Leaf diseases have begun to appear on corn in many parts of the state. Leaf spots on the lower leaves of plants have caused concern among some corn producers, but relatively dry weather has kept the diseases from becoming serious. Several diseases have been observed, but gray leaf spot is the only one that appears to be a potential threat to yield this year.
Anthracnose leaf blight
|||Anthracnose leaf blight on corn.|
Anthracnose leaf blight (caused by the fungus Colletotrichum graminicola) is usually the first leaf disease to appear. Symptoms are brown, oval, or elliptical spots (up to about 1/2-inch long) with a dark brown or purplish border, often surrounded by a yellowed zone. There may be black speckles within the dead tissue. Sometimes the symptoms are limited to the leaf margins. Anthracnose can cause significant damage to very young plants, but I did not see it this year until the last two weeks. Now it is causing some of the lower leaves to wither in some fields, but this will not affect yields.
|||Common rust on corn.|
Common rust (caused by the fungus Puccinia sorghi) has been very light so far. Conditions have not been favorable for this disease.
|||Holcus spot on corn.|
Holcus spot (caused by the bacterium Pseudomonas syringae) has appeared in a number of fields. Symptoms are light tan (sometimes almost white) round or oval spots on the lower leaves, usually about 1/4-inch or less in diameter. The spots may appear water-soaked at the margins or have a light-brown border. This disease is not known to cause economic damage, but it can look serious when spots are numerous.
Gray leaf spot
|||Early symptoms of gray leaf spot.|
Gray leaf spot (caused by the fungus Cercospora zeae-maydis) has started to appear. In a tour of southeast Iowa, we found the disease in most fields from Indianola to Mt. Pleasant. The good news is that there was an average of less than one lesion per plant in each field. This is a low level of disease, but it is too soon to tell whether gray leaf spot will be a significant problem. In seed corn fields, Tilt applications are already underwayon susceptible inbreds. Those considering Tilt on field corn should be scouting to determine if an application is justified. See the June 9  and June 23  issues of Integrated Crop Management for guidelines on controlling gray leaf spot.
This article originally appeared on pages 138-139 of the IC-478(17) -- July 14, 1997 issue.