Gray leaf spot, caused by the fungus Cercospora zeae-maydis, has become the most significant corn leaf disease in Iowa, especially in fields south of Highway 30. Wet conditions earlier this month followed by the current warm, humid weather will probably be favorable for gray leaf spot this year. Extension field specialist Mike White (south central) has reported that the disease has appeared already in southern Iowa. Controlling this disease requires an integrated strategy that includes hybrid selection, crop rotation, and possibly tillage and a fungicide.
||Early symptoms of gray leaf spot.
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 nonhost crop (any crop other than corn is a nonhost) if surface residue is less than 35 percent, and two years or more if surface residue is 35 percent or higher;
- tillage--reducing surface residue to 35 percent or less will significantly decrease disease severity;
- timely planting; and
- scouting for symptoms--this should be done starting at V8 in field corn (earlier in seed corn) if a fungicide application is being considered.
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 buildup 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 field corn production, Tilt can be considered an option if the strategy described above cannot be followed. The chance of profitability by using fungicidal control is dependent on several factors, including cropping history, tillage practices, location, the price of corn, yield potential of the field, 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 presilking disease levels, a disease threshold alone 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.
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 (also see table):
Susceptible hybrids. A Tilt application can be used profitably on these hybrids if the disease is present on the third leaf below the ear (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 favorable for 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, (2) the previous crop was corn and there is 35 percent or more surface residue, (3) the field itself has a history of gray leaf spot problems, (4) the disease is present on the third leaf below the ear (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! Leaving an unsprayed check strip is a good way to gauge disease intensity and effectiveness of the fungicide. Be legal and safe. For more information, see Corn Gray Leaf Spot (IPM 49) and 1997 Iowa Corn Yield Test Gray Leaf Spot Ratings (Pm 660 GLS-97). These publications are available from any county extension office.
||Severe gray leaf spot damage.
Gray leaf spot ratings for grouping corn hybrids by using four different rating scales.
||1 (good)-9 (poor)
||1 (poor)-9 (good)
||1 (good)-5 (poor)
||1 (poor)-5 (good)
||7 or greater
||3 or less
||4 or greater
||2 or less
||4 or less
||6 or greater
||2.5 or less
||3.5 or greater
This article originally appeared on pages 129-130 of the IC-480(17) -- July 6, 1998 issue.