Tuesday, July 09, 2013
Gray Leaf Spot on Corn Leaf

Kentucky Corn: Keep Watch for Gray Leaf Spot

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This is just a reminder of the factors that favor a positive yield response from a strobilurin fungicide in corn. Figure 1 lays this out graphically.

Figure 1. Factors that increase the probability of benefit from a foliar fungicide in corn. The top five factors are often the most important.

Figure 1. Factors that increase the probability of benefit from a foliar fungicide in corn. The top five factors are often the most important.

Disease pressure is, of course, the primary factor determining the probability of a benefit from a foliar fungicide in corn. With all the rain this season, gray leaf spot (Figure 2) may be widely active. Producers may wish to scout fields for evidence of disease activity.

Scouting guidelines for gray leaf spot originated from Iowa State University, and many universities have adopted these with little to no modification. They are:

1. Consider a fungicide application if:
a. The hybrid is rated as susceptible or moderately susceptible, AND,
b. 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling.

2. Consider a fungicide application if:
a. The hybrid is rated as moderately resistant, AND,
b. 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling, AND,
c. Additional factors or conditions that favor disease development are present (See Figure 1)

These are approximate guidelines, and the disease can be unpredictable. Nevertheless, these guidelines can be helpful tools in making rational spray decisions.

Figure 2. Young lesions of gray leaf spot of corn. Image of Doug Jardine, Kansas State University.

Figure 2. Young lesions of gray leaf spot of corn. Image of Doug Jardine, Kansas State University.

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