Nature always finds a way and that’s why the Mid-South is facing not only herbicide resistance in weeds but also an upswing of fungicide resistance in some soybean diseases.

While frogeye leafspot hasn’t yet been a huge problem in Louisiana, it may be on the rise. The big headache for the state’s soybean producers is cercospora leaf blight, said Trey Price, LSU field crops pathologist at the 2014 Mid-South Farm and Gin Show.

“In the early 1990s, we had benzimidazoles and other products that were primarily used in Louisiana,” said Price. “There are a lot of gaps in knowledge and the fungicide use records. Nobody really keeps up with it. Every now and then, USDA provides some estimates.”

The strobilurin fungicides came out in the late 1990s. Shortly after, in 2004, Asian soybean rust arrived on the scene.

“That perked everyone’s ears up and they began paying more attention to soybean diseases,” said Price. “Right now, we estimate anywhere from 40 to 75 percent of the soybean acres in the Mid-South are treated with a fungicide. Personally, I’d estimate around 60 percent are treated – and that’s probably conservative.

“Another way to look at this is fungicide use has increased along with the soybean price. That makes sense.”

However, alongside increased use comes news that the efficacy of some fungicides have decreased. And that decrease has happened while cercospora leaf blight has solidified its status as the major problem in Louisiana soybeans.

“And we’ve confirmed fungicide resistance from that pathogen.”

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There are many options when choosing a fungicide, said Price. “The take-home is that three of the four fungicide types for soybean have a high risk for resistance. That means we need to be conservative with these products. The reason for the resistance risk is most of the products have a very specific mode of action and the pathogens are capable of mutating rapidly to overcome that.”

Fungicide resistance isn’t new, Price pointed out. “It’s been documented since fungicides came out. Benzimidazoles came out in the late 1960s and resistance was confirmed soon thereafter. With strobilurins, there are 56 species in 20 different crops. Triazoles don’t have a high risk to resistance -- I’d say the risk is medium. There are a few more modes of action with the triazoles.

“Even the newer SDHI fungicides have already documented 12 cases of resistance.”

Soybeans aren’t the only crop being affected by fungicide resistance.

“What happens with corn is we put out plant health applications when there are low levels of disease in the fields. You may be selecting for resistance in corn pathogens, as well.”

How it occurs

So, how does resistance happen in the field? “You put out a strobilurin application on a majority of sensitive pathogens. After the spraying, you’ve eliminated all the sensitive individuals and leave behind resistant individuals. Keep spraying with that same chemistry type and eventually you’ll end up with a majority of the population being resistant to fungicides.”

This happened in Louisiana with cercospora leaf blight. “This disease is a bit different than frogeye. It causes more blight on the leaves, purple seed stain and, sometimes, premature defoliation of beans.”

Cercospora is now widespread in Louisiana and strobilurin resistance has been confirmed in most parishes.

“Remember this: 85 percent of the isolates I screened were resistant to strobilurin fungicides. That’s a clear indication that the majority of the pathogen population is resistant to that chemistry type.”

Price has also found thiophanate-methyl resistance in cercospora leaf blight. “It was found in 19 out of 27 parishes, so far, in about 30 percent of the isolates I ran. We also have pathogens that are resistant to both thiophanate-methyl and strobilurins.”

Frogeye leaf spot is more of a problem farther north in the Mid-South. “It can affect seed as well as leaves. Usually, in the middle of lesions you’ll see sporulating. In Louisiana, we’ve had light disease pressure from frogeye in the past couple of years. Nonetheless, we’ve confirmed strobilurin resistance in the pathogen in nine parishes.”

A good thing with frogeye is that there are resistant soybean varieties. “There are more resistant varieties than susceptible varieties. That’s a good way to avoid the disease.”

Another positive with frogeye, said Price, is “we’re seeing efficacy with fungicides.” At a 2013 trial at the Dean Lee Research Center in Alexandria, La., researchers looked at various fungicide types on frogeye. The trial showed “good efficacy with triazole fungicide types.”

One interesting approach was Headline plus Incognito. “We don’t see activity with (Incognito) with cercospora. But we are seeing it in frogeye. A management option might be a thiophanate-methyl mixed with a triazole product.”

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Producers should remember these things, said Price:

  • Plant a resistant cultivar.

“Avoid disease all together. That’s our best defense in any disease control situation.”

  • Apply products that contain a mixture of active ingredients, particularly a triazole if dealing with frogeye leaf spot.
  • Mix it up.

“Change your mode of action. Don’t put out the same chemistry one after another.”

  • Only apply a fungicide if needed.