Summer Patch is a destructive, soil-borne fungal disease that attacks the root systems of cool-season turfgrasses, primarily affecting Kentucky bluegrass and fine fescues. The fungus, Magnaporthe poae, infects the roots and crowns of the grass, but the visual symptoms only appear later in the season when the turf is under heat and drought stress. This disease can weaken a lawn significantly, leading to unsightly dead patches that persist and often recur in the same areas year after year. Managing Summer Patch requires a two-pronged approach: immediate, preventative chemical treatment and a long-term commitment to improving the lawn’s cultural health.
Identifying Summer Patch Symptoms and Causes
The most obvious signs of Summer Patch are the circular, straw-colored patches of dead or dying grass that typically appear in mid-summer, between July and September. These patches can range from approximately six inches to three feet in diameter, and they may merge to form larger, irregular areas. The grass within the affected area first turns yellow before wilting and collapsing into a dark, straw-brown color.
A characteristic symptom that helps distinguish Summer Patch is the “frog-eye” pattern. This occurs when a small tuft of healthy, green grass remains in the center of the dead patch. The fungus begins colonizing the turfgrass roots and crowns in the spring when soil temperatures at a two-inch depth reach 65°F (18°C) or higher. The damage to the vascular system prevents the plant from taking up water and nutrients. The grass eventually succumbs when subjected to high temperatures (above 82°F) and drought conditions.
Applying Fungicides for Immediate Control
Since the fungus damages the roots long before visible symptoms appear, fungicide application must be preventative to be effective. Curative treatments applied after patches are visible are much less effective because extensive root damage has already occurred. The first application should be timed to coincide with the spring soil temperature threshold of 65°F at a two-inch depth.
Effective chemical control relies on systemic fungicides that can be absorbed by the plant and reach the root zone. Recommended options for preventative control include DMI (Demethylation Inhibitor) class fungicides, such as propiconazole, and QoI (Quinone outside Inhibitor) or strobilurin fungicides, like azoxystrobin. These treatments typically require follow-up applications every three to four weeks until the peak summer stress period subsides, often totaling two to four applications.
Proper application technique is necessary to ensure the active ingredient reaches the root zone and crown tissue. Fungicides should be applied with a high volume of water (four to five gallons per 1,000 square feet) or immediately watered in with a light irrigation cycle. This process washes the chemical through the thatch layer and down to the point of infection, maximizing efficacy. Always follow the product label instructions precisely regarding rates and safety.
Adjusting Lawn Care Practices to Prevent Recurrence
Long-term management of Summer Patch depends on improving the overall health of the turfgrass root system to help it withstand summer stress. The disease often takes hold in lawns with poor cultural practices that restrict root growth. Adjustments to watering, aeration, fertilization, and mowing height are necessary.
Watering practices need to shift away from frequent, shallow cycles that promote shallow roots. Lawns should be irrigated deeply and infrequently to encourage roots to grow further down into the soil profile. Watering should occur early in the morning to allow the leaf blades to dry quickly, reducing surface moisture that supports fungal growth.
Soil health is a major contributing factor, as compacted soil and excessive thatch accumulation encourage the disease. Core aeration removes small plugs of soil, improving drainage, reducing compaction, and allowing for better air and water movement in the root zone. Performing this procedure in the late spring or early fall promotes deeper, healthier root growth that resists infection.
Fertilization adjustments are important because excessive quick-release nitrogen in the spring and early summer can stress the grass and fuel the pathogen. Utilize slow-release nitrogen sources and avoid heavy nitrogen feeding just before the summer heat arrives.
Using nitrogen sources that acidify the soil, such as ammonium sulfate, can help reduce disease severity. Summer Patch is most severe in soils with a pH of 6.5 or greater.
Raising the mowing height, particularly during the hot summer months, reduces stress on the turf and promotes a healthier canopy and deeper root system. For susceptible grasses like Kentucky bluegrass, a mowing height of three inches or higher is advised to minimize vulnerability to heat and root damage. In areas with persistent outbreaks, overseeding with resistant species like perennial ryegrass or tall fescue can help reduce future symptom expression.