Corn School: Weather calls the shots on disease

"In Iowa in 2025, historic levels of southern rust developed when increased inoculum met three times the normal precipitation from July through mid-August, combined with tropical-like temperatures, notes Robertson. These conditions were ideal for disease development and helped drive an explosion of the disease across the state. For susceptible hybrids under high disease pressure, yield losses ranged from 25 to 80 bushels per acre where fungicide was not applied."

"Ontario's experience with tar spot in 2021 followed a similar pattern: storm fronts, prolonged leaf wetness and conducive environmental conditions drove infection and spread. The longer plants remain wet and infected, the greater the risk of yield impact - especially in late-planted corn, says Tenuta. So what can growers do to mitigate disease risk when the weather turns in favour of pathogens? First, adopting earlier planting dates, where practical, can shorten the window of peak disease pressure."

"Second, targeted fungicide applications within the R1-R3 window remain one of the most consistent management tools for foliar diseases such as tar spot, particularly when forecasts indicate prolonged wet conditions."

Weather conditions strongly influence foliar disease development and resultant yield losses in corn. Increased inoculum combined with three times normal precipitation and warm temperatures in Iowa in 2025 produced historic southern rust outbreaks, causing 25 to 80 bushels per acre losses on susceptible hybrids without fungicide. Ontario saw tar spot outbreaks in 2021 driven by storm fronts, prolonged leaf wetness and conducive environmental conditions, with greater yield risk for late-planted corn. Mitigation strategies include earlier planting where practical, targeted fungicide applications in the R1–R3 window, and use of forecasting tools that combine regional weather data and pathogen dynamics.

Read at Realagriculture