Farmers who spray fungicide on corn “just as a precaution,” don’t have to worry this year about Extension specialists applying a guilt trip to them. 

Plant pathologists from several Corn Belt universities are suggesting fungicide applications on corn and others are recommending it. But farmers who apply a fungicide are doing it because there is more of a threat this year, than just doing it because it might boost the yield.

It has been a fun dance to watch for a number of years. Farmers have increasingly sprayed fungicides on corn because chemical dealers have suggested it might boost yields. There has never been proof of that, and somewhere between 40 percent and 60 percent of the time it will pay for the cost of the chemical and the application. However, there seems to be sufficient threats to corn this year that Extension specialists are suggesting that a fungicide application is a good idea. So, if you have already done it, you don’t have to go to bed with a guilty feeling (as if you really every did!)

The issue this year is delayed planting and a wet spring which favored the development of some foliar diseases in the corn canopy says Iowa State Plant Pathologist Alison Robertson

Looking at the state of Iowa where corn has some significant delays, she says it will be late July before most of it tassels and when diseases start during the early grain fill stage, there is an increased risk of reduced yield. But she says consider whether your field is at risk:

  1. Hybrids vary in their tolerance to diseases and fields with a susceptible hybrid should be sprayed with a fungicide.
  2. If scouting has found either gray leaf spot or northern corn leaf blight in the lower canopy, that is a signal for spraying. You might find either common or southern rust in the mid to upper canopy and those are not as serious.
  3. Certain environmental conditions will foster development of one disease over another.

So what do you use to control fungal issues? Since last year researchers from across the U.S. have created a decision guide for farmers on what is available for use and what should be used. The recommendations are available from the Corn Disease Working Group, which produced a Fungicide Efficacy Table, with recommendations that are applicable to most corn producing states.  

Is this table the final authority? No, says Ohio State University corn specialist Pierce Paul says, “This is not a complete list of all the fungicides labeled for use in corn nor is it a complete list of all the diseases managed with foliar fungicides; it is a list of some on the most marketed products, with ratings of their efficacy against some of the most common and economically important foliar diseases.” 

Paul says there is a likelihood of problems in Ohio with the excessive rain, and if that has been an issue for your farm, he says look for lesions on the leaves, “If your hybrid is susceptible, a fungicide is usually recommended when lesions are observed on the ear leaf and the leaves below the ear on 50 percent of the plants - 50 plants with lesions out of 100 plants examined across the entire field.”

At Purdue, Plant Pathologist Kiersten Wise agrees with Robertson at Iowa State and says, “Many fields across Indiana are currently at a younger growth stage than normal due to delayed planting, and therefore may be at greater risk for yield loss due to disease development. “ 

And she says don’t delay in scouting for fungal problems, “Research in Indiana indicates that fungicides are most effective at preventing yield loss due to disease when applied at the tasseling to early silking (VT-R1) growth stage. Scouting fields around V14, or just prior to tassel emergence, can help determine the level of disease pressure in a field.”

Wise is most concerned about gray leaf spot, and says if your corn is susceptible, take precautions now:

  1. Consider a fungicide application if the hybrid is rated as susceptible or moderately susceptible AND 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling. (watch her video).
  2. Consider a fungicide application if the hybrid is rated as moderately resistant AND 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling AND additional factors or conditions that favor disease development are present (residue present, favorable weather conditions)

If northern corn leaf blight is your issue, Wise says there are no treatment thresholds, but apply the same recommendations for gray leaf spot. And she says use the fungicide efficacy table for treatment recommendations.

Another issue that may have escaped you is any damage to corn from wind or hail. Those can open wounds in corn, and soybeans as well, and allow fungal spores to begin multiplying. Iowa State University plant pathologists say they do not require such wounds, but based on 2012 research, there were some conclusions that were drawn:

  1. Non injured plots yielded more than those injured by simulated hail.
  2. For soybeans there was more damage from hail at the R1 stage than at the R4 stage.
  3. For corn hail at R2 was more serious than hail at the VT state.
  4. Fungicides applied 7 days after a hail event recorded better yields than fungicides applied 2 days after a hail event.
  5. Fungicide application on corn usually resulted in better yields over corn that did not have a fungicide applied.


While university-based researchers may not be ready yet to endorse crop fungicide application “as a precaution,” this year does get their nod due to the likelihood of seeing fungal issues develop from the environmental issues impacting crops this year. Cool and wet problems can create leaf blights and scouting for them should be a priority.

Source: FarmGate blog