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Summer Patch & Necrotic Ringspot of Turf

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Summer patch and necrotic ringspot of turf are common fungal diseases in the lawns of Utah. Summer patch is cause by Phialophora graminicola and necrotic ringspot by Leptosphaeria korrae.

Necrotic ringspot is a serious problems particularly on 2 to 5-year-old Kentucky bluegrass lawns established from sod. Lawns established from seed also show the disease but much less frequently.


Necrotic ringspot of turf is a cool weather disease. It usually occurs during the months of March through May, and September through November. Symptoms consist of dead circles or arches that range in size from several inches to several feet in diameter. Often, the disease will create a "frog-eye" or "doughnut" in the lawn. A "frog-eye" is a small, circular patch of green lawn surrounded by a ring of dead or dying grass. Microscopic examination of the grass crown and leaf sheaths reveals the presence of dark hyphae of the fungus.

Summer patch is a warm weather disease. Symptoms are present during the months of May through September. The symptoms are similar to those of necrotic ringspot, consisting of the dead circles and arches that range in size from several inches to several feet in diameter. The summer patch fungus also produces the microscopic, dark hyphae on the surface of the grass crowns and the leaf sheaths. These hyphae tend to be larger than those found with necrotic ringspot but are much less common on the plant tissue.


Cultural techniques that prevent the lawn from becoming stressed will help prevent these two diseases.

1.Use mixtures of two or more grass species and two or more resistant cultivars to reduce the possibility of disease.

2. No Kentucky bluegrass cultivar is immune to this disease; however, cultivars differ in susceptibility. Classic, Eclipse, Majestic, Merion, Midnight, Mystic, 'Alpine', 'NE80-88', 'Princeton-104', 'Mystic', 'Joy', and 'I-13' consistently have less disease than Baron, Cheri, Columbia, Glade, Ram I, 'HV-97 (Cocktail)', 'Annika', 'Amazon', 'Opal', 'Trampas', and 'Sydsport'. Avoid the latter cultivars.

3. In early spring or fall, aerate the lawn. Aeration improves water penetration and reduces thatch, reducing conditions that cause stress.

4. Avoid high-nitrogen and quick-release fertilizers. Apply nitrogen fertilizer frequently at low rates or in slow release forms such as sulfur coated urea or IBDU. This avoids undesirable flushes of growth which are more susceptible to disease.

5. Avoid over-watering turf especially when using automatic sprinkle systems that come on every day. In most soils, water the lawn deeply and infrequently. Sandy soils require more frequent, light irrigation. Wet the entire rot zone when irrigating.

6. Mow the lawn 2-3 inches high. Mow often enough so that only 30-40% of the leaf length is removed per mowing.


Cultural techniques provide the most reliable control of these diseases, but in difficult situations fungicides may be necessary. Summer patch can be controlled with Tersan 1991, Fungo 50, Rubigan, and Cleary's 3336. Irrigate the turf area to be treated the night before. The next day apply the fungicides and irrigate them in before they dry on the foliage. These fungicides are best used as a preventative. Bayleton can also control summer patch, but it must be applied before the fungus becomes active in early summer.

Fungicides available for the control of necrotic ringspot of turf include Tersan 1991, Fungo 50, Rubigan, and Chipco 26019. These fungicides should be applied in the same manner as those mentioned for summer patch control.

Commercial products listing implies no endorsement by Utah State Cooperative Extension Service. Criticism of products not listed is neither implied nor intended. Those using products are responsible for their use according to the current label directions of the manufacturer. Pesticide labels are legal documents, and it is a violation of federal and state laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally responsible for its proper use. Always read and follow the label. Larry A. Sagers Regional Horticulturist Utah State University Extension Thanksgiving Point Office

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