Are soybean diseases getting worse?

In spite of all the year-to-year variations, some worrying trends are developing

Ask a dozen farmers, advisors or industry reps to identify the worst soybean disease this year and it’s possible you’ll get a dozen different replies. Much of it depends on location, but there are concerns that some diseases are increasing in severity, if not frequency or geographic area.

Some of the headline diseases of 15 to 20 years ago have faded. There aren’t many reports these days of powdery mildew or stem canker, although there’s still some soybean mosaic virus and Pythium damping off.

But other diseases, particularly soybean cyst nematode (SCN) and sudden death syndrome (SDS), are increasing in frequency and severity.

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The two have been linked and SCN is well-recognized as the primary yield-robber, not just in Ontario or Eastern Canada but across most of the U.S. Midwest. Sudden death is the more worrisome, though, given its collaborative spread alongside of SCN.

A rundown of diseases would also be incomplete without mentioning white mould that is now a perennial challenge for growers in Eastern Ontario.

“I find it hard to say we’re seeing a trend in some of these diseases but last year was definitely an outbreak of SDS and it was more widespread than I’ve ever seen it,” says Shawn Brenneman, agronomic sales representative with Syn­genta Canada. He attributed that to early planting and cool conditions, followed by a drier July in 2018. “To me, it was the environment that did it. But at the heart of it, because of more frequent soybean rotations and shorter overall rotations, the inoculum is always there.”

Although SCN and SDS are on the rise, diseases like Phytophthora and white mould (pictured) can still challenge growers.
photo: Supplied

Brenneman thought growers would be seeing more SDS in 2019 because of the inoculum that was left behind last year, plus the fact that more is known about the link between SDS and SCN. He believes that SCN is going to be the primary issue and will spread faster. It will continue to be an issue and as a result, more SDS is likely to show as well.

“I’d expect we’ll see more frequency of SDS in future years because of its link and spread with SCN, but it is more a case of having a one-in-five outbreak like 2018,” says Brenneman. “I can’t call that a trend because it’s usually more of an outbreak, the same with phomopsis in 2018. I saw more phomopsis seed mould last year than I had in a long time and it was because of all that moisture we had in August and September, finishing off that crop.”

It became particularly troublesome for growers who didn’t apply a fungicide.

Seed treatments and screening

As for the disappearance of some diseases, Brenneman credits the use of fungicidal seed treatments along with year-to-year screening of varieties done by the seed companies. That’s probably one of the lesser-acknowledged success stories in the seed trade, where companies are doing more to screen for susceptible varieties and purge them from their seed catalogues. That’s reduced the outbreak potential for diseases like SDS and SCN as well as Phytophthora root rot and white mould.

If there’s a wrinkle in that process it’s the pace of bringing new varieties to market. New trait platforms and breeding practices mean there can be less screen time for rarely seen diseases like pod-and-stem blight, phomopsis, cercospora or frogeye leaf spot. Those can show up simply because the pathogen is always present and will show with the right environmental conditions.

“The adoption of seed treatments has also helped with the emergence of soybeans, and if you get those seed diseases like phomopsis, you get a good-quality seed treatment on it and it’s fine,” says Brenneman. “You can take a lot of seed that’s maybe at 50 per cent germ and move it up to 90 per cent with a seed treatment.”

Don’t forget fungicides

The application of fungicides is another change in management practices that’s having a positive effect. Mervyn Erb, a certified crop advisor from Brucefield, north of London, concedes he hasn’t seen much in the way of disease in soybeans. Some of that he puts down to most of the customers he serves having standard three-crop rotations and some will add in edible beans as a fourth.

“I’ve never seen a lot and I’m not seeing more,” he says. “That being said, the most prevalent soybean disease that I see — and everyone has it everywhere — is septoria brown spot that shows up on the unifoliate leaf early on. It bounces up off the ground and usually, the unifoliate leaves die off because of it. But it’s not an issue or a problem.”

Some soybean diseases can be vectored by bean leaf beetle (left): Japanese beetles and stink bugs are also appearing earlier.
photo: Courtesy of Cara McCreary

Occasionally, in a wet year and with a variety that doesn’t have a lot of natural resistance, Erb has had to spray a fungicide to stop it because it can work its way up to one of the trifoliates. Other than that, he’s done some work in St. Thomas for a couple of large clients and in that region, they get a fair bit of cercospora, which damages leaves later in August during dry weather, causing mottling and discolouration on the soybean seed.

As for Phytophthora, SDS or Rhizo­ctonia, Erb sees very little of those.

“Part of that is that right now we have some of the greatest fungicide and seed treatments we’ve ever had,” he adds. “Vibrance-Max, Quatro, they’re all fantastic seed treatments and look after a lot of early seedling diseases and plant diseases, and those have really helped us out. Certainly we have tremendously good seed treatments on the fungicide side.”

Unfortunately he does find more SCN each year and just as Brenneman states, where there’s SCN, SDS is beginning to appear more often as well. Erb has also found that more growers are opting for fungicides on their soybeans. Some are livestock farms with good rich soils that can get a little mouldy at times. Those get a treatment with fungicides which can also help with seedling diseases like phomopsis, bean pod mottle virus and cercospora.

“Even if I had something happening, I likely wouldn’t see it because most of my growers are applying those fungicides,” says Erb. “Another thing is that there’ve been some really nice yield increases from a fungicide at that stage. The worst thing is you break even, but that management technique is looking after those leaf diseases.”

“It’s always there”

One of the key messages, regardless of the disease, is that once the inoculum has entered a field, it’s always there, whether it’s SDS or SCN or Phytophthora or white mould. Erb agrees that white mould has been an issue for many years, particularly for those of his customers who are dairy farmers. They spray a mould fungicide at R2.5, receiving a yield bump as well as protection from the mould.

“Mould’s always been a problem, it’s not going away and although I wouldn’t say it’s getting worse, it’s just not getting better,” says Erb, who says he’s also seeing more vectoring of diseases by insects. “That’s come with the demise of neonic seed treatments. We’re seeing more insects earlier on, like bean leaf beetle, Japanese beetles and stink bugs.”

Stink bugs, leafhoppers and thrips are being blamed for bringing bacterial brown spot into adzuki beans. And alfalfa mosaic virus is coming in with leafhoppers or thrips.

Brenneman also reminds growers of the importance of scouting fields. It’s not just a matter of looking for weeds, it’s also keeping an eye out for brown leaves or spots on the leaves. With most diseases, once it appears in the field, it’s too late because yield loss has likely already occurred. That’s the benefit of applying fungicides, and Brenneman cites many of his customers for their applications at R1 or R2.

“We’re not alone,” says Brenneman. “The environment’s going to change, we’re going to continue to have new diseases show up and see shifts in races or biotypes in the field and it’s just a matter of staying on top of that.”

This article was originally published in the October 2019 issue of the Soybean Guide.

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