Leaf spot and melting out in turf
Leaf spot and melting out are two turf diseases with similar symptoms, caused by different fungi. Melting out is caused by Drechslera poae and leaf spot by Bipolaris sorokiniana. Both fungi survive winter in infected plant material. In the spring and later in the fall, during cool, wet weather, melting out becomes a problem, while leaf spot usually occurs in mid-summer, when the weather is warm and damp. Melting out becomes a problem during cool, wet weather in the spring and fall.
Symptoms of both diseases start out as water-soaked (dark green) spots on the grass blades. The spots later turn tan with purple to dark brown margins. Spots are elliptical and may extend the width of the leaf. Multiple spots on a leaf often cause death of the grass blade. Leaf infections may eventually progress down the blade and into the crown and roots. Once the fungus reaches the crown and roots, the entire plant may be killed, causing large areas of grass to thin. Infected areas are irregularly shaped, and range in size from several inches to several feet in diameter. Severe infections may cover the entire lawn.
Proper maintenance is the most effective way to control melting out and leaf spot. Water lawns during dry periods and properly fertilize. Dethatch lawns as needed, maintaining a thatch layer of less than ½ inch. Shade, heavy nitrogen fertilization, excess thatch, mowing too low, and overuse of broadleaf herbicides can stress turf and stimulate plant disease. Control is difficult once the disease becomes established. Fungicides may aid in control if applied in early spring or when leaf spots first become apparent. Chlorothalonil (Daconil 2787) and thiophanate-methyl (Cleary's 3336) are currently labeled for homeowner use.
Lamey, H. A., Ash, C. L., and Steinstra, W. C. 1983. Lawn Diseases. Minnesota Extension Service publication AG-FO-3386. pp 8.
Turgeon, A. J. 1991. Turfgrass Management, third edition. Prentice Hall. pp 418.
Chad Behrendt, Crystal Floyd