Level of disease should determine corn fungicide use

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Most university plant pathologists continue to contend that foliar fungicide applications to corn should be driven by disease present on the corn leaves and risk factors for the disease pressure increasing after the tasseling (VT) stage.

Carl Bradley, Department of Crop Science, University of Illinois, in speaking to attendees of the University of Missouri Crop Management Conference, said he looks at fungicide use as a tool that should increase yields enough to more than pay for the cost of the fungicide and application.

“For foliar disease control and yield response, we tend to see more consistent profitability when fungicides are applied because of a disease threat,” Bradley said.

From research run the last five years, he said, “Less than one-half of the time we would have been profitable when spraying fields that had a relatively low level of disease.”

The main discussion about fungicide use on corn is for an application between the VT and brown silk (R2) stage late in the growing season.

Bradley’s recommendation continues to be for making a fungicide application based on scouting. “For hybrids that are susceptible to moderately susceptible, consider a fungicide application when disease is present on the third leaf below the ear or higher on half of the plants in the field,” he explained.

“We are not talking about a certain level of disease or a certain level of severity. We are talking about where is the disease on the plant and how many plants are affected prior to tassel with the idea being that if there is disease present on that third leaf below the ear on half the plants that disease will develop into levels that will be damaging to yields,” he continued.

Disease resistant hybrids can also have disease pressure requiring fungicide application when the same scenario of third leaf below the ear and half the plants are infected, but discovering a level of infection of a disease such as gray leaf spot that requires treating isn’t common with resistant hybrids, Bradley said.

If a corn field is sprayed with a foliar fungicide and there is little or no disease present, but a yield bump occurs, then the fungicide is having an affect other than disease control on the plant. “There are some other factors that can occur with these fungicides that are not related to disease control,” Bradley said.

But Bradley still contends that the unexplained yield bump will not be sufficient to show profit for a corn grower unless the fungicide application is also controlling some level of disease that was occurring in the field, too.

In fields infected with gray leaf spot or southern rust, Bradley has also looked at a late-season fungicide application reducing stalk rot. Again, in general, if disease pressure justified a fungicide application then stalk rot problems could be reduced compared to untreated disease infected corn plants. But hybrids resistant to the disease were less likely to show an improvement in stalk quality, he explained to the Crop Management Conference crowd in Columbia, Mo.

As for applying a fungicide when corn is in the V6 stage of growth, Bradley isn’t sold on that application, and if an ag retailer is going to sell a farmer on trying an application at that stage then it should be done in a trial situation with treated and untreated corn next to each other in a field, he suggested.

He said, “My take on the V6 application is that there really is not much level of disease out there at that time, and a V6 application is not going to give protection throughout the entire season. So, I think the verdict is still out a little on those V6 applications.”

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