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.”

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.