Necrotic ring spot, Ophiosphaerella (syn= Leptosphaeria) korrae, and summer patch, Magnaporthe poae, are two serious, fungal root and crown rots of Kentucky bluegrass, annual bluegrass, fescue, and bentgrass. These diseases were previously referred to as Fusarium blight syndrome. Necrotic ring spot and summer patch cause identical symptoms and cannot be easily distinguished in the field.

Symptoms and Diagnosis

NECROTIC RING SPOT is most common in cool weather. It usually occurs during 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 a donut-shape 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 crowns and leaf sheaths may reveal the presence of the dark hyphae of the fungus.

SUMMER PATCH is most common in warm weather. Symptoms are present from May through September. The symptoms are similar to those of necrotic ring spot, consisting of dead circles and arches that range in size from several inches to several feet in diameter. The summer patch fungus also produces 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 ring spot, but are much less common on the plant tissue.

When plants with summer patch are dug up, the roots are dark brown to black, hard, and dry-rotted in contrast to the white color of healthy crowns and roots.

Life Cycle

Necrotic ring spot fungus survives in soil and infects the crown and roots of grass plants. It commonly occurs when wet weather is followed by hot, dry periods. The range of temperature and moisture conditions at which this disease develops is much broader than that for summer patch, so disease outbreaks can occur from mid-spring through late fall.

Integrated Pest Management Strategies

1. Fertilizer. Use 1 to 3 lbs. of actual nitrogen per 1000 sq. ft. Apply nitrogen fertilizer in the fall (September through November). Do not apply even small amounts of fertilizer during the June- August stress period because this will tend to stimulate the disease. Therefore, fertilize only in autumn (September through November). Maintain a pH above 6.2.

2. Thatch. Thatch (the layer of organic matter between the mineral soil and the green grass) should be no more than 1/2 inch in thickness. Thatch can be removed by vertical slicing machines and/or aeration during the spring and early fall. Over a longer period, thatch will be reduced by using good cultural practices.

3. Mowing. As the cutting height of turf is decreased, its susceptibility to disease increases. Consequently, cut lawns at 2- to 3-inch height and often enough so that less than 1/3 of the leaf blade is removed during each mowing.

4. Watering. Deep watering is essential for proper root growth. More frequent watering provides an ideal environment for disease development. Soaker hoses are very useful for supplementary watering on steeper slopes where sprinklers are inefficient and apply water too fast. The harmful effects of excessive temperature can be reduced by a light sprinkling of the surface at midday.

5. Cultivars. Select resistant cultivars. Before seeding, consider recommended cultivars that are resistant to necrotic ring spot and summer patch.

6. Fungicides. Chemical treatment is efficient only when good cultural practices are first used. Thoroughly water areas with a history of these diseases several days before applying a fungicide. Apply in early to mid-June and a second application two to three weeks later. Use a preventive fungicide program with recommended fungicides. Read labels for proper fungicides and always adhere to the rates and procedures recommended on the label. Pesticides registered for use include azoxystrobin (Heritage), chlorothalonil (Daconil), iprodione (Chipco), thiophanate methyl (Cleary 3336), and ziram.

Organic Strategies

Strategies 2, 3, 4, and 5 are strictly organic approaches. Using an appropriate organic fertilizer would be a viable organic approach to Strategy 1.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against necrotic ringspot/summer patch. Not following the pesticide label before usage is a violation of federal law.