No. 9 Article 3/June 3, 2011

Holcus Leaf Spot of Corn in Southern Illinois

Holcus leaf spot, a somewhat uncommonly observed corn disease, was diagnosed last week from samples sent from a couple of fields located in southern Illinois. Holcus leaf spot is caused by the bacterium Pseudomonas syringae. Symptoms appear as light-colored round spots that can be observed on the foliage of corn and a few other grassy weed hosts. Some spots may have edges with a "water-soaked" appearance. The symptoms can be easily mistaken for injury caused by herbicide drift, and the only way to get a confident diagnosis is to send samples to a plant diagnostic lab (such as the University of Illinois Plant Clinic).

Like most bacterial diseases, holcus leaf spot may be more severe when wounds are created on the leaves by high winds and/or hail. The pathogen is most effective at infecting corn and causing disease during warm, wet weather (75 to 85°F).

Symptoms of holcus leaf spot on corn leaves. (Photo courtesy Dr. Jason Bond, Southern Illinois University.)

Most historical observations of holcus lead spot have indicated that the disease will not spread to non-infected leaves and that yield losses generally are negligible. Because of the low risk of yield loss, no management recommendations are needed. Foliar fungicides are not effective against holcus leaf spot, since the disease is caused by a bacterium rather than a fungus.--Carl A. Bradley

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