Integrated Crop Management

Leaf diseases on small corn

Now that the corn has started to grow again, producers and crop scouts are starting to notice disease problems other than poor emergence, seed rot, and seedling diseases. We have had a couple of reports of leaf diseases and one sample was received at the ISU Plant Disease Clinic with symptoms of holcus spot, caused by the bacterium Pseudomonas syringae. Symptoms of holcus spot are light tan (sometimes almost white) round or oval spots on the lower leaves, initially about 1/4 inch in diameter, but sometimes growing larger and coalescing into irregular spots and streaks of dead tissue. The spots may appear water soaked at the margins or have a light brown border. Another disease that looks similar is bacterial leaf blight, caused by the bacterium Acidovorax avenae. This little-known disease was prevalent for a short time in early 1998. It has not been common since then, but it might be present this year as a result of the wet weather. Symptoms are elongated, water-soaked lesions from about 1/2 inch to several inches in length that turn yellow then brown and dry out. Individual lesions often are clustered together in a band on the leaf where infection took place in the whorl. Neither disease is known to cause economic damage in dent corn, but they can look serious when spots are numerous. Both bacteria survive in plant residue and splash onto leaves where infection takes place after a heavy rainfall. Symptoms often appear suddenly after a heavy rain but do not spread to new leaves. Because most of the corn has not been growing vigorously, some plants currently look very poor with leaf damage due to wind, blowing soil, cold temperatures, or holcus spot. But as the weather improves and the plants put on new leaves, these problems will likely disappear.

Anthracnose leaf blight on young corn leaves.

We also have started to see anthracnose lesions on small corn plants. Anthracnose leaf blight (caused by the fungus Colletotrichum graminicola) is usually one of the first leaf diseases to appear. The fungus survives in crop residue and its spores are splashed onto the leaves. Anthracnose is definitely more severe where corn follows corn. Symptoms are brown, oval, or elliptical spots (up to about 1/2 inch in length) 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 and contributes to postemergence stand loss, but it appears that it has been too cold for this disease to develop much until now. As with the bacterial leaf diseases, we do not expect any economic impact of these early-season leaf infections this year.

Holcus spot on corn.

Holcus leaf spot symptoms on corn.

This article originally appeared on pages 111-112 of the IC-486(14) -- June 18, 2001 issue.

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