Fungicide applications of strobilurins to corn and soybeans when there is little disease

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Some growers may be contemplating the use of foliar fungicides on hybrid corn this season, due to higher market prices for corn, marketing of fungicide products, or concern over increased disease risk from planting corn-on-corn. Growers may have similar thoughts of applying foliar fungicides to soybeans due to higher market prices.

Conditions are not favorable for the development of most foliar diseases in corn. In many parts of the state, rolled leaves can be seen during the middle of the day, clear evidence that corn is experiencing moisture stress. Conditions for foliar diseases of soybean are similarly unfavorable. Substantial areas of Michigan are experiencing droughty conditions, and the extended forecast is for above normal temperatures and below normal rainfall through the end of July.

In Michigan, foliar diseases have not commonly been serious enough in field corn or soybeans to warrant the use of a fungicide, although they are used regularly in seed corn. The 2007 Insect, Nematode and Disease Control bulletin in Michigan Field Crops, E-1582 contains a complete listing of fungicides registered to control foliar diseases in both corn and soybeans .

However, the issue of applying foliar fungicides to hybrid corn or soybeans [primarily Headline and Quilt (corn) and Headline (soybeans), which contain an active ingredient in the family of fungicides known as strobilurins] when there is little disease continues to come up. In many parts of Michigan, corn is starting to tassel, and so we are entering the critical period of development that determines yield. Similarly, soybeans are flowering and nearing their critical period of development. With high prices for both corn and soybeans, growers may be looking at avenues for increased yield. Certain strobilurins can change the plants physiology in a manner that sometimes enhances yield, even when diseases are not present, but there is no cookbook formula to consistently achieve this result.

So far, there has been little university-conducted research in Michigan to evaluate the yield effects of using strobilurins on field corn where there is little disease present, and this is an area in need of greater study. Our field research with soybeans has not demonstrated significant increases in yield using strobilurins in soybean where little disease is present. In some other states, university research has found increased yield in soybeans, even when diseases are not present. However, research results in corn have been much more mixed. Most of the studies done on field corn show no statistically significant improvement in yield from foliar fungicides applied to corn in trials when there are minimal amounts of disease through the grain fill period. A few studies showed yield increase when strobilurin fungicide disease activity was low, and a fraction showed yield increases that would more than pay for the cost of the application at $4.00/bu corn prices.

In the article “Can foliar fungicide raise corn yield when there’s little disease?”(The Bulletin, University of Illinois, Extension, No. 14 Article 10/June 29, 2007 http://ipm.uiuc.edu/bulletin/) Emerson Nafziger, Professor of Agronomic Extension writes, “Because we are talking here about the decision to apply fungicide for something other than disease control, hybrid traits such as foliar disease resistance might not help us make a better decision. Previous crop might also be of little help, since the effect of previous crop on current crop stress level might be more important than its effect on disease. The fact that we saw some yields above 240 bushels per acre and no effect of fungicide indicates that high yields are not always associated with a positive effect of fungicide on yield, either. We simply do not have a way to predict when there might be a yield benefit from the use of a foliar fungicide when there is not a disease present. We have clear evidence that such an effect can take place, but because it’s likely to be a product of complex interactions between crop stress level, weather, and previous growing conditions, we do not know in advance, based on current knowledge, when it will happen.”

Resistance is an inherited change in a plant pathogen’s susceptibility to a fungicide. Intensive use, overuse or misuse of certain fungicides can result in the development of resistance. Using fungicides containing strobilurins as a product for yield enhancement when disease activity is below levels that threaten yield, contributes to the risk of development of fungicide resistance. Spores can be transported over long distances and millions of spores are produced. Remember, these products are fungicides. Strobilurins are in the “high risk” category for resistance. Resistance has been already documented in several crops for several different pathogens.  

If you still plan to “experiment” with the plant health aspect of strobilurins, Ohio State developed the following guidelines for setting up a fair evaluation in your field to determine whether or not these products are truly providing any benefit. (From the C.O.R.N. OSU newsletter July 7, 2007 “So you are trying fungicides on corn and soybeans, how to evaluate if they are worth the cost?” - Anne Dorrance, Pierce Paul, Dennis Mills.)

  1. Know what the variety or hybrid is. For both corn and soybeans only the moderately susceptible and highly susceptible hybrids and varieties have demonstrated yield impacts in fungicide applications
  2. Have more than one check strip, wider than your combine, and space them across the field. Three should be plenty, but two is not enough
  3. In your comparisons, do not include the parts of the field where you have weed escapes or along tree lines. These areas are going to yield less, so this is false data whether it was treated or not. Make sure other variables such as soil types, soil fertility variations and pest issues are not biasing your field experiment
  4. Approximately three weeks after applications, walk some of the strips. For corn, look at the ear leaf; what percent leaf area has got lesions in the treated vs nontreated? Is gray leaf spot, anthracnose, or northern leaf blight present? For soybeans, look at the upper canopy for frogeye or downy mildew and on the lower canopy look at how much brown spot is present.
  5. Take averages. When you do harvest your fields, take several strips, both untreated and treated. Then take the average of the untreated strips and compare that average to the treated. Fields are not uniform and with our stand issues and unevenness across many fields this year, there is going to be even more variability.

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