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Soybean sudden death syndrome

 Soybean leaves affected by sudden death syndrome tend to curl inward.     Photo by Albert Tenuta, OMAFRA.

December 1, 2014 - Plant pathologist Albert Tenuta with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) is collaborating with researchers in the United States to fine-tune management options for soybean sudden death syndrome (SDS).

“[The disease] is definitely on the rise and has considerable economic implications. So growers need to watch for it and address it,” says Tenuta.

SDS is caused by Fusarium virguliforme. This fungal pathogen overwinters on crop residue and in the soil as survival structures called chlamydospores. In the spring, the chlamydospores germinate and infect soybean roots.

“The pathogen colonizes the roots early in the season, but we don’t usually see foliar symptoms until later on when it warms up, either when the plants are under stress or as they get into the reproductive stages in mid to late July and August,” explains Tenuta.

The pathogen causes root rot but it also produces a toxin. When that toxin moves up into the leaves, it causes the disease’s characteristic foliar symptoms. “The leaves will show an interveinal chlorosis, yellowing between the veins, while the veins will stay green. The affected tissue will turn brown and then disintegrate, leaving just the veins. In severe cases, the leaves will fall off, but the petioles – the stalks that attach the leaf to the stem – will remain attached to the stem.”  

The disease is a growing problem in Ontario and the north central U.S. In Ontario, SDS symptoms were first noticed 20 years ago in the Chatham area. These days, the disease continues to spread, primarily in southwestern Ontario. Tenuta notes, “Both the incidence and severity continue to increase on an annual basis, and the disease continues to expand into new areas and new fields each year.”

According to Tenuta, one of the key factors in the spread of SDS is the presence of soybean cyst nematode (SCN). “Soybean cyst nematode damage to soybean roots can aid the SDS pathogen in its ability to get into the plant. As well, our studies and others’ have shown that the SDS pathogen moves around in the cyst of soybean cyst nematode. So as soybean cyst nematode moves into new areas, then closely after that sudden death syndrome is found.”

Weather is a crucial factor in the impact of SDS on soybean yields. “Cool, wet conditions at planting are ideal for sudden death syndrome infection. We have those conditions in most years in Ontario and we had them in 2014. Also, as growers are planting earlier and earlier, we’re seeing a greater likelihood of cool, wet conditions at or shortly after planting. The pathogen also likes wetter conditions throughout the growing season,” says Tenuta.

Like Ontario, Iowa had high levels of SDS in 2014, due to early wet weather followed by rainy weather later in the season. “When you get years like this with a lot of rain in August and September it can definitely finish off your crop early,” says Dr. Daren Mueller, an assistant professor and plant pathologist at Iowa State University.

SDS yield losses also depend on the soybean variety. Tenuta notes, “Overall in most cases, it’s not unexpected to see a 10 to 20 per cent yield reduction in parts of fields with moderate levels of SDS infection. The greatest yield losses we see in Ontario are on varieties that are susceptible to soybean cyst nematode and have little tolerance to sudden death syndrome. In those fields, you can get losses of 40 to 50 per cent or more. In some parts of those fields, you can have up to 100 per cent loss, particularly if the disease starts early.”

Advancing SDS management
Mueller is leading a collaborative project with Tenuta and university researchers in several states in the north central U.S. to develop improved strategies for SDS management. Now in its second year, this three-year project is actually the third in a series of three-year SDS projects. 

Mueller explains, “The first six years involved a lot of neat work on the pathogen’s biology – how does it infect, what does it infect, how does it survive in a corn-soybean rotation, and so on. That work has allowed us to better understand the disease at a level where we can really make forward progress. So we’ve taken all of that information and put together some experiments to fine-tune management options for farmers.”

This series of SDS projects is part of the multi-state North Central Soybean Research Program and is funded by the Soybean Checkoff. The Ontario portion of the current project is funded by Grain Farmers of Ontario (GFO) through Growing Forward 2 funding.

The researchers are evaluating a variety of practices for managing SDS. One of those is planting date. “Until now, all of the literature showing that planting ‘early’ increases the SDS risk has been based on comparing mid-May to mid-June planting dates. But farmers are planting into late April now, so we want to see how these earlier dates affect the SDS risk,” says Mueller. The researchers are also hoping to work with a climatologist to develop some risk assessments for earlier planting as the climate becomes more uncertain.

They are also comparing a wide range of fungicide treatments. Mueller notes. “There are a lot of new products – foliar fungicides, in-furrow fungicides and seed treatments – and we’re assessing them to see if any of those affect SDS.”

As well, they are examining SDS levels in relation to SCN-resistant soybeans. “Soybean cyst nematode is the number one soybean disease. Over the last several years there has been increased reproduction of soybean cyst nematode on the most popular source of SCN resistance, although it is still considered the best source of resistance. We want to see if that increased reproduction will affect SDS, since one of the ways you manage SDS is by controlling soybean cyst nematode,” explains Mueller.

The project also involves some side studies, including a recently completed study about herbicides and SDS. In addition, Mueller is conducting some Iowa-specific research with his colleague Dr. Leonor Leandro to assess the effects of practices like tillage and cover crops on SDS.

Findings so far
The researchers are already gleaning valuable information from the project. For example, only one of the many fungicide treatments looks promising so far. “It’s a new seed treatment from Bayer CropScience; it seems to be holding up well in different environments,” says Mueller.

Tenuta adds, “This seed treatment doesn’t eliminate SDS, but it delays development of the disease until later crop growth stages and decreases the amount of SDS. This product is available in the U.S. and hopefully it will be available in Ontario soon.”

The researchers also tried using this seed treatment’s active ingredient for in-furrow and foliar applications, although these uses aren’t on the label. The in-furrow application worked very well, but the foliar applications did not work.

They are also finding that slightly delaying the planting date helps in some cases. “Our research here as well as in the U.S. shows that the earlier we plant, the more favourable the conditions are for infection and higher levels of SDS,” says Tenuta.

But he doesn’t recommend really delayed planting. “Although we can avoid or minimize sudden death syndrome by planting into warm conditions in early to mid-June, the problem with that is the loss of yield potential, so waiting that late to plant is not a viable option.”

As well, Mueller notes, “Very early planting did increase the SDS risk in some fields, but this year we had fields with just as much SDS in the latest planting as in the early planting because we had so much rain in late August. So the driving factor is moisture, as long as the inoculum is there.”

One of the project’s side studies examined whether glyphosate affects the occurrence of SDS. “Some people have said that sudden death syndrome increases in the presence of glyphosate use. So we conducted a three-year study at multiple locations in six states and provinces, comparing various applications of glyphosate and other herbicides to assess the effects on sudden death syndrome levels,” explains Tenuta. “The results, which were recently published, found glyphosate did not increase SDS infection and hopefully will end the discussion.”

Mueller adds, “Where we had no SDS, none of various Roundup rates and timings caused SDS to appear. And where we had any levels of SDS, none of those glyphosate regimes caused SDS to be increased.”
This fall and winter the researchers will complete their analysis of the 2014 data. Mueller says, “We’re hoping to be able to finalize some of the recommendations this year, and then start tweaking treatments and maybe combining practices together for 2015.”

Tips for managing SDS
Based on their findings so far and previous studies, Mueller and Tenuta offer several tips for managing soybean sudden death syndrome.

“The number one thing is to make sure you actually have SDS. We had a lot of fields this year that had brown stem rot or stem canker, and they were both misidentified as SDS,” says Mueller.

“Stem canker is caused by a whole complex of fungi called Diaporthe and Phomopsis, which cause stem canker, pod and stem blight, and Phomopsis seed decay. I think that whole complex was confused with SDS in some fields this year. So look at the outside of the stem to see if there is a canker, which is where stem canker gets its name. Also, the initial foliar symptoms for stem canker are more of a general yellowing, rather than the yellowing between the veins that you get with SDS.”

Tenuta notes, “The interveinal chlorosis symptoms of SDS are very similar to the foliar symptoms of brown stem rot. The main way to distinguish between the two diseases is to split the stems. If the pith, the centre portion of the stem, is brown, then that is brown stem rot. If the pith is white, but there’s browning or discoloration of the tissue just below the stem surface, and there’s also root rot, then that is often SDS.”

If you diagnose the problem as SDS, then also look at the roots for soybean cyst nematode symptoms or test for the nematode.

Although there aren’t any in-crop treatments for SDS, you’ll need the SDS diagnosis for crop planning. “As you evaluate your fields and yields and plan for next year, note any disease or insect pest problems in each field and target varieties or hybrids that have resistance or tolerance to those problems,” says Tenuta. “If the field has SDS, look for soybean varieties with good tolerance to both SDS and soybean cyst nematode. And be on the lookout for new SDS seed treatments. Our results show that the SDS seed treatment in conjunction with a soybean cyst nematode-resistant variety and even an SDS-tolerant or moderately susceptible variety is a benefit.”

On fields with a history of SDS, Tenuta also suggests considering planting those fields last and using other practices such as tile drainage and tillage to promote warmer, drier conditions early in the season.