Leaf Spot / Melting-Out Disease (Helminthosporium)

At one time, melting-out disease was considered to be one of the most important diseases effecting Kentucky bluegrass and other cool-season turfgrasses, as well as bermudagrass. Although it is still a problem, many resistant cultivars and management strategies have been developed for melting-out, thus making it easier to manage. This fungal disease affects all parts of the plant, including leaves, shoots, and roots.

Melting Out Disease

Melting-out disease first forms leaf spots in a “bulls-eye” pattern before it spreads to the leaf sheaths, crown and roots, ultimately killing the plant. During hot weather, the leaf symptoms may not occur, but the rest of the plant is affected. Multiple dead plants form a patch and several patches will combine as the disease spreads causing the turfgrass to appear to be melting-outwards.

The disease is caused by the fungi drechslera and bipolaris. These fungi attack living tissue under specific environmental conditions, typically in the spring. When the fungi spores are exposed to appropriate temperatures and moisture on leaf blades for a few hours, they germinate and penetrate the leaf tissue. Once this happens, the cells of the turfgrass are killed, causing the spots to form. Asexual spores, called conidia, are produced on the infected tissue within a few days to repeat the cycle.

Controlling Melting-Out Disease

Cultural Controls include improving soil aeration and water drainage along followed by re-seeding with resistant grasses. Avoid using an excess of nitrogen in the spring. If watering is required, water early in the day.

Chemical Controls include using fungicides if they are applied when the disease first begins to appear. If the disease repeatedly appears in the same area, then a combination of chemical and cultural controls should be considered.

Avoid applying broadleaf herbicides such as 2,4-D and plant growth regulators when leaf spot or melting-out disease is present, they encourage its growth, development and spread.