Get a jump on resistance

Plant frogeye leaf spot-resistant varieties and avoid solo QoI applications if fungicides are needed.

frogeye leaf spot
Frogeye leaf spot typically attacks foliage. In severe cases, it can cause premature defolliation, significantly reducing photosynthesis and yields. — Photo courtesy Mississippi State University

By Vicky Boyd

As strobilurin-resistant frogeye leaf spot (FLS) continues to expand across much of the Southern soybean production region, Extension specialists say using resistant varieties remains a grower’s best option.

“The No. 1 recommendation I will always make to growers is to use host-plant resistance,” says Travis Faske, University of Arkansas Extension plant pathologist based in Lonoke. “That’s the best, most economical option they have.”

If scouting indicates a fungicide is warranted for FLS, Faske and colleagues now recommend against using strobilurins — also known as QoIs — as a stand-alone treatment.

“What I’ve been telling producers, county agents and crop consultants I believe every field (in Tennessee) most likely has some level of resistance, so don’t plan on using a solo QoI in soybeans to control frogeye leaf spot,” says University of Tennessee Extension plant pathologist Heather Kelly.

Instead, she and others advise growers to switch to a fungicide with another mode of action, such as an MBC (methyl benzimidazole carbamate) or DMI (demethylation inhibitor).

Better yet, they say, is to use a premix or tankmix that contains two different modes of action, one of which can be a strobilurin.

“Even though the QoI won’t control frogeye leaf spot, there are other pathogens that it will take care of and help manage,” says Kelly, based at the West Tennessee Research and Education Center in Jackson. Growers also should apply fungicides at labeled rates.

In addition, Kelly recommends incorporating other non-chemical resistance-management practices, such as crop rotation and debris management, into production regimes.

Fungicide resistance continues to expand

Strobilurin-resistant FLS has been found in 178 counties in 12 states east of the Mississippi River, as of April 1. The region includes most of the Southeast where soybeans are grown.

The fungal pathogen that causes FLS is called Cercospora sojina. It primarily attacks soybean foliage, causing circular or angular spots on the leaf surface that resemble frog eyes. Symptoms begin as dark brown, water-soaked spots and mature into lesions with tan or brown centers and a narrow reddish-brown to purple margin.

In severe cases, the disease can cause premature defoliation, significantly reducing photosynthesis and yields.

Although less common, disease symptoms also may be found on pods, seeds or plant stems. Optimum conditions for FLS infection and development are temperatures between 77-85 degrees Fahrenheit and high moisture, such as from heavy dews or prolonged periods of light rains. The organism also can survive from one season to the next on infected seed and plant material.

Although host-plant resistance tops Extension recommendations for managing FLS, many growers in the past have shied away from these varieties because they didn’t yield as well as those without the genetic attribute.

But Kelly says that is changing, and she cites yield data from the past three years of variety trials conducted at the UT Ag Research & Education Center at Milan — considered an FLS hotspot.

“In general, I’ve seen some very high yielding, very highly resistant frogeye leaf spot varieties of soybean in my trials over the last three years, so more breeders are incorporating FLS resistance into their genetics,” she says.

Faske agrees and notes that Asgrow 4632, a variety with FLS resistance, also produced more than 100 bushels per acre in the Arkansas Soybean Yield Challenge last season.

“I think they’re improving in that (yield) area, and I hope the trend continues,” he says.

Under severe disease pressure, the resistance trait still may not protect the soybean plant entirely from infection, but it should help.

“If you invest in the genetics and a resistant FLS variety, you’re less likely to need a fungicide application to protect yield from FLS,” Kelly says. “It depends on how the year plays out. In drier years, we don’t always have fungal diseases develop that will rob yield.”

Scouting remains the best way to catch the disease early, Kelly says. Fungicides are most effective when applied preventively or shortly after infection. So it’s best if growers apply them before fungal infection or at the onset of symptoms to protect the plant. Based on averages from numerous field trials, the “sweet spot” timing to apply an FLS treatment is R3 when pods are just beginning to form, she says.

“When you start to see the disease, you try to figure out what level and how quickly the disease will develop,” Kelly says. “In some instances, we can find the disease and it very slowly develops. Other times after finding it
at low levels and the right conditions occur, it’s really blown up.”

Disease risk model in the works

To help growers and consultants better gauge the risk of FLS developing in their soybean crops and when to spray, Kelly is leading a research project to develop a forecasting model.

Her first attempt looked at using a simple model to predict when weather conditions would be conducive for disease development. The results were inconsistent.

But Kelly is working on a more complex model that will factor in field disease history, field cropping history, varietal genetics, weather parameters and possibly spore trap catches. Assisting her is Jamie Jordan, a research associate also pursuing his Ph.D., and Dr. Binbin Lin, a post-doctoral researcher.

Spore traps work on the same premise as other pest traps. In this case, they measure the amount of inoculum present and can alert growers seven to 10 days before FLS symptoms appear in the crop.

“We know to use a moth trap around non-Bt corn — once you get to a certain level, then it triggers an insecticide application,” she said. “With frogeye leaf spot, it will be more complex. You have the level of resistance in the variety to consider as well as cropping and disease history of the field. We’re finding higher disease pressure in sites that had continuous soybeans with highly susceptible varieties planted.”

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