hard problem to solve.

The other question farmers want to know is: When more sources of resistance will be coming out? Again, it takes a really long time to breed new materials. Fortunately, we are seeing more Peking varieties all the time and there is progress being made. But it doesn't seem fast when you need something new.

Nearly 100% of the commercial soybean varieties contain varietal resistance to SCN. Probably 90% of those contain PI88788. The good news is soybean breeders seem to have done a good job getting rid of the yield lag that was associated with alternative sources of resistance. Or maybe we're seeing those alternative sources yield better because the nematode is increasingly adapted to PI88788. Even so, it is still better than planting a susceptible variety.

Q: What is the most fascinating thing about nematodes that people don't know?

A: That's hard to pick because they are so interesting. But in the stage called second stage juvenile, when they set up their feeding site on the root, the nematode produces roughly 50% males and 50% females. Both stages start feeding and stop moving. They feed from one location. What we've found is their body wall muscles degenerate while this is happening. They couldn't move even if they wanted to. The males then have to move in order to reach the females. And so, in the last stage of becoming adults, the males actually regrow their muscles to be able to move and find females.

Q: What's the coolest fact about nematodes?

A: The largest nematode ever found was 25 ft. long and found in the placenta of a sperm whale.

**

To see more in the Stomp on SCN Yield Losses series:

-- Stomp on SCN Yield Losses - 1, "Take an HG Type Test to Avoid Varieties Vulnerable to Soybean Cyst Nematode," at https://www.dtnpf.com/…

-- Stomp on SCN Yield Losses - 2, "Consider These 5 Steps When SCN Threatens to Reduce Soybean Yields," https://www.dtnpf.com/…

-- Production Blog, "Start Digging for Answers on Nematodes," at https://www.dtnpf.com/…

Pamela Smith can be reached at pamela.smith@dtn.com

Follow her on social platform X @PamSmithDTN

 
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