Corn and Corn Fungicides

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

The following was written by Dr. Arv Graubaskas, Extension Field Crops Plant Pathologist at the University of Maryland in his Maryland Field Crop Disease Notes. The issue of applying fungicides to corn when part of the field is replanted is addressed. This question has been asked and this is the best answer we know.

In some parts of the state the earliest planted corn is close to developing a tassel. There has also been a lot of corn that had to be replanted due to drowning. In many other cases corn was only recently planted because the soil finally dried out enough to get equipment onto the field. There is thus a tremendous range of growth stages of corn out there and in many cases in the same field. Considering the weather pattern is still pretty wet the legitimate question of whether or not a fungicide program is warranted is raised.

First don’t get sold on the plant health or plant physiology response to strobilurin fungicides (Headline, Quadris, and to a lesser extent Quilt and Stratego) alone. Key word here is alone. The usual sales pitch involves stating that millions of acres have been treated and the average response has been 8, 10 or even 15 bu/A. These averages include cases that had significant foliar diseases as well as cases with little or no disease. These fungicides are great disease management tools and when diseases like gray leaf spot are significant will outperform other classes of fungicides. The performance of these fungicides when diseases are clearly yield limiting is so outstanding that it skews the average response number.

Let me illustrate with my data from last season. I had 16 replicated “trials” across the state across a range of crop histories, planting dates and hybrids that compared untreated corn vs. a strobilurin fungicide. The average response across these trials was 7.3 bu/A, but in only five of these trials was the positive (yield beneficial) response statistically significant. More importantly in those five trials gray leaf spot was a serious problem and the actual responses to the fungicide in those five cases ranged from 24 to 38 bu/A. In other words, where I did not have enough gray leaf spot to cause losses and thus the fungicide could only provide a yield advantage through alteration of plant physiology the average response was -0.7 bu/A. The overall average looks good but it is skewed by the cases that really benefited which were those cases where the fungicide primarily worked as a disease control agent. The plant physiology or plant health type effect occurs in concert with disease control to often outperform other classes of fungicides when diseases are a problem. Where there is no disease the plant physiology side benefits have little or no effect on yield or stand on a consistent basis. The bottom line is an insurance program where there is no need for insurance will only cost you money. Use a fungicide when you have a real risk of a foliar disease. The highest risk of getting gray leaf spot, the primary foliar disease of corn, involves three factors: 1) a susceptible corn hybrid, 2) no-tilling corn into corn stubble, and 3) a relatively wet season.

There is one additional factor that needs to be considered regarding fungicides in corn, especially this season. It gets us back to the comment that there is quite a range of growth stages out there and in many cases in the same field. Fungicides applied by air in a fullgrown corn crop generally perform better if surfactants are used. The surfactants help the fungicide to penetrate the canopy and be better distributed throughout the canopy. Arrested ear syndrome, where the development to corn ears is damaged resulting in a percentage of small malformed ears, has been associated with fungicides applied with a non-ionic surfactant (NIS) or sometimes other products (certain formulations of tankmixed products) especially if the corn is in the late vegetative stages of development just before tassel. Most of the damage seems to be associated with NIS surfactants but the fungicide formulation and tank-mixed products are not completely exonerated. It is therefore important that if one decides on using a fungicide that it is applied at or after tassel formation is completed. If it is close or there are parts of the stand that are not yet in tassel then NIS surfactants should not be used and be wary of any tank-mix products. One final comment on fungicides in corn, these products have not been shown to directly reduce stalk rots. Fungicides affect stalk rots and therefore improve stand, when there is a foliar disease. There is no direct action of the fungicides applied at or near tassel on the stalk rot pathogens that develop much later in the season. However, significant loss of effective leaf area from foliar diseases predisposes plants to stalk rots. Therefore, when there is a foliar disease problem and you manage it with a fungicide you get the additional benefit of reducing stalk rots.

The bottom line is fungicides have been proven to be excellent disease management tools for foliar diseases in corn and have an indirect effect on lodging due to stalk rots of the crop. The additional benefits of strobilurin fungicides are the reason they often outperform other fungicide classes with regard to yield when diseases are an issue. They should be considered when the risk of a foliar disease is high. Otherwise they are expensive insurance with a relatively low chance of a return if significant foliar diseases do not develop.

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