By Emily Unglesbee, DTN Staff Reporter

How common is fungicide-resistant frogeye leaf spot? So common that susceptible strains of the pathogen are now extremely hard to find.

“It’s almost like they’re going extinct,” said Carl Bradley, a University of Kentucky Extension plant pathologist who first confirmed QoI-fungicide-resistant frogeye leaf spot in 2010 in soybean fields in Illinois, Kentucky and Tennessee. In the decade that followed, resistance has spread like wildfire. “We’ve seen a complete flip-flop, where it is increasingly hard to find the sensitive [strains] and the resistant ones are the vast majority,” Bradley said.

Carl Bradley, University of Kentucky Extension

The frogeye leaf spot pathogen is resistant to the QoI class of fungicides (Group 11), which many farmers know as strobilurins, although other fungicides fall into the QoI class.

Scientists have confirmed this kind of fungicide-resistant frogeye leaf spot pathogen in 19 states:

–Alabama –Arkansas –Delaware –Illinois –Indiana –Iowa –Kentucky –Louisiana –Mississippi –Missouri –Nebraska (confirmed, but not published yet) –North Carolina –Michigan (confirmed, but not published yet) –Minnesota (confirmed, but not published yet) –Ohio –Tennessee –Virginia

Where the resistance has been found, it is dominant. Iowa State Extension plant pathologist Daren Mueller led a team that collected samples of frogeye leaf spot from Iowa soybean fields in 2019. It was not a hard hunt. “We could find the disease in every field we looked,” Mueller noted.

They tested the samples against a single product, azoxystrobin (Quadris), a QoI fungicide. All 79 strains they collected for testing were fully resistant. That means these strains are resistant to the entire QoI class of fungicides — not just that active ingredient, Mueller said.

This type of fungicide resistance is neither partial nor gradual. The most common mutation that allows the frogeye leaf spot pathogen to survive QoI fungicides, known as G143A, confers total resistance as soon as it emerges.

Resistant strains with this mutation are, at a minimum, hundredfold less sensitive to QoI fungicides, with some showing six hundredfold to seven hundredfold less sensitivity, Bradley said.

“Once you have this mutation in the field, spraying a QoI fungicide is like spraying water on that field,” explained Mueller.

“You can’t use a high enough rate of the fungicide to fix this,” Bradley added.

Scientists don’t usually like to speak in extremes, but this type of resistance is becoming so widespread that it’s hard not to, Mueller said.

Frogeye Leaf Spot on Soybeans

“I have been telling farmers that if you have frogeye leaf spot in Iowa, there is a 99% chance it is resistant to QoIs,” he said.

QoI fungicides are an old class of chemistry and commonly used in row-crop agriculture. They include fungicide-active ingredients like pyraclostrobin (Headline) and picoxystrobin (Aproach). Companies are moving toward selling mostly pre-mix fungicides containing more than one fungicide class, such as a DMI (Group 3) or an SDHI (Group 7) fungicide, which still have activity on frogeye leaf spot.

But it is still possible to find single-ingredient QoI fungicides, especially in generic formulations, Bradley and Mueller noted.

“With some QoI fungicides going off-patent, it can be pretty enticing financially to use a generic, but it won’t be effective on resistant frogeye if it’s just a single QoI in the jug,” Bradley cautioned.

QoI fungicide active ingredients commonly used in row-crop agriculture include: azoxystrobin, fluoxastrobin, picoxystrobin, pyraclostrobin and trifloxystrobin.

Frogeye leaf spot is not the only foliar disease pathogen to develop resistance to this class of fungicides. Scientists have found QoI-resistant strains of Septoria brown spot in Illinois, Iowa, Kentucky and Tennessee, and QoI-resistant strains of target spot have also been found in Alabama, Bradley said.

“It’s part of a trend,” he said.

See more on fungicide classes and their respective resistance risks from the Fungicide Resistance Action Committee, an industry group that tracks resistance in fungicides, here: https://www.frac.info/…

See more on fungicide resistance in row crops from the Crop Protection Network here: https://crop-protection-network.s3.amazonaws.com/…

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/BAS)

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