Leaf diseases appear to be widespread on corn already this season. This is not too surprising, considering the amount of rain we have had. Fortunately, most of the symptoms so far are due to holcus spot or anthracnose, which do not tend to spread rapidly among the plants during mid-season. These diseases currently seem severe in some fields following the heavy rains, but the new leaves should escape infection (if the rain lets up). No in-season control measures are recommended for these two diseases. Other diseases are appearing, and some of these are a bigger concern, especially in seed corn.
||Anthracnose symptoms usually include a dark border and leaf yellowing.
Anthracnose leaf blight (caused by the fungus Colletotrichum graminicola) is often 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 it is limited to the leaf margins. Anthracnose can cause significant damage to very young plants. Now it is causing the lower leaves to wither in some fields and is even showing up on some of the newer leaves.
||Holcus leaf spot symptoms are pale round spots with or without a border.
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 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. It often appears suddenly after a heavy rain but then does not spread to new leaves.
Common rust (caused by the fungus Puccinia sorghi) has started to appear in some seed corn fields. Our weather has been favorable for rust. This disease is potentially very damaging to seed corn, and seed producers need to keep a close eye on it.
Northern leaf spot (caused by the fungus Bipolaris zeicola) also has been found in many seed fields, in some cases severe enough to warrant a fungicide application. The symptoms of this disease differ among the races of the fungus, but Race 3 seems most common, with roughly rectangular lesions about 1/8 inch wide by 1/2 inch long. This race greatly resembles gray leaf spot and sometimes they cannot be distinguished in the field.
So far no gray leaf spot, but it won't be long now. Seed corn producers need to consider fungicide options at this time (see last week's ICM newsletter article, Controlling leaf diseases in seed corn, pages 115-116). It's still too soon to tell if the gray leaf spot will be severe enough to use a fungicide on hybrid corn, but the wet conditions indicate that it might be worthwhile in some fields.
This article originally appeared on page 124 of the IC-480(16) -- June 29, 1998 issue.