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Corn foliar diseases and fungicides in Minnesota - delving into the uncertain

Dean Malvick, Extension Plant Pathologist

Interest in using foliar fungicides on hybrid corn has recently reached unprecedented levels in Minnesota. Why, when, and where should fungicides be applied to hybrid corn for grain production? No easy answers are available. Moreover there are no answers that apply to every field in all the different cropping regions in Minnesota, but some of the elements that go into this question can be discussed.

First, why are fungicides applied to corn in Minnesota, if they are applied? The products are generally very good fungicides that are designed to control foliar fungal diseases. This means that if they are applied properly at the right time when foliar diseases are present, they can be very effective for disease management. But how often do we have significant levels of foliar disease on corn in Minnesota? The answer seems to be rarely. This is especially true this year when dry conditions have reduced the likelihood of foliar diseases.

Foliar diseases occur on hybrid corn in Minnesota at low levels every year, however, comprehensive scouting has rarely been done to get a good understanding of which diseases occur and at which levels. Some of the common fungal leaf diseases in Minnesota are eyespot, northern corn leaf blight, anthracnose, common smut, and common rust, although gray leaf spot and northern corn leaf spot also occur. For identification of corn diseases, an excellent book Field Guide to Corn Diseases with multiple color photos of many diseases is available through the University of Minnesota Extension.

More disease could occur on corn in the future as corn-on-corn acreage increases and tillage decreases in some areas. Given that foliar diseases seem at present to rarely occur at levels that cause yield loss on corn in Minnesota, and that scouting for disease before applying fungicides is not a common practice, why are fungicides applied? There is another reason.

Prophylactic application of fungicides on corn in the absence of much disease is a new approach that is being promoted because it may provide yield benefits that are independent of disease control. One argument is that some fungicides reduce plant stress or improve the physiological functioning of plants, thus increasing yields. It is difficult to know if this occurs in the field. Another idea is that some diseases may occur at low levels that result in yield loss, and these diseases may be reduced by fungicides. If this is the case, scouting should help in determining if fungicides are likely to provide a significant yield benefit.

To some, the question about fungicide use on corn simply comes down to the idea that fungicides may be good if they can routinely provide an economic benefit beyond the cost of the product and its application. Do they provide this benefit? There is very little data available to support widespread use of foliar fungicides on hybrid corn. Not many replicated research trials have been done in the Midwest, and the same is true for Minnesota. Two University of Minnesota trials were conducted in 2006 with multiple corn hybrids, one at the Southern Research and Outreach Center in Waseca and one at the Southwestern Research and Outreach Center near Lamberton (results, pdf). These trials demonstrated no significant yield benefit after application of Headline® fungicide. Additional trials will be conducted in 2007 in Minnesota, as well as in other Midwestern states, to build our base of information on when and where corn foliar fungicides are most likely to provide a significant benefit.

On-line newsletters from Illinois (Foliar Fungicides, Prophylactic Fungicides), Iowa (Fungicides increasing), and Indiana (Are Fungicides valuable?) provide other perspectives on the use of fungicides on corn. These articles also suggest that the greatest benefit will occur when fungicides are applied to control disease. Foliar diseases are typically more common in these states than in Minnesota. This does not negate the possibility of fungicidal benefits beyond disease control under some situations.

Several points should be kept in mind when considering the use of foliar fungicides on corn in Minnesota. First, these fungicides will generally perform best when they are controlling significant levels of foliar diseases. Second, if foliar fungicides are used, it is recommended to leave untreated check strips in the same fields to help assess whether the fungicides improved corn yields. Third, corn foliar fungicides may result in yield increases under some conditions and in some locations even when disease levels are very low, but minimal solid information is available yet to determine when and where they are most likely to provide this benefit.

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