Consider All Angles in Fungicide-Resistant Disease Management
Travis Faske is quite familiar with fungicide-resistant frogeye leaf spot. The University of Arkansas Agriculture Cooperative Extension plant pathologist works with soybean farmers in their fields to confirm disease resistance to QoI fungicides (group 11).
“Products containing QoI fungicides as the only mode of action no longer control the disease in Arkansas,” says Faske. “Resistance develops in many cases, such as when fungicides are applied in the absence of disease and when single modes of action are used repeatedly to control disease.”
Faske says using cut rates on susceptible cultivars grown where disease pressure is frequent and relying on fungicides as a primary tactic for disease management is another way for resistance to develop.
Current State of Resistance
Trey Price, associate professor of agronomic crop pathology at Louisiana State University, has confirmed QoI resistance in frogeye leaf spot, aerial blight and Cercospora leaf blight. In the case of aerial blight, QoI resistance seems to be concentrated in southern Louisiana, where rice-soybean rotations are common. Price adds QoIs still work in north Louisiana on aerial blight, likely because the area has less selection pressure with more crop rotation options.
“However, any foliar or soilborne disease caused by fungus has the potential to develop resistance,” Price warns. “I suspect we have resistance with target spot and Septoria brown spot, also.”
Like Faske in Arkansas, LSU AgCenter researchers travel to soybean farms throughout their state, collecting pathogen samples and testing them in the lab for resistance. To minimize the development of highly resistant populations, farmers should use multiple modes of action or broad-spectrum fungicides, says Vinson Doyle, LSU AgCenter assistant professor of mycology.
Faske notes DMI fungicides (group 3) are one fungicide class that continues to have efficacy now that QoI fungicides no longer control frogeye leaf spot.
“Many of the fungicides used by farmers are premixed, usually a QoI and DMI, which reduces resistance pressure compared to using just one mode of action,” Faske says. “However, use of these two chemistries where QoI-resistant frogeye leaf spot is present puts all the pressure on the DMI fungicide.”
Mix Up Practices Before Mixing the Tank
The best strategy may be taking action that does not rely on fungicide use as the first control option.
“The less fungicides are used, the longer it will take for resistance to become a problem,” Doyle says. “Farmers should scout fields weekly and apply fungicides only as needed. When disease is present, use appropriate water volumes and nozzles and apply by ground, if possible.”
The best first option may be to utilize crop genetics to reduce disease pressure, says Faske. Other options he recommends include:
• Changing the planting date to avoid sensitive growth stages and historical disease pressure.
• Planting on wider row spacing to allow airflow to reduce the environment for disease development.
‚Ä¢ Rotating to a crop that isn’t susceptible to the fungus, such as corn or small grains.
Price also encourages farmers to weigh the economics of fungicide applications. It comes down to common sense and dollars and cents, he says.
“What are the odds a fungicide application will provide economic returns over the break-even point?” Price asks. “Just because your neighbor is spraying doesn’t mean you need to spray. Be cautious of the rumor mill. Don’t panic. And if you don’t know what you’re looking at in terms of possible resistance, call a specialist.”
For more information on fungicide resistance as well as best management practices, download the Take Action app or visit www.IWillTakeAction.com/diseases.