Farmers have seen wide range of yield responses to foliar fungicide on corn
The results of these research plots generally mirror what corn farmers see in their fields when they use foliar fungicides—a wide range of yield responses. "I can't claim to have it all figured out, but field experience along with what we've learned from conversations with other agronomists, researchers and farmers have provided some perspective," says McGrath. He writes the CSI:Iowa (Corn Soybean Insight) column each month in Wallaces Farmer magazine.
Most everyone agrees that the No. 1 factor impacting fungicide application profitability on corn is management of common diseases—such as gray leaf spot, common rust, etc. "If corn diseases are present, yield responses to foliar fungicide applications are typically higher on corn hybrids that have low disease resistance scores," he notes. "If disease levels are high enough, hybrids with solid disease resistance may respond well, too."
Warm, humid conditions around the time of grain fill favor the development of diseases. "We don't hope for diseases, of course, but we do hope that we get some much needed moisture from timely rains, even if it increases the humidity," says McGrath. Crop history of the field and the amount of crop residue left on the soil surface can also contribute to development of corn disease. "With several pathogens that survive in corn residue, corn-on-corn and other high-residue systems can increase disease levels," he adds.
Timing of fungicide application can influence odds of getting positive return
Geography can also influence the amount of disease present in a cornfield. For example, southeast Iowa tends to be warmer and more humid than much of the rest of the state and historically has had higher levels of diseases. "While sometimes we see fungicide applications increase yields in fields with low disease pressure, increasing disease pressure is a better indicator to the potential profitability of treating," says McGrath.
Application timing can influence the odds of getting a positive return to the application of corn fungicide. Taking into consideration label recommendations and your own field observations is critical in deciding when to apply foliar fungicides on corn. "If applied too early, the residual effects of the product may be gone as diseases set in," says McGrath. "If applied too late, the fungicide may not effectively control the diseases already established. Pay attention to the growth stage of the corn plant. Most agronomists agree that the full tassel stage or VT through the blister stage or R2 is the optimum timing if a fungicide is needed."
You need to scout some cornfields more closely than others to make decisions
You will need to scout corn hybrids more intensively if they have moderate-to-low disease resistance. Scout more often if the weather's warm and humid and if rainy weather is present or predicted for July and August. And watch corn-on-corn, high-residue fields closely. Late-planted corn often is more susceptible to diseases.
"You also need to confirm that the fungicide is labeled to control the diseases present in the particular field you are considering treating," says McGrath. Read a little more about ISU's corn work with fungicides here. What about soybeans? Some farmers are asking about the timing of application of foliar fungicides for soybeans. McGrath suggests you read this article online--here is a good article on fungicides in beans.