DEVELOPMENT OF IPM-BASED CORN FUNGICIDE GUIDELINES FOR THE NORTH CENTRAL STATES

In 2007, it was estimated that 10% of corn acres in the U.S. (~ 9-10 million acres) were sprayed with a fungicide. This equates to $225 to 250 million when using an estimated application cost of $25/acre. For Illinois, Iowa, Ohio, and Wisconsin, where 38% of corn acres are grown, this would be approximately 3-4 million acres at an estimated cost of $75 to 100 million. Foliar fungicide applications for corn grain production have been rare because the value of the yield response may not be sufficient to offset the cost of chemical control. The probability of profitable fungicide use in corn for grain is strongly dependent on yield potential and susceptibility of the hybrid planted. Furthermore, for example, with an estimated application cost (product plus application) of approximately $25.00 per acre and corn price of $5/bu, a yield increase of at least 5 bu/A would be needed to offset the cost of fungicide application. Reports from fungicide industry-based research suggest that the application of fungicides leads to yield increases as high as 20 bu/A in some years. Most of these reports, however, were provided without a complete characterization of the conditions under which the trials were conducted. Very little information was given about disease onset relative to grain fill, the overall level of disease, edaphoclimatic conditions, and hybrid susceptibility, tolerance, and yield potential. Results from university-based replicated trials suggest that the effect of fungicide application on yield is less clear-cut than industry data seem to suggest. In some university trials, positive yield responses were associated with disease control, in others there were no yield benefits even when diseases were controlled, yet in other trials positive yield responses were observed in the absence of high disease pressure. Unwarranted use of foliar fungicides may increase the development of fungicide resistance. Quinone outside inhibiting (QoI) fungicides are especially at risk for fungal pathogens to develop resistance to them. The Fungicide Resistance Action Committee (FRAC; www.frac.info) characterizes QoI fungicides as having a HIGH RISK of resistance being developed. To date, over twenty phytopathogenic fungal species have developed field resistance to the QoI fungicides. It is important that these fungicides are used properly and only when needed. The majority of foliar fungicide applications to corn in the 2007 growing season were with a stand-alone QoI fungicide (pyraclostrobin; Headliner), which can lead to increased selection for Cercospora zeae-maydis isolates with reduced sensitivity to QoI fungicides. Finally, current fungicide timing and thresholds are based on results from past studies conducted using triazole-based fungicides with modes of action and pre-harvest intervals that are different from those of currently recommended fungicides. Uniform trials, using standard protocols, are needed to reevaluate current recommendations and to develop thresholds for the current group of fungicides. The combination of factors necessitates the need to provide a cohesive set of recommendations to growers.