Issue No. 11, Article 6/June 8, 2007
Prophylactic Fungicide Applications on Corn: A Good Practice?
If rumors hold true, Illinois will likely see higher use of foliar fungicides on hybrid corn in the 2007 growing season than in any of the previous seasons added together. Three factors seem to be driving this potential increase in fungicide use: market price of corn; increased corn-on-corn acreage, which increases foliar disease risk; and marketing of fungicide products. Although some of these factors may be good reasons to consider purchasing a corn fungicide, additional factors should be considered: the disease susceptibility of your hybrid; the weather (i.e., has conducive weather, namely rainfall and humidity, been present, or has weather been forecasted to be conducive for disease at tassel emergence and beyond?); and your scouting observations.
Scouting observations are very important and can be used to help make fungicide decisions. Below are some fungicide guidelines based on scouting observations (adapted from G. Munkvold, Iowa State University):
- Susceptible or moderately susceptible hybrids. A fungicide application should be considered if the disease is present on the 3rd leaf below the ear or higher on 50% of the plants before tasseling.
- Intermediate hybrids. A fungicide application should be considered if conditions and factors are favorable for disease. These include the following situations: there is a history of problems with gray leaf spot or other disease; the previous crop was corn and at least 35% surface residue; disease is present on the third leaf below the ear leaf or higher on 50% of the plants before tasseling; the weather is warm and humid through July and August.
- Moderately resistant or resistant hybrids. A fungicide application is generally not recommended, but field scouting is still important.
Prophylactic fungicide applications without scouting observations that favor an application are less likely to provide a yield benefit, and there are potential secondary adverse effects of applying an unwarranted fungicide, including the development of fungicide resistance.
A little background on the fungicides that will be applied to corn this season: these products contain a fungicide chemistry known as the "strobilurins." Some fungicides, like Headline and Quadris, contain only a strobilurin component; other fungicides, like Stratego and Quilt, contain a strobilurin component plus propiconazole, which is from another fungicide chemistry known as the "triazoles." The strobilurin fungicides have been characterized as being at high risk for the development of resistance to them, and resistance has been documented in several fungal pathogens that cause diseases of different crops.
Some of the same factors that encouraged the development of glyphosate herbicide-resistant palmer amaranth and waterhemp in the "weeds world" may also encourage the development of strobilurin fungicide-resistant pathogens in the "disease world." The pollen grains from palmer amaranth and waterhemp are extremely small, which allows their transport--and hence transport of potentially resistant genetic material--across long distances. Palmer amaranth and waterhemp also produce hundreds of thousands of seeds within about 40 to 60 days of germination. Similarly, many fungal pathogens of corn produce very mobile spores that can travel relatively long distances, and the number of spores that can be produced by fungal pathogens under suitable conditions are far greater than any weeds.
To reduce the risk of fungicide resistance development, follow these practices:
1. Apply a fungicide only when it is warranted. Use IPM practices, and base fungicide applications on good scouting observations.
2. Observe recommended fungicide rates. Applying a fungicide at a sublethal rate can increase the risk of fungicide resistance development.
3. Mix or alternate fungicides with different modes of action.
4. Do not make back-to-back applications of fungicides with the same mode of action.
For more information about fungicide resistance, go to the Fungicide Resistance Action Committee (FRAC) Web site.--Carl A. Bradley and Matt Montgomery