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Necrotic Ring Spot- A Cool Weather Fungus

Posted - Oct. 6, 2001 at 8:33 a.m.

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Necrotic ring spot (NRS) is one of several fungal patch diseases caused by fungi. NRS often appears as scattered small, circular, light green to straw-colored patches. Necrotic ringspot is a cool weather disease. It usually occurs during March through May, and September through November. Microscopic examination of the grass crown and leaf sheaths reveals the presence of dark hyphae of the fungus. CULTURAL CONTROLS Cultural techniques that prevent stress on the turf will help prevent these two diseases. Kentucky bluegrass is the primary host of NRS. Environmental stresses such as heat and lack of moisture, weaken the turf and make it more susceptible to the disease. NRS often shows up two to three years after sod lawns are established. It is more severe on Kentucky bluegrass that is established from sod on sites without proper soil preparation. The most common problem leading to NRS is laying sod on top of hard, compacted clay soil. This inhibits root development, enhances thatch buildup, and is water-resistant. This creates drought-stressed grass even though it is watered frequently. Proper site preparation before sodding a lawn seems expensive at the time but results in a healthier lawn with less maintenance costs. Control As with most turf diseases, NRS is principally a disease of stressed turf. The disease is best controlled by proper cultural practices, which maintain the turfgrass at optimum vigor, and application of a fungicide. Use resistant varieties when establishing or re-establishing a lawn. Use mixtures of two or more grass species and two or more resistant cultivars to reduce the possibility of disease. No Kentucky bluegrass cultivar is immune to this disease; however, cultivars differ in susceptibility. : Adelphi, Eclipse, Midnight, Park and Wabash I-13, 'Alpine', 'NE80-88', 'Princeton-104', 'Mystic', 'Joy', and 'I-13' consistently have less disease than 'HV-97 (Cocktail)', 'Annika', 'Amazon', 'Opal', 'Trampas', and 'Sydsport'. Avoid the latter cultivars. Check local nurseries for the availability of the seed varieties or check with Steve Reagan company in Salt Lake City or Granite Seed company in Lehi for specialty varieties. Mow grass as necessary to maintain a height of 2 1/2 to 3 inches. Make sure mower blades are sharp. Never remove more than one-third of the grass blade at a time. Water to a depth of 6 to 8 inches as infrequently as possible without creating water stress. Water in the morning or midday so the leaf blades dry as quickly as possible. Avoid excessive applications of nitrogen fertilizer that induce tender, succulent growth and more susceptible tissue. Use fertilizer frequently at low rates or in slow release forms such as sulfur-coated urea or IBDU. This avoids undesirable flushes of growth that are more susceptible to disease. Never apply more than 4 pounds of nitrogen per 1,000 square feet in an entire year Overseeding with perennial ryegrass (this grass that is not susceptible to NRS) also may be done. However, the result may produce patches of perennial ryegrass mixed in with the existing Kentucky bluegrass. Colors and textures of these two grasses are not the same. CHEMICAL CONTROLS Cultural techniques provide the most reliable control of these diseases, but in difficult situations fungicides may be necessary. The turf area to be treated should be irrigated the night before. The next day the fungicides should be applied and irrigated in before they dry on the foliage. These fungicides are best used as a preventative. 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. Larry A. Sagers Regional Horticulturist Utah State University Thanksgiving Point Office


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