Koehler reviews disease presence in soybeans

Dr. Alyssa Koehler, University of Delaware Extension plant pathologist, demonstrates soil sampling for soybean cyst nematode during an Aug. 19, 2021 field day at the Carvel Research and Education Center. (Delmarva Farmer file photo)

GEORGETOWN, Del. — Dr. Alyssa Koehler, Extension plant pathologist at the University of Delaware’s Carvel Research and Education Center, reviewed the past year’s disease presence in soybeans and small grains and discussed her research during Delaware Ag Month’s Agronomy session Jan. 13.
It was a calm year so far as foliar pathogens go, Koehler said, so the full season soybean foliar fungicide efficacy trial did not show any readable disease. However, her team was able to look at the effect of various fungicide treatments in the foliar study on green stem (a disorder that causes soybean stems to remain green even after pods and seeds are mature), measured two weeks before harvest. They also looked at purple seed stain and Diaporthe/Phomopsis seed decay. With some of the treatments, a three- to four-bushel increase in yield was noted.
Fields across Delaware and Maryland were surveyed in 2019 through 2021 for soybean cyst nematode, the most damaging soybean pathogen across North America. SCN can persist in the soil for a long time and can produce three to six generations per year. Each female can produce 200 eggs which survive in cysts up 10 years or more. Because all this happens underground, SCN is called “the silent yield robber,” Koehler said. You may not know it’s there and may attribute losses to another factor such as soil type or variety.
Planting “resistant” lines is a great way to control SCN.
“We had pretty excellent resistance that came from the PI 88788 gene,” Koehler said. That gene has been used for decades and now is less effective, but 95 to 98% of soybeans on the market are PI 88788. New resistance genes are coming, but it will be a while.
Koehler’s goal is to get a handle on SCN distribution as well as root knot and any other nematodes that might be out there. Her studies are supported by the Maryland and Delaware Soybean Board, the Atlantic Soybean Council and the SCN Coalition.
There are two approaches to sampling: predictive and diagnostic, Koehler said. With predictive sampling, you work your way through an entire area collecting samples in a zig zag pattern. She said they typically sample at the end of the season when nematode populations are at their peak and are forming on the root system.
Diagnostic sampling is used when you’re interested in a particular “hot spot” and sampling is limited to that area, as well as from a healthy area nearby for comparison.
The survey was done in all of Delaware and on Maryland’s Eastern Shore from Cecil County south to Somerset, and in Western Maryland’s Garrett County.
Just because nematodes are present doesn’t always mean there are problems, she said. Virginia Tech established economic damage thresholds, which Koehler’s team is in the process of reevaluating. SCN was in almost 57 percent of samples taken, and within that 57 percent, about 66 percent were at the high threshold.
They are finding a variety of nematodes, some above thresholds, including root knot nematode at 36-percentabove. Other nematodes found were lesion, lance, stubby root and stunt. Spiral nematode was really abundant, found in 85% of the samples, but it is usually thought to have to be extremely high before an impact is seen, she said. A summary of the study will be issued as soon as results are finalized.
In addition to resistant varieties, options for dealing with SCN include seed treatments. The most common on the market are Ilevo (active ingredient fluopyram) and Saltro (pydiflumetofen).
“These are actually fungicides with nematicide activity which we see on SCN, root knot, reniform, lance and lesion,” Koehler said.
Both fungicides target Fusarium virguliforme, the causal agent of Sudden Death Syndrome (SDS), which researchers said they did not see much of in 2019 and 2020, but there was quite a bit in 2021.
Research has shown the earlier you plant, the more SDS infection there is in the crop’s early days — but the symptoms are not seen until the plant starts reproducing.
“Soybeans only have so many ways of telling us they have a problem,” Koehler said. “Just because you see a bright leaf doesn’t necessarily mean it’s SDS.”
Koehler also noted both treatments have some early season septoria brown spot control listed on the label.
“So, especially when you’re planting early, the value of these seed treatments really seems to come from helping to get that seed up out of the ground.” Untreated seeds lost leaves early. The plants from treated seeds had brown spot control, and an increase of 2 to 3 bushels per acre.
She added that initial populations of SCN had been very high in 2020, which she attributed to a mild winter.
“The product wears off eventually, so as we learn with a lot of nematode things, there’s usually not that silver bullet. It’s just things that we can add to try to chip away at nematode problems and add a little bit of yield back in,” she said.
The seed treatment trial was repeated in 2021 in Georgetown. Some drop in SCN numbers was seen with both treatments compared to the plain seed.
The yield increase was about six bushels. In summary, the seed treatments aided both the speed of emergence and percent of emergence. The Saltro-treated seed had significantly fewer females 30 days after emergence and the lowest number of nematodes in the soil at the end of season in 2020. Differences in nematode populations among treatments were not observed in 2021.
This trial is part of a multi-state trial being conducted in 12 other states and one Canadian province.
Koehler put in a plug for the SCN Coaltion, referring viewers to the website thescncoalition.com for more information.
Briefly, she described work on Fusarium head blight in wheat and barley trials. FHB spores land on wheat heads and infect the plant during flowering. The spores cause “scabby” or shriveled, wilted kernels that are usually contaminated with mycotoxin deoxynivalenol, or DON.
In a 2021 wheat fungicide efficacy trial, university researchers looked at fungicide timing. This year, however, there was little Fusarium around, Koehler said. There were minuscule counts on heads and they were counting individual grains on large numbers of heads.
The take away, she said, is that a lot of work is being done in many states looking at timing of fungicide application. There is a new product, Miravis Ace. Pre-anthesis treatments are effective at reducing FHB and DON, but anthesis application is better. And applying a bit late also works.