Leaf spot and melting out are among the most frequent and destructive lawn diseases of common bluegrass. Melting out is caused by the fungus Drechslera poae and leaf spot by the fungus Bipolaris sorokiniana. Excess thatch, heavy nitrogen fertilization, excess shade, mowing too close, and broadleaf herbicides promote these diseases. The melting out pathogen, D. poae, can also cause leaf spots. Conversely, the leaf spot pathogen, B. sorokiniana, can also causing melting out symptoms.
Leaf spot occurs in warm weather and is easily recognized. Spots on the leaves develop purplish-red to purplish-brown borders and brown to tan centers. The spots may extend the width of the leaf and are somewhat longer than wide. Leaf spots may cause the death of leaf tips. Leaf sheaths are also infected, and may die, resulting in thin stands of grass. Melting out begins as spots on the leaf blades and rapidly moves down the leaf sheath and into crowns and roots. In advanced stages, when many plants die in a large irregular patch, it is known as "melting out". These patches may range in size from several inches up to many feet and may produce an irregular patchwork across an entire lawn.
Once established, melting out is difficult to control. Proper watering and fertilization for bluegrass varieties in your lawn will reduce the danger of melting out. Common Kentucky bluegrass should not be fertilized as heavily as the elite blue grass varieties or high maintenance diseases will develop. It is particularly important to avoid excess use of nitrogen fertilizer and evening watering. Remove excess thatch — the layer of plant material tightly interwoven with living tissue between the soil surface and the green vegetation. Fungicides may be needed to control leaf spot disease. Their effect is temporary and beneficial only when combined with a change in cultural practices. Fungicides are most effective if the initial application is made when leaf spot first becomes serious and less effective when small patches of lawn begin to die. When large areas of a lawn are dying or dead from melting out, dead patches should be reseeded with a resistant variety. Bluegrass varieties differ in their susceptibility to melting out. Older common bluegrass types are often very susceptible while newer elite varieties are usually quite resistant.