No. 17 Article 3/July 17, 2009

Effects of Foliar Fungicides on Corn Stalk Quality

Note--be sure to read "Considerations for Applying Foliar Fungicides to Corn" in the July 10 issue of the Bulletin, which contains important information about risk factors and disease observations that can be used to make fungicide application decisions.

In the last week, I've received a few questions about the effect of foliar fungicides on stalk quality of corn. In 2008, stalk quality was evaluated in some of my corn fungicide research trials. When the black layer was evident in corn, stalks in each plot of these trials were split open with a knife and evaluated for stalk rot severity using a 0 to 5 scale (0 = no stalk rot evident and 5 = complete destruction of the pith with lodging below the ear; this scale was developed by Ron Hines, formerly with the University of Illinois).

Sample stalks with different values using the 0 to 5 stalk rot severity scale. (Photo courtesy of Ron Hines.)

Belleville and Dixon Springs trial. One of the research trials in which stalk rot was evaluated was planted at both Dixon Springs and Belleville. The Dixon Springs site was planted in early May, while the Belleville site was not planted until June because of heavy rainfall throughout May. At Dixon Springs, foliar disease pressure was low, and no statistical differences in foliar disease severity were observed between the untreated control plots and the fungicide-treated plots (Headline at 9 fluid ounces per acre, in this case). At Belleville, common rust severity was extremely high and caused considerable foliar disease severity in the untreated control (72% severity on the ear leaf). Headline fungicide was able to provide adequate protection, and treated plots had low foliar disease severity (less than 10% severity on the ear leaves of treated plants). Stalk rot severity followed a similar trend; it was relatively low at Dixon Springs, with fungicide having no effect. In contrast, at Belleville, stalk rot was statistically less severe in Headline-treated plants vs. non-treated plants.

Effect of Headline fungicide on stalk rot severity and foliar disease severity at Belleville and Dixon Springs, IL in 2008.

Urbana trial. A fungicide research trial conducted at Urbana included one hybrid considered to be susceptible to gray leaf spot (GLS) and another considered to be moderately resistant to it. In addition, this trial was mist-irrigated throughout the season to help ensure a favorable environment for GLS. Foliar disease severity was greatest on the susceptible hybrid, and the foliar fungicide (Headline at 6 fluid ounces per acre) reduced disease severity compared to the untreated control on this hybrid. On the moderately resistant hybrid, no statistical difference in foliar disease severity occurred between non-treated and Headline-treated plots. Similar to the foliar disease ratings, stalk rot severity was greatest on the GLS-susceptible hybrid, and Headline fungicide reduced stalk rot compared to the untreated control in this hybrid. On the hybrid that was moderately resistant to GLS, no differences between non-treated and Headline-treated plants occurred for stalk rot severity.

Effect of Headline fungicide on stalk rot severity of a hybrid considered to be susceptible to gray leaf spot and one considered to be moderately resistant to gray leaf spot at Urbana in 2008.

Relationship between foliar disease and stalk rot. Based on the results I've presented above, I think it is fair to state that there is a relationship between foliar disease severity and stalk rot severity. This relationship has been observed before by others and has been studied in the scientific literature. When foliar disease pressure is severe, the "blighted" leaves cannot produce enough photosynthates (sugars) to adequately fill the ear. When this happens, the plant may "rob" the stalk for additional sugars, which can damage the integrity of the stalk and allow additional colonization by stalk rotting pathogens. So foliar fungicides can impact stalk rot, but they likely do not directly control stalk rot pathogens; rather, they control foliar pathogens, which allows the plant to get more photosynthates from leaf photosynthesis because of reduced foliar disease severity. Knowing this, it's likely that we will only see improved stalk quality with fungicides when foliar disease pressure is high.

Funding for some of these research trials was provided by the Illinois Department of Agriculture Fertilizer Research and Education Council (FREC) and the USDA-CSREES North Central Regional Integrated Pest Management Program.--Carl A. Bradley

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