By Emily Unglesbee
DTN Staff Reporter
ST. LOUIS (DTN) -- Rain gets the blame for a lot of crop injury this year, but don't assume those yellow patches in your soybean field are all water damage. Those same symptoms could be evidence of sudden death syndrome (SDS) this summer, plant pathologists told DTN.
Patches of the disease are starting to surface in central and southeastern Iowa soybean fields, and conditions this spring and summer have been conducive for an SDS outbreak year, Iowa State University plant pathologist Daren Mueller reported in the university's Integrated Crop Management newsletter.
Much of the Midwest and South saw plentiful rainfall this past spring alongside moderate planting temperatures. Add plenty of disease inoculum lurking in fields after a significant outbreak of the disease in 2014, and many growers may be facing a perfect recipe for the disease, said Southern Illinois University plant pathologist Jason Bond.
"All the components for having a major SDS year are in place -- the only thing that could slow down SDS is a very dry July and August," he told DTN.
No in-season treatment can save a field with SDS, but now is the time to carefully map out which fields and varieties are hardest hit by the disease and plan ahead for the 2016 planting season, Bond said.
IN THE FIELD
SDS is most easily identified by a phenomenon known as interveinal chlorosis -- when leaves turn yellow but the veins remain green. Leaves will eventually turn brown and the plant can die abruptly, hence the disease's name.
The distinctive interveinal yellowing of SDS can help growers distinguish it from a disease lookalike, stem canker, which has also been on the rise in recent years and produces similar symptoms, said Iowa State University plant pathologist Daren Mueller.
Splitting the soybean stalk of the infected plant can also help growers differentiate SDS from brown stem rot, which also has similar symptoms. The "pith" or center of SDS-infected stalks will be white rather than brown, which indicates brown stem rot. See this video from University of Wisconsin plant pathologist Damon Smith for more details on how to identify SDS in the field: http://bit.ly/….
When scouting, prioritize early planted fields, because cool spring weather can keep young soybean roots within the disease's infection zone for longer. Fields that suffered SDS damage in past years should also be scouted early and often, Bond said.
The disease generally becomes visible around the R2 to R3 growth stage, as the plant starts piling on pods. Once your fields near the R6 stage, pull out your notebook and start evaluating your varieties, Bond said. "Scientists have found that how bad the plant looks at the R6 growth stage is most indicative of how much yield loss is going to occur," he explained.
Plants that are suffering leaf death, defoliation and pod abortion should be considered severely infected, he said. Make a point to evaluate the yield of these damaged patches at harvest, Bond added. This can help you avoid varieties that performed poorly due to disease, as well as determine if some varieties yield better than their symptoms would suggest.
PLANNING FOR NEXT YEAR
SDS remains one of the most difficult soybean diseases to protect against, but research and new tools hold some promise.
Selecting resistant hybrids is still a grower's best option for control, plant pathologists agree. However, growers now have on-seed fungicide treatments that target SDS, such as ILeVO, a product from Bayer CropScience that can provide some protection against the initial disease infection.
University trials of ILeVO showed yield benefits in fields infested with SDS last year, but growers shouldn't bank on the treatment alone, said University of Illinois plant pathologist Carl Bradley. "In cases where we've paired up the most resistant variety we can get along with ILeVO, that's where we see really high levels of SDS control," he explained. "If you have a fairly susceptible variety and you have conditions that are favorable for disease, you are still going to see some symptoms, but it's going to be a lot less severe than without the ILeVO."
Studies led by Iowa State University last year showed that long-term crop rotations -- where farmers planted crops other than beans for two or three years in a row -- could also lower inoculum levels, Mueller said. The effect of tillage remains uncertain, he added.
If your fields remain a healthy blanket of green for now, or you've never had SDS before, don't assume the disease should be off your radar, Bradley said. "The problem with SDS is it is so environmentally driven," he explained. "That's the hard part about making the decision about using the seed treatment."
Emily Unglesbee can be reached at firstname.lastname@example.org
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