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Spraying fungicide on beans? Get ’em early

Trey Price, LSU AgCenter, Bugwood.org frogeye leaf spot on soybean pod
FROGEYE LEAF SPOT: A foliar fungicide treatment can be effective against frogeye leaf spot in soybeans, the most prevalent fungal disease of soybeans in Maryland.
A three-year study in Maryland shows fungicide treatments between R1 and R3 can enhance yields.

There is a time to pay and a time to spray. When it comes to fungicide applications on soybeans, Andrew Kness thinks he has the answer for the time to spray.

“A foliar fungicide really has the best odds of return on investment when you apply them to susceptible varieties of soybeans around the R1 to R3 growth stage,” he told a group of growers at the recent Maryland Commodity Classic.

How does he know this? It’s based off a three-year study he led on soybean fungicide applications at multiple locations in the state from 2019 to 2021. His results might be too late for this season, but it’s good to keep in mind when planning for next year.

Kness planted full-season soybeans on 7.5-inch rows in soybean residue at two locations — in western Maryland at the Western Maryland Research and Education Center, and on the Eastern Shore at the Wye Research and Education Center.

He planted soybeans in existing residue, he said, to induce higher disease pressure during the season. He sprayed fungicide between R1 and R3 — typically around R3 — at 20 gallons per acre.

Frogeye leaf spot is the most common fungal disease of soybeans in Maryland and will appear in dry or wet conditions. Other ailments like target spot, septoria brown spot and cercospora leaf blight are also common but less prevalent.

In 2019, the first year of the trial, two fungicides — Miravis Top and Priaxor — were tested alongside a control plot. The Miravis treatment also included a treatment that was done 14 days later to coincide with the R3 growth stage.

With the exception of the untreated control plot, all treatments saw less incidence of frogeye leaf spot, leading to yields ranging from 44.1 bushels in the untreated plots to 56.9 bushels in the double-treated Miravis plots.

Oddly enough, Kness said the treated plots enhanced incidence of green stem, a disorder that causes soybeans to stay greener for longer and can delay maturity. “We measured that with a hand-held green seeker device. Basically the higher the number, the greener the plants. That may or may not be a problem because if they stay greener longer, it will take longer to dry out, and you might have to cut them later or wait for a frost,” he said.

An additional Priaxor treatment at R3 was added in 2020. Similar results were seen that year, and green stem also increased, especially in the Miravis double treatment.

Last year, seven treatments were tested, most of them at R3 and 14 days after R3. But a soybean variety that was resistant to frogeye leaf spot was also used. “So the plants didn’t see any disease at all, but there was still green stem,” he said, adding that the fungicide may be delaying plant senescence.

Overall, the fungicide treatments helped improve overall yields by 4 to 9 bushels per acre due to its control of frogeye leaf spot. Whether a treatment like this is economic for your farm, though, depends on your own situation.

“I do think that the biggest part of economics is to know your cost of application and to figure out how many bushels you need in order to make that work,” he said.

The graphic below shows the number of additional bushels needed to break even, based on the cost of soybeans and the fungicide application’s cost per acre. Anything over those numbers would be a profit; below that you would be losing money.

Kness calculated the numbers by taking the yield from the treatment multiplied by the cash price for soybeans, then minus the application cost. You will have to modify the formula for your specific operation.

Number of additional bushels needed to break even table

Preventing SDS

One ailment fungicides won’t be effective against is sudden death syndrome.

“Foliar fungicides only work on fungal diseases that attack the foliage. Sometimes things could look like a foliar disease but are actually not, and SDS is one of them,” Kness said.

Sudden death syndrome is a necrosis virus, although it is caused by a fungal pathogen that is soilborne. It can be a big issue in fields that are planted early, something Kness said has been the trend on many farms.

“It seems like there has been a trend over the past several years that folks want to plant soybeans earlier and earlier, and in those cool, wet soils, this is where we get the infection occurring,” he said. “We had folks planting soybeans at the end of March this year. Cool, wet soils favor it and other root-borne issues.”

Sudden death syndrome survives year to year in old soybean residue. It hangs out in the taproot of the soybean plant until R1, he said, when symptoms then start appearing in the upper part of the plant. The disease causes necrotic lesions in between the leaf veins.

"So the fungus produces a toxin that will be transported to the upper part of the plant, which is what produces those striking symptoms that you see," Kness said. “The pathogen is not even in the leaves; it’s way down in the roots of the plant. Symptoms just manifest in the foliage.”

Leaflets will then drop off the stems, leaving the plants without petioles, which is one of its most distinctive symptoms.

Crop rotation can help manage the disease, he said, but the best control is paying close attention to planting condition and planting date.

Good drainage is also key, he said, because moisture is needed for SDS to manifest itself. Even tillage can be effective to help dry out the soil.

Variety selection should also be considered, as some soybean varieties are resistant to SDS and provide resistance to soybean cyst nematode.

The Crop Protection Network has more information on preventive measures, including seed treatments that could be effective.

TAGS: Fungicide
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