Digging Into SDS

AMES, Iowa (DTN) -- Iowa State plant pathologist Daren Mueller had a tough task: Keep freshly fed folks awake in the soft seats of a dimly lit auditorium well into the afternoon napping hour.

Fortunately for Mueller, the topic of conversation was Sudden Death Syndrome in soybeans -- a fresh wound for the Iowa farmers and crop advisers in attendance at Iowa State's Integrated Crop Management Conference on December 4.

His audience stayed wide awake as Mueller laid out the most recent research insights from a series of studies into the damaging soybean disease, funded by the North Central Soybean Research Program. The results included a promising new soybean seed treatment, benefits from long-term crop rotation and cover crops, as well as insights into how planting date and long-term tillage affect the disease's development.

PLAN ON PLANTING EARLY

One 2013-2014 study examined 11 planting date trial plots in Illinois, Iowa, Indiana and Ontario. Researchers planted soybeans on dates ranging from April 15 to June 15 and then compared the development of SDS among them.

The results were quite clear: Early planted soybeans did show more SDS development, but they still yielded consistently higher than late-planted soybeans with less disease.

"Our recommendation remains that you don't ever delay planting just to avoid SDS," Mueller concluded. "If you have a field that has SDS, maybe put it at the bottom of your list of fields to plant, but don't go out and deliberately delay planting because you're going to be sacrificing that yield even before you run the risk of getting SDS."

The studies confirmed that weather remains the largest determining factor in SDS development, Mueller added.

"SDS did increase as you planted earlier, but the driving factor that trumped planting date was moisture," namely heavy summer rainfalls, he said.

NEW PRODUCTS MAY HELP

Another 2013-2014 study examined the effect of in-furrow and foliar fungicide applications, as well as a new seed treatment from Bayer CropSciences called Ilevo, in SDS plots in Iowa, Indiana, Illinois, Michigan and Ontario.

Commercially available in-furrow and foliar fungicide applications had no effect on the disease, but Ilevoshowed promise, Mueller said.

Across all trials, Ilevo averaged a 3.6 bushel bump in fields with SDS symptoms and a 3.4 bushel increase in fields with few or no SDS symptoms. In-furrow applications of Luna Privilege also added 3.4 bushels in fields with SDS development, but had only a 0.7 bushel increase in fields without significant SDS symptoms.

In the planting date trials, Ilevo performed best in early planted fields with the most severe SDS development, Mueller added. The seed treatment added 5.7 bushels on average to those fields.

CROP MANAGEMENT TACTICS

Most tillage and no-till studies on SDS only evaluate one year of data, so Mueller and his colleagues set out to conduct the first long-term tillage study after the 2010 outbreak of the disease. The study involves three plots -- a no-till corn and soybean rotation, a no-till corn and a tilled soybean rotation, and a tilled corn and soybean rotation.

Four years later, the data shows ... nothing, Mueller concluded. "There was no significant difference in disease severity index or yield in 2014" among the plots, he said. "But we're going to continue this tillage study and see if we can get any longer-term effect of tillage on SDS."

Long-term crop rotation studies were more promising. At Iowa State's Marsden Farm in Boone Co., Iowa, researchers evaluated SDS development in three plots -- a 2-year rotation with corn and soybeans, a 3-year rotation with corn, soybeans, oats and red clover, and a 4-year rotation with corn, soybeans, oats and alfalfa.

"There was more disease incidence and severity in the 2-year rotation compared to the 3-year and 4-year rotation," Mueller noted, which suggests the longer a field is away from soybeans, the better.

However, knowing that many farmers are unlikely to stray so far from a standard corn and soybean rotation, some researchers are examining the role cover crops can play in breaking up the disease cycle during winter fallow periods.

University of Illinois plant pathologist Darin Eastburn conducted a 3-year study on the use of rye, rapeseed, mustard and canola. He found rye and rapeseed do have the potential to suppress development of the SDS pathogen, Fusarium virguliforme, and Rhizoctonia rot in the soil, although results were not consistent across his plots. Rape, rye and canola also significantly decreased soybean cyst nematode egg counts in the study.

Back at Iowa State, Mueller oversaw another study that evaluated 15 cover crops as possible hosts for the SDS pathogen.

They found the fungus survived and rotted the roots of one grass, sorghum, and all the legumes -- alfalfa, crimson clover, hairy vetch, pea and red clover. None of the brassica cover crops showed any root rot.

"It showed that some of these legume cover crops could be possibly helping build up that Fusarium virguliforme population in the field, not that it needs any help," Mueller warned.

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

(Ps/CZ)

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