Checking the Crystal Ball on Fungicide Resistance

While no one can predict the future with complete certainty, farmers must look to the future to attempt to determine what the next growing season holds. There are many decisions to consider, from seed selection, equipment and land decisions, fertilizer application timing, and now fungicide application management.

While fungicide resistance has not reached the same level of concern for farmers as herbicide resistance, it’s only a matter of time before it becomes a greater issue, says Daren Mueller, Ph.D. Iowa State University Extension plant pathologist.

But agronomists, researchers and farmers have an opportunity to prevent the issue from becoming worse by looking for clues from 2016 to make predictions on possible fungicide resistance during the 2017 growing season.

“We’ll likely continue to see issues with the pathogen that causes frogeye leaf spot, which was the first soybean pathogen with documented resistance to the QoI fungicides (strobilurins),” says Carl Bradley, Ph.D., University of Kentucky Extension plant pathologist.  “With checkoff support, we have found resistance in 12 states, so far.”

QoIs– which include products like AboundTM and ReasonTM –  are effective, but also susceptible to resistance developing, because it requires only a single-step mutation of the pathogen, which can happen almost overnight, says Bradley. He says the good news is more fungicides are starting to feature multiple modes of action (MOA), which will help slow down resistance.

“We don’t expect much fungicide resistance this year in Iowa, because currently the only fungal disease of concern is frogeye leaf spot,” says Mueller. “Frogeye leaf spot does not show up very frequently in Iowa, though we did have it in 2016 and we are testing isolates, but do not have results yet.”

Frogeye leaf spot targets western Kentucky, southern Illinois, western Tennessee, southeast Missouri and areas further south, and conditions were favorable in this area for the pathogen, so more foliar fungicides to combat frogeye leaf spot were warranted, says Bradley. “If farmers relied solely on foliar fungicides and made the wrong decision on which product to use, they may not have gotten great control, partially due to fungicide resistance,” says Bradley.

But that doesn’t necessarily mean there will be more fungicides applied in 2017. Mueller says lower commodity prices mean farmers will likely cut back on fungicides, which could help lower the risk of fungicide resistance in the short term. However, he expects development of fungicide resistance will only be a matter of time. Bradley and Mueller recommend the following management options to slow the advance of fungicide resistance:

  • If you are in an area affected by frogeye leaf spot, you should select varieties based on susceptibility and resistance to the pathogen. “If frogeye leaf spot is a continuous problem in your area, you should select varieties based on yield, soybean cyst nematode resistance and then frogeye leaf spot resistance,” says Bradley.
  • Practice crop rotation.
  • Scout and properly identify diseases to avoid unnecessary sprays. “There are so many different ways an application can fail, so looking at a field can provide clues to determine if it is truly failure of fungicide effectiveness,” says Mueller.
  • Spray fungicides when needed. “We need to limit fungicide applications to when we really need them, and that’s where scouting is important,” says Bradley. “If we take the approach to spray everything whether it is needed or not, we see more selection pressure. Scouting helps make those decisions, if we need to apply a fungicide or not.”
  • Continue to select products with multiple modes of action, even when genetic options become available. “If you’re using a product that contains two MOAs or active ingredients, both need to be effective,” says Bradley. “If only one has good efficacy, it’s really not helping you. As we get more MOAs, we need to make sure all are effective.” If you are using a single mode of action fungicide, rotating to a different FRAC code will certainly be beneficial, says Mueller.

“There are no silver bullets, and if we start using products like they are silver bullets, that’s when they’ll break down and have issues,” says Bradley. “Keep in mind all management options, not just foliar fungicides. That will matter in the long run to manage these diseases.”

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