Preventing Summer Patch of Kentucky Bluegrass Now

Summer patch is one of the most destructive diseases of Kentucky bluegrass on lawns, commercial turfgrass areas, athletic fields and greens surroundings. The impact of this disease can be reduced by using several management strategies this spring.

The symptoms of summer patch first appear in the summer months as circular to irregular grayish patches of wilty turf, two to six inches in diameter. Individual leaves quickly turn tan to straw- colored. Large circular or crescent shaped patches of affected turfgrass one to several feet in diameter appear as the disease progresses. There is often unaffected turfgrass in the center, giving the patches a frogeye appearance.

The disease is more common on older, established Kentucky bluegrass and tends to be more severe on slopes with southern exposure. Alternating periods of moisture stress followed by heavy rainfall favor disease development.

Although summer patch symptoms do not occur until the summer months, the causal fungus Magnaporthe poae actually infects bluegrass roots in the spring. The aboveground symptoms of summer patch occur because the root systems are severely damaged by the summer patch fungus. Diseased root systems cannot provide enough water to maintain turfgrass quality in the summer. Plants have difficulty outgrowing the infection because little root replacement occurs under high soil temperatures in the summer.

On high profile turfgrass areas with a history of summer patch, it may be desirable to apply a preventive fungicide. There are a number of fungicides that can be applied in the early spring to reduce root infection. Labelled materials include most of the dmi type fungicides such as Banner Maxx, Eagle, and Bayleton and the stobilurin material, Heritage. To be effective in reducing summer patch severity, the fungicide of choice must be applied early as soil temperature at the two inch depth approaches 65 F. In most areas of Missouri this typically occurs in mid to late April. The fungicide should be reapplied one month later.

Other practices that can reduce the severity of summer patch include maintaining the height of cut on lawns above 2.5 inches and avoiding the overuse of nitrogen in the spring. As we approach periods of low rainfall, water deeply and infrequently to maintain even moisture.

Barb Corwin , Extension Plant Diagnostic Clinic, UMC (573)882-3019