Controlling gray leaf spot in field corn

Gray leaf spot, caused by the fungus Cercosporazeae maydis, has become the most significant corn leaf disease in Iowa, especially in fields south of Highway 30. The elongated rectangular lesions are now familiar to many producers throughout the state. Controlling this disease requires an integrated strategy that includes hybrid selection, crop rotation, and possibly tillage or a fungicide.

Gray leaf spot symptoms

Ideally, a gray leaf spot management plan should include the following:

  • a moderately resistant hybrid (always check yield performance!),
  • crop rotation--one year in a non-host crop if surface residue is less than 35 percent, two years or more if surface residue is 35 percent or more (any crop other than corn is a non-host),
  • tillage reducing surface residue to 35 percent or less will significantly decrease disease severity, and
  • timely planting.

The fungus survives in crop residue on the soil surface, and spores can be blown from field to field. These characteristics influence the effectiveness of crop rotation and tillage as disease management methods. No-till or minimum-till fields in continuous corn are at the greatest risk for disease losses. In minimum-till fields, it may be desirable to perform tillage every few years to interrupt the build-up of gray leaf spot inoculum. Even where crop rotation and tillage are used, the disease may develop because of wind-blown spores. However, losses in these fields are less severe than in fields where corn residue is on the soil surface.

Fungicidal control of gray leaf spot (with Tilt) has become a common practice in seed corn production. In commercial corn production, Tilt can be considered an option if the strategy described above cannot be followed. The chance of profitably using fungicidal control is dependent on several factors including cropping history, tillage practices, location, the price of corn, and the weather. Fungicides should not be viewed as the first option in gray leaf spot management in field corn.

The decision to use Tilt to control gray leaf spot should be based on several factors, not just a disease threshold. Because Tilt cannot be sprayed after silking, and we cannot predict yield loss based on pre-silking disease levels, a disease threshold is not accurate for predicting the economic benefit of a fungicide application for gray leaf spot. In fact, there is currently no way to predict the economic benefits of a fungicide application for gray leaf spot on field corn. We can make research-based recommendations, but the weather will ultimately determine whether a fungicide is profitable.

The plants on the left were unsprayed; the plants on the right received a single application of Tilt at tasseling. The yield difference was 27 bushels per acre

If a fungicide is used on field corn, we have found that the most profitable approach is a single application of Tilt at tasseling. This should be considered under the following conditions (see table).

Gray leaf spot ratings for grouping corn hybrids using four different rating scales.

Group 1 (good)-9 (poor) 1 (poor)-9 (good) 1 (good)-5 (poor) 1 (poor)-5 (good)
Susceptible 7 or greater 3 or less 4 or greater 2 or less
Intermediate 5-6 4-5 3-3.5 2.5-3
Moderately resistant/Resistant 4 or less 6 or greater 2.5 or less 3.5 or greater
  • Susceptible hybrids--A Tilt application can be used profitably on these hybrids if the disease is present on the third leaf below the ear leaf (or higher) on 50 percent of the plants before tasseling.
  • Intermediate hybrids--A Tilt application can be used profitably on these hybrids only if conditions are very favorable for the disease. This is likely if:

    1. the field is south of Highway 30 or in an area with a history of gray leaf spot problems, and
    2. the previous crop was corn and there is 35 percent or more surface residue, and
    3. the field itself has a history of gray leaf spot problems, and
    4. the disease is present on the third leaf below the ear leaf (or higher) on 50 percent of the plants before tasseling, and
    5. the weather is warm and humid through July and August.
  • Moderately resistant or resistant hybrids--We do not recommend using a fungicide on these hybrids.

Always follow label directions when using a fungicide! Be legal and safe. For more information, see these publications available from any ISU Extension county office: Corn Gray Leaf Spot, IPM-49, and 1996 Iowa Corn Yield Test Gray Leaf Spot Ratings, Pm-660-GLS-96.

This article originally appeared on pages 87-89 of the IC-478(12) -- June 9, 1997 issue.

Updated 06/08/1997 - 1:00pm