Forage grasses, including crabgrass, can be affected by a fungal disease caused Helminthosporium leaf spot. Symptoms of this disease vary by grass species and are usually observed when leaf lesions are mature, causing round to oval, gray to tan lesions surrounded by a dark halo. Younger lesions may be darker with a yellow halo, depending on grass species. Affected leaves can turn yellow or brown, and severe infection can cause plants to die and cause thin stands.
The fungi that cause Helminthosporium leaf spot survive on grass residue, crop debris, and can also be seed-borne. Fields that have thick thatch layers will be at greater risk for infection and disease development. Wet weather also favors disease development, and the disease will establish and spread more quickly with frequent rainfall.
Management of this disease is not always needed but there are several ways to minimize disease impact. Maintain adequate fertility and avoid over-fertilizing with nitrogen. Timely mowing or grazing to the recommended height will also reduce disease impact. When establishing a new forage stand, select varieties with good disease resistance packages and seed at recommended rates. No fungicides are currently labeled for use on Helminthosporium leaf spot.
For more detailed information on Helminthosporium leaf spots on various forage grasses, see the following University of Illinois publication: http://ipm.illinois.edu/diseases/series300/rpd309/ ~ Dr. Kiersten Wise, University of Kentucky