Resistant Frogeye Leaf Spot Control Strategies

Resistant Frogeye Leaf Spot Control

Fungicide resistance is spreading across several states, making resistant frogeye leaf spot control a concern for many soybean growers, especially those in the mid-South.

Warm temperatures and high humidity provide the right environment for most soybean diseases to develop and thrive, especially when pathogens are already present in the field.  Frogeye leaf spot (FLS), which overwinters on field debris, can be a soybean grower’s biggest yield robber and is more challenging to control each year.

“FLS affects the green leaf area of soybean plants, which reduces the photosynthetic rate,” says Heather Marie Kelly, Ph.D., extension plant pathologist and researcher, University of Tennessee.  “The reduction in photosynthesis and premature defoliation of soybeans caused by FLS can reduce soybean yields from 10 to 60 percent.”

Frogeye Leaf Spot Resistance Expands

Complicating FLS control is development of tolerance to strobilurin-based fungicides, which are commonly used to control the disease.

“By the end of 2012, tolerance was confirmed in 44 counties across eight states,” Kelly says.  “After new resistance discoveries are tallied for 2013 that number will grow.  And it’s not just happening under typical resistance-development scenarios.  We’ve observed situations where growers were doing all the right things — regularly rotating crops and applying fungicides properly — and fungicide tolerance was still found.  This may be due to the very diverse pathogen populations causing FLS.”

Growers shouldn’t abandon those strategies, Kelly says. She notes that prudent FLS management involves a combination of crop rotation, planting soybean varieties with built-in resistance to the pathogen and judicious fungicide use.  To support her recommendations, Kelly conducts field research to rate FLS resistance levels of soybean varieties and evaluates how well various fungicides and application timings protect soybean crops against disease.

Trial Results Spotlight Solutions

New fungicide active ingredients are providing more options for growers working to manage fungicide resistance development.  For the past two seasons, Kelly’s fungicide trials have included evaluations of new DuPont™ Aproach® Prima fungicide, which combines the unique strobilurin in DuPont™ Aproach® fungicide, introduced in 2012, with a triazole for a second mode of action to control FLS and help prevent development of fungicide resistance.

In a 2012 trial at Milan (Tenn.) Research and Education Center, which included a relatively low level of resistant FLS strains but relatively heavy disease pressure, Aproach® Prima application increased yield by 4 bushels per acre and reduced FLS severity by 20 percent versus the untreated plots.  In an on-farm trial site, with a relatively high level of tolerant FLS strains, Aproach® Prima application increased yield by 7 bushels per acre and reduced FLS severity by 30 percent compared to untreated plots.

In a 2013 trial at the Milan site, applying Aproach® Prima reduced FLS disease severity from 50 percent to 15 percent.  Yield data is still being compiled.

“Strobilurin fungicides are still very effective tools for managing soybean diseases,” Kelly says.  “However, as with any resistance management program, it’s important to use multiple modes of action to control disease, avoid resistance development and provide more strength for overall disease management.

“With two modes of action, Aproach® Prima fungicide provides good disease control for tolerant frogeye leaf spot and other diseases, too. It’s definitely a good tool for resistance management.”

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