COLUMBIA, Mo. – University of Missouri Integrated Pest Management coordinator Mandy Bish often hears farmers say, “A treated acre is an acre I’m not losing sleep over.”

That is a good mindset for residual herbicide applications, she says, but it may or may not pay off when it comes to foliar fungicides, depending upon circumstances.

Use of foliar fungicides in soybean has increased across the southern states, including Missouri, since 2005. The uptick followed Hurricane Ivan’s wind and rain, which likely brought spores of soybean rust into the U.S.

The Missouri Soybean Merchandising Council has helped fund 43 Missouri foliar fungicide strip trials through the MU Certified Strip Trial program since 2018. See results at striptrial.missouri.edu.

In those trials, Missouri soybean growers saw an average yield increase of 1.8 bushels per acre over nontreated acres when applying fungicides at R3 development stage. The increase was not unique to Missouri. Researchers in 240 small plot trials in nine states and Ontario, Canada, reported a 2.7% percent yield increase. “This corresponds to a 1.6-bushel-per-acre bump in 60-bushel bean,” says Bish.

However, not all fields responded the same. Researchers continue to look at how disease pressure, extreme precipitation and other factors such as row spacing affect response.

But one thing is certain, Bish says. “Fungicides work when we need them. We do not want to lose that effectiveness.”

Producers must weigh the yield increase against potential risk of fungicide resistance in the long term.

Bish also shares updates on two other issues affecting row crops.

Tar spot

Missouri producers reported more tar spot cases in northern Missouri cornfields in 2022. MU’s Plant Diagnostic Clinic confirmed cases in northern counties in August 2022. Bish estimates that it is present in the northern third of the state. “We’re not the hot spot yet but continue to creep closer,” she says.

Tar spot’s growing presence calls for frequent scouting, Bish says, since it spreads quickly. In years with severe outbreaks, tar spot can cause yield losses of 20-60 bushels per acre due to reduced grain fill, kernel abortion and reduced kernel weight, as well as late-season lodging.

First discovered in 1904 in the country of Mexico, the disease reached the Midwest by 2015. Tar spot, which spreads quickly through fields, appears as circular or oval raised black dots on both sides of corn leaves. A brown or tan halo may surround the spots, which can also appear on the sheaths and husks. It overwinters on soil surface residue and spreads by wind and heavy rain.

Scout often, choose less susceptible corn varieties and apply residual and multiple modes of action foliar fungicide treatments to reduce losses, Bish says.

Timing of application also matters. Current research shows no benefit to R4 or R5 applications. The most effective applications, given current disease pressure, appears to be at VT/R1, Bish says.

SCN management

Soybean cyst nematode (SCN) costs the soybean industry $1.2 billion annually. Eggs persist in the soil for years. Found in 1956 in Missouri, it appears in all soybean-growing counties. “It’s not going anywhere soon,” Bish says.

More troubling is that the effectiveness of SCN-resistant soybean varieties is breaking down, Bish says. Commercially available SCN-resistant varieties, using P188788 and Peking, were first introduced in the late 1960s. More than 95% of the resistant varieties contain P188788.

“Basically, we’ve relied on one resistant variety for more than 30 years,” Bish says.

SCN can rob yields without any visual indicators. Roots sometimes show infestation during the growing season, but soil tests that include SCN egg counts tend to be better indicators of SCN densities. (See www.scndiagnostics.com for information on how to sample for SCN eggs.)

ILeVO seed treatments offer protection against SCN in greenhouse studies. MU Certified Strip Trials with ILeVO showed an average increase of 2 bushels per acre in treated strips compared to nontreated strips across 20 locations, Bish says.

Researchers are also studying how cover crops affect SCN numbers, Bish says. “We have a lot to learn in this area, but most preliminary data indicate our most popular cover crop, cereal rye, is not influencing SCN populations,” she says. Rotation with corn and other nonhost crops still appears to be the best approach to manage SCN until breeders can develop new resistant varieties.

For more information, contact Bish at bishm@missouri.edu or 573-882-9878.