Sudden death syndrome threat in soybeans is climbing

SDS is attracting more attention as a companion of soybean cyst nematodes

Interveinal colouring and chlorotic tissue are clear indications of sudden death syndrome (SDS) in soybeans.

In the list of pests and diseases affecting soybean production in Eastern Canada, there are widespread culprits — like soybean cyst nematode (SCN) — and regionalized, almost annual challenges, such as white mould in Eastern Ontario. Nor to be forgotten are diseases like Phytophthora and Rhizoctonia root rots, which have been less frequent in the past 10 years. 

Sudden death syndrome (SDS) is by no means a recent arrival. It was first confirmed in Arkansas in 1971 and has since spread across most of the soybean-growing regions of the U.S. and Canada.

While in some years SDS can be third behind Phytophthora root rot, extension personnel and certified crop advisors (CCAs) say sudden death now ranks behind only SCN as the greatest overall pest threat to soybean yields.

A story in a U.S. publication last September questioned whether SDS was more prevalent in 2020 compared to 2019. Given the wet start to the 2019 planting season, some might be surprised to hear that 2020 was, in fact, worse for SDS in many parts of the U.S. Midwest. Yet it was that wet start to 2019 that actually helped growers, not just in the U.S. but also in Ontario. That’s because growers had to wait until later in May, and sometimes into June, to get their soybeans in the ground.

The recent trend in soybean planting is to go as early as possible, with some advisors suggesting it’s even preferable to interrupt corn planting if the right conditions open the door to soybeans. More and more, we’re hearing “you can always go back and finish corn after.”

Unfortunately, the move to push maturities in soybeans in an effort to maximize yields is lending itself to more disease outbreaks. And an emerging link between SDS and SCN is creating more headaches for growers at a time when SCN resistance is breaking down and SDS is spreading. 

“We’re starting to see ideal conditions for sudden death syndrome development,” says Albert Tenuta, field crops pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). Environmental conditions often start out cool and wet then turn cooler and wetter later in the summer. “Then you tie that in with soybean cyst nematode populations and SDS has become a disease that’s well-adapted to our conditions.”

Although it’s becoming more prevalent, Tenuta maintains awareness is also on the rise. Growers should be diligent about sudden death syndrome, particularly in those parts of southern Ontario where they’ve seen SCN the longest and where populations are a problem. 

The SDS basics

The cause of SDS is Fusarium virguliforme, a soil-borne pathogen that infects soybeans within the first two or three weeks, colonizing roots and moving into the developing seedling and then into the plant. Cool, wet conditions, which often come with the trend towards earlier planting, promote the SDS infection.

Above-ground symptoms are more familiar with SDS but below-ground stem-browning is another indicator. photo: Courtesy Albert Tenuta, OMAFRA

In 2020, the weather turned hot and dry and there were early SDS symptoms in some fields, yet there were other root rots and diseases at play, as well, notes Tenuta. But what drives those foliar symptoms — not just root rots — are those late-season rains, and in 2020, it was the rain events in July that helped the corn crop while promoting above-ground foliar SDS symptoms in soybeans. 

“As the fungus colonizes the root system, it also produces a toxin, which is mobile in the plant and as the plant takes up that necessary moisture and nutrients, it’s also moving up the toxin,” explains Tenuta. “That’s where you see those individual yellow or chlorotic spots which become larger or brown in colour and then the tissue between the veins starts to disintegrate.”

In tandem

Why SDS becomes a bigger problem is because of its tag-team approach with SCN. If SCN is present in the soil, the nematodes will create the entry point in the roots, allowing the SDS pathogen to move in. Most growers, advisors and agronomists are familiar with the interveinal colouring and chlorosis that takes place later in the season, but Tenuta states this early stage is important to SDS’s later above-ground visual symptoms. 

“What really promotes that infection is an environment of cool, wet conditions,” Tenuta adds. “One thing that’s helped SDS or made it more problematic is that by planting earlier, at some point, you’re going to have those cool, wet conditions in Ontario and the pathogen needs those and it just sits in the root system after infection.”

Although the interveinal colouring is a clear and visual symptom, some growers seeing it for the first time may mistake it for something else, particularly a phosphorus deficiency or a disease like brown stem rot. But as Jillian Ball notes, the root system remains largely untouched with those conditions. With SDS, the roots will undergo as much discolouration as the above-ground appearances, so growers or their advisors should be slicing open plant stems to check for those symptoms.

“Producers who have never seen or haven’t had a problem with SDS before may mistake it for something that it’s not,” says Ball, customer service manager with Haggerty Creek Ltd., near Bothwell, Ont. “Usually, once a farmer sees it, they won’t mistake it again.”

Ball believes the identification process also speaks to the value of the relationship between a grower, their CCA, retailer or local co-op representative. Even if the grower hasn’t encountered SDS on their farm, it’s likely their advisor has.

“The other thing to be careful of is using triazole fungicides,” says Ball. “If they’re sprayed at temperatures above 35 C, the damage can look very similar to the SDS leaf symptoms.”

Ball echoes Tenuta’s assessment about SDS being more prevalent in regions where SCN is a longer-term issue and also where he states it’s definitely spreading. Although she covers an area that’s commonly a sandier soil base bounded by Chatham-Kent, Lambton, Middlesex and Elgin counties, SDS isn’t staying in one spot. 

“We see it more prevalent on those soils and some of our sales representatives have more growers who are on sand,” says Ball. “We didn’t think it was as much of an issue as in 2019 but we’re seeing it in more places now — it’s not just strictly staying in the sands.”

She’s also seen the trend towards earlier planting in soybeans and a more recent move to shorter corn-soybean rotations. In the past, it was thought that corn provided a sufficient break from SDS or SCN. Now that just isn’t the case.

“We always thought that corn was a non-host crop but we’re noticing that it’s starting to survive on corn trash, so that’s a little worrisome in a corn-soybean rotation,” says Ball. “There won’t be anything to keep your inoculum numbers down but if you throw in a cereal of some type in there, SDS and SCN can’t sustain off cereal trash, so it’ll keep your numbers at bay.”

Heading east

Just as SCN did in the 2000s, SDS appears to be heading east in Ontario, and Adam Pfeffer agrees the incidence of SDS tended to be higher in 2020 due to an earlier planting season, with cool, wet conditions that are more conducive to infection. Pfeffer had a field of his own with a sand-loam soil that had fairly heavy SDS pressure, which was a bit of a surprise, but coincided with a little drought stress and a manganese deficiency.

“The disease trend and the spread are slowly progressing eastward,” says Pfeffer, agronomic systems manager with Bayer. “It’s certainly tied to SCN and that’s not going away anytime soon, but I would agree that in the last 10 years, SDS has certainly expanded its range. Whether that’s environmentally driven or tied to SCN’s presence and pressure increasing, I don’t know. I can’t see it being more sensitive genetics because every company’s trying to screen and breed for varieties that are more tolerant to SDS.”

Adding cereals to the rotation provides a break in the disease cycle — for both SDS and SCN. photo: Supplied

In addressing the identification issue, Pfeffer agrees that to those who’ve never seen it, it may be a challenge. Yet he likens SDS to finding waterhemp in a field and misdiagnosing it as redroot pigweed for a couple of years until it can’t be ignored. 

“Growers first experience it in a new area, where SDS might catch them off-guard, and if it’s just a small patch here and there, they might buzz through it with the combine,” he adds. “It shows up late-season and by the time you get to harvest, you don’t necessarily see the same visual symptomology.”

Again, that speaks to the value of a good relationship between a grower and their CCA or agronomist: they’ll catch it fairly quickly.

In terms of improving management of SDS, Pfeffer adds his voice to the chorus that recommends longer rotations, noting that prices for winter wheat in the past two years have provided more of an incentive for growers. But shortening rotations falls into the same category as continuous corn or relying on glyphosate only to control weeds: at some point, it’s going to catch up to growers.

“It’s always Mother Nature’s job to adapt and survive and if we don’t keep throwing diverse rotations and everything we can at it, we’ll run into problems regardless of what it is,” he says. 

It’s similar to eastern Ontario growers who are dealing with white mould on an almost annual basis. They’re dropping their populations and planting to wider rows. The same applies to growers in southern Ontario with SDS and the use of seed treatments and delaying planting until the soils warm.

Help or hindrance

The presence of SCN can worsen the effects of SDS, but the other way of looking at it is that the right selection of SCN-resistant varieties can help fend off the advance of nematodes and lessen the impact of the SDS pathogen. Use of an effective fungicidal seed treatment (ILeVo or Saltro) also helps, as does reducing stress during the growing season, ensuring fertility and effective weed management provide conditions for optimum plant health.

Tenuta’s research trials, sponsored by the Grain Farmers of Ontario in conjunction with the Crop Protection Network, looked at different planting, management and agronomic practices to help manage SDS. By delaying planting and waiting for drier, warmer soils, it was found to be a method of bypassing infection.

“The research shows it can be done. The problem is, I don’t know many growers who will wait until the middle of June or later to plant their soybeans,” says Tenuta. “The benefits from planting late can be achieved by using all the SDS management tools available, thereby reducing the potential losses you’re going to get from SDS. If you can keep the plant healthy for even an extra two to three weeks, it’s better able to maximize its efforts. Seed treatments will help with the delay but they won’t eliminate the infections.”

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CG Production Editor

Ralph Pearce

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