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Factors to Consider in Your Corn Fungicide Decision!

The interest around fungicide application in corn is not only on disease control but on "plant health" impacts as well. Here are a few factors that you should include in your corn fungicide decision process:

Disease Potential

First and foremost, we cannot emphasize enough the importance to scout your fields for the presence of disease. This can be done by the grower or through the use of crop consultants / ag retailers, etc. You will need to assess the levels of each disease present. Knowing which disease is present is important because some have a greater potential of impacting yield than others. The "big three" in terms of corn leaf diseases for Ontario are Northern Leaf Blight (NLB), Common Rust (CR) and Gray Leaf Spot (GLS). Other diseases such as eyespot can be at high levels, but their overall yield impact is much lower then NLB, CR and GLS. In our inoculated trials at Ridgetown, it has been common to lose more than 20 bushels from NLB and CR, but for eyespot it is a vastly different story (Figure 1).

Hybrid Susceptibility to Disease

It is important to remember that hybrids have differential responses to the presence of pathogens or disease pressure. Some hybrids have the ability to protect or minimize the development of disease symptoms, but others, unfortunately, are not well suited for an attack by many of our common pathogens. Good examples are trials we conducted in 2008. We evaluated four different hybrids with varying disease tolerances from "high to very low". As expected, hybrid "D", which was the most susceptible, had the highest disease levels and largest yield loss. Consider hybrid susceptibility (or corn type such as seed corn) when selecting hybrids during the winter and during your fungicide decision process.

In 2008, five commercial seed corn fields were selected in the Chatham area in southwestern Ontario. This area represents the primary seed corn production area in the province. At each location, four treatments were established which included 3 fungicides (Headline, Quadris and Quilt) and an untreated control in which no fungicides were used. Fungicides were applied with the high clearance John Deere Field Research Sprayer maintained by the University of Guelph Ridgetown Campus. Plots were allowed to be naturally infected and disease ratings were recorded pre fungicide application.

Yield increases were observed for all five fields individually regardless of which of the three fungicides were applied when compared to the untreated control. All three fungicides resulted in a significant yield increase when the data from all five locations were combined as compared to the untreated control. Although the difference between the fungicide treatment and untreated control was significant, there was no significant difference between the three fungicides when averaged across fields.

Hybrid Responses to Fungicides

The interactions among foliar fungicides and corn hybrids are not well understood; they most likely play a critical role in yield responses. In order to answer this question, a multi-year study was started in 2008 and continued this year. We found significant differences in how hybrids responded to Headline. The differential hybrid responses to the fungicide could explain why there has been such variation or inconsistency in results from on-farm strip trials conducted in Ontario and the US.

The trials were conducted at three locations in southwestern Ontario (Ridgetown, Exeter and Wingham Ontario) during the 2008 growing year. At each location, 21 commercial corn hybrids appropriate for that location were selected and planted. The 21 hybrids at each location were treated with or without the foliar fungicide Headline (BASF) at the tassel stage (VT) for a total of 168 plots per location. Headline application was done with the high clearance John Deere Field Research Sprayer maintained by the University of Guelph Ridgetown Campus. Headline was selected in this trial since much of the data generated in Ontario and the US has been associated with this product plus space and resources were limited. Plots were allowed to be naturally infected and disease ratings were recorded pre and post fungicide application.

As expected, fungicide response varied among hybrids and the percentage of hybrids that had a statistically significant response (p<0.05) varied by location as well (Wingham - 38%, Ridgetown - 33%, Exeter - 19%). Interestingly, there was little relationship between fungicide yield response and the yield potential of the hybrid and or the hybrids susceptibility to leaf diseases. This is in contrast to other studies which have found a greater economic response to fungicides in hybrids which lack a good disease package. One reason for our difference in results from these studies could be the low disease levels we had in these plots. In other studies we conducted (see point 3), where a susceptible check was used, we found the greatest yield response occurred where foliar leaf disease levels were high. We now know that both the hybrid and the presence of leaf disease are important factors in the response to fungicides.

Other Factors that Must be Taken into Consideration

1. Crop Value - is it high or low - do the math!

2. Application Cost - ground vs air. Remember to include tramp damage as well

3. Future Weather - is the long-term environmental conditions favourable for continued disease development?

Figure 2. Greatest disadvantage level was shown on the susceptible Hybrid D at both locations

Figure 3. Significant yield losses were found again on the most susceptible corn Hybrid D at both locations

Figure 4. All 3 fungicides provided signifcant disease and yield compared to untreated control