• By Adam Probst, Extension Agent

Agriculture & Natural Resources -Corn disease scouting

Sources: Dr. Kiersten Wise; UK Plant Pathologist Farmers are annually concerned about corn disease, and this year will be no exception. Corn is moving through growth stages quickly, and the warm, humid weather in many parts of Kentucky has been conducive for foliar disease development. Fungicides are commonly promoted to reduce the impact of foliar disease in corn, but with tight margins and low crop prices, it is important to scout fields this year and check hybrid resistance ratings prior to fungicide application. Also, consider crop production factors and their impact on disease development. Planting continuous corn or conservation tillage programs increase the risk for foliar disease development since the fungi that cause these diseases survive in residue. Additionally, irrigated fields are at higher risk, since irrigation creates an environment favorable for disease development.

Scouting over the next few weeks and just prior to tasseling can help determine if fungicide applications are needed. Although disease levels will continue to build over the course of the season, University research indicates that foliar fungicides applied at tasseling or early silking (VT-R1) provide optimal foliar disease control and also the best chance for seeing a yield response, compared to applications that occur after blister (R2).

It is important to accurately identify foliar diseases before deciding if a fungicide application is needed. There are several diseases appearing in western Kentucky, some of which do not warrant fungicide application.

Gray leaf spot (GLS) is caused by the fungus Cercospora zeae-maydis. Early symptoms of gray leaf spot are observed on leaves as tiny lesions surrounded by a yellow halo. It is difficult to diagnose GLS when the lesions are very small, but these lesions will elongate into narrow, rectangular, brown to gray spots, expand parallel to the leaf veins, and may grow to over one inch long on susceptible hybrids.

Symptoms vary depending on hybrid susceptibility, and hybrids with some level of resistance to gray leaf spot may only have small, jagged lesions rather than the long, rectangular shape characteristic of lesions on more susceptible hybrids. The fungus that causes gray leaf spot survives winter in residue, and typically lesions are first observed on the lower leaves, and move up the canopy as the season progresses. Iowa State University developed guidelines to determine when a fungicide may be necessary to prevent yield loss from gray leaf spot.

These thresholds incorporate hybrid susceptibility ratings to gray leaf spot and disease levels prior to tasseling:

1. Consider a fungicide application if the hybrid is rated as susceptible or moderately susceptible and 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 the hybrid is rated as moderately resistant and 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 additional factors or conditions that favor disease development are present (residue present, favorable weather conditions). Scout resistant hybrids for disease problems, but in general, fungicide applications to resistant hybrids are not recommended and will not consistently result in increased yield. Fungicide efficacy of specific fungicide products for gray leaf spot are described in the updated fungicide efficacy table for management of corn diseases, which is developed by the national Corn Disease Working Group, and posted on the Crop Protection Network website.

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