Select hybrids or varieties that are resistant or less susceptible to yield-limiting fungal diseases. “Fungicides should never be used as the first line of defense against diseases,” says Alison Robertson, an Iowa State University Extension plant pathologist. Instead, disease-resistant or disease-tolerant hybrids and varieties should form the foundation of disease management plans, she adds.

Use cultural methods where possible to reduce crop disease risk. These may include crop rotation, residue removal or methods that encourage residue decomposition.

DISEASE TRIANGLE: The emergence of white mold is a good example of the disease triangle at work. It consists of pathogen presence, a susceptible host and the right environment. All three must converge for disease to surface in crops, says Alison Robertson, an Iowa State University Extension plant pathologist. (Gil Gullickson)

Apply fungicides only in response to disease or a disease threat. “Every time we spray a fungicide, we put selection pressure on all fungi in that field,” Robertson says. Fungicides work well to control disease-causing fungi, such as those that spur tar spot in corn. Every time a fungicide is used, however, resistant biotypes develop that can overcome a fungicide over time. That’s what’s happened to frogeye leaf spot in Iowa, which resists fungicides belonging to the strobilurin class. “Those organisms can evolve and eventually overcome the fungicide, and we will not have that tool,” Robertson says. “It not only places selection pressure on other fungi that cause plant disease, but also fungi that cause disease in humans.” Limiting applications to disease management can extend effective fungicide use, she says. The Crop Protection Network has a tool to help farmers determine corn fungicide return on investment.

Monitor disease forecasts to help determine if environmental conditions increase disease risk and warrant the use of foliar fungicides. Tarspotter for tar spot in corn and Sporecaster for white mold in soybeans can help with these fungicide application decisions. The National Predictive Modeling Tool Initiative is currently funding research on corn, wheat and cotton diseases that will help create future disease-forecasting models to aid farmers in fungicide application decisions. “One thing to remember with these models is they are not perfect, just as weather models are not perfect,” Robertson says. “But each year, we add more data to them to make them better.”

Use products that contain multiple fungicide classes and/or rotate fungicides of different classes. This helps reduce the selection of fungicide-resistant strains of fungi and prolongs the efficacy of individual fungicide classes.

Don’t cut fungicide rates. Using recommended label rates is crucial to slow resistance to fungicides classes.

Maximize fungicide efficacy by using appropriate nozzles and carrier volume.