By Emily Unglesbee
DTN Staff Reporter
ST. LOUIS (DTN) -- Growers who spot yellow patches in their green soybean fields this year should suspect sudden death syndrome (SDS). Plant pathologists are starting to see the disease symptoms crop up throughout the Soybean Belt, and damage may become more widespread in the next couple weeks.
Sudden death syndrome (SDS) is aptly named because plant death is swift and certain. Although farmers can do nothing to stop the development of this fungus-driven disease, now is the time to evaluate the resistance levels of varieties and plan for next year, plant pathologists told DTN.
"I think we're set up for a good SDS year," Iowa State plant pathologist Daren Mueller said. "We have everything in place: We have good moisture, and early on we had good infection rates, so the last hurdle would be having normal rain in the next month."
Mueller said growers in southeast and south-central Iowa are already seeing symptoms of the disease and more infected plants will become visible if the state receives rain in August.
SDS is caused by a soil-borne fungus that needs cool, wet soils to initially infect young plant roots. Soybeans planted before June are at a greater risk for the disease, because their slower-growing roots stay vulnerable to the pathogen for longer, said Mueller.
After a plant's roots are infected, the progression of the disease relies on good rainfall, which much of the Soybean Belt has seen this year.
"There was a study done comparing SDS and non-SDS years, and they found that a lot of rainfall in June was one of the driving factors, when plants are in vegetative and early reproductive stages," Mueller explained. "What [the rain] is doing is sort of flushing the toxin to the upper parts of the plant."
WHAT TO DO NOW
Because SDS is driven by weather and is untreatable after the initial infection, farmers can feel helpless and frustrated as they watch the disease develop in July and August. Still, these months are actually the best time to scout your fields and analyze the effectiveness of your management strategy, Mueller noted.
"Walk your fields and see which varieties are getting the disease," he urged. Evaluating the level of infection between different varieties can not only help you better plan your seed selection for next year, but it can also help seed companies adjust their labeling, Mueller noted.
"As much as seed companies want to release resistant varieties, SDS is a real [challenge] to screen for because it's so inconsistent," he explained. "Anytime you can gather information on your variety and get it back to the companies, that's valuable and that helps them correct their resistance ratings."
When scouting, look for the distinctive yellow, then brown coloring between the veins of soybean leaves. In severe cases, the leaves will curl up and drop off.
"It is a patchy disease that actually favors the windshield scouters, because it does occur first on the edges of fields, where there's more traffic and compaction" Mueller noted.
Because SDS symptoms can be mistaken for other diseases, University of Wisconsin-Madison Extension field crops pathologist Damon Smith put together a video to help growers scout and diagnose the disease.
Smith recommends pulling up plants that display SDS symptoms and examining their roots for rotting and poor nodulation. Splitting the stem down the middle and looking for a white-colored "pith," or core of the plant, can also help confirm SDS. See Smith's video here: http://goo.gl/….
Any soybean growers with good moisture right now should scout, even if they don't have a history of the disease in their fields, Mueller warned. "Anyone that gets rainfall from here on out, they should be out scouting," he said. "SDS is a soil-borne pathogen and it doesn't move fast ... but it can transfer pretty easily on a tractor tire or muddy shoes. Even if you didn't have it in the past, I would still be on the lookout for it."
Don't panic if you find spots of SDS -- the patchy nature of the disease can mean the total yield loss across a field may be smaller than expected. "On average, in a patch that looks pretty severe, you're probably looking at 10% to 25% yield loss in that patch," Mueller estimated. "Even in 2010, across Iowa, when we had a fairly significant epidemic of SDS, it was estimated to be only a 3% yield loss across the state."
Picking resistant varieties is the biggest factor in avoiding SDS outbreaks, but other management strategies such as improving drainage, breaking up compaction and a more diverse crop rotation can help limit the disease's damage.
However, Mueller warned growers against automatically pulling the plow out of the shed next year if they have a bad outbreak. "The answer on tillage isn't very clear," he said. "If there is an actual compaction zone in your fields, which forces roots to stay where fungus is, then it might help. But if you're just tilling to respond and feel better, there are reports that show long-term no-till is actually the best thing for (SDS). It can help break down the fungus quicker, among all the other benefits of no-till."
Rotating soybean fields back into corn is also of little help, because the fungus can survive and even increase on corn roots, Mueller added. Subbing in even just one year of an alternative crop like clover, alfalfa, or rye can significantly reduce dense build-ups of the fungus, he said.
For more information on SDS and management practices to discourage it, see this Purdue guide: http://goo.gl/… and this Iowa Soybean Association factsheet: http://goo.gl/….
Emily Unglesbee can be reached at firstname.lastname@example.org
Follow Emily Unglesbee on Twitter @Emily_Unglesbee.
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