In the past week, the Iowa State University Plant Disease Clinic has received corn leaf samples from northwest Iowa with Holcus leaf spot. Holcus leaf spot is caused by the bacterium Pseudomonas syringae. Light tan (sometimes almost white), round, oval spots, which may appear water soaked at the margins or have a light brown border occur on the lower leaves. The spots are initially about 1/4 inch in diameter, but sometimes grow larger and coalesce into irregular spots and streaks of dead tissue. Later the lesions dry out, turn light brown, and have a papery texture.
Holcus spot symptoms can resemble chemical injury to leaves, similar to paraquat drift. In addition, they often are confused with Eyespot, a fungal disease caused by Aureobasidium zeae. Eyespot symptoms are small round spots with a distinct brown border and a yellow halo.
|Holcus leaf spot symptoms on corn.|
|Eyespot causes small, round spots with a brown border surrounded by a a yellow halo.|
Wounds caused by hail, blowing soil or wind can transmit the disease. Warm (75-85°F), wet, windy conditions early in the season favor infection and the development of Holcus leaf spot. Symptoms often appear suddenly after a heavy rain, but then do not spread to new leaves. Todd Vagts, Extension field crop specialist, who sent in one of the diseased corn samples, reported that they have had several days of warm (temperatures in mid-80s), wet, windy and humid weather in Carroll County.
Holcus leaf spot is mostly cosmetic and does not result in yield loss. Therefore there are no practical management tactics for holcus spot. Probably the most important take home message is not to mistake this disease for a fungal leaf disease, such as Eyespot. There have been instances where unnecessary (and ineffective) fungicide applications have been made against Holcus leaf spot.
This article originally appeared on page 81 of the IC-492(14) -- July 5, 2004 issue.