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Necrotic Ring Spot

Symptoms
ring spotIn the early stages, symptoms of NRS often appear as small, scattered, circular, light green to straw-colored patches. As the symptoms progress, the patches become sunken orcrater-like and appear as rings or arcs of dead turf. These arcs range in diameter from a few inches to 3 feet or more. Often tufts of apparently healthy grass remain in the center of this  circular area, producing  a “frog eye” pattern. Symptoms may increase in severity at particular sites and then decrease.

About the Disease
ring spotThe pathogen responsible for NRS survives year to year as mycelia (fungal threads) in dead plant debris, in the thatch layer, and in infected plant parts. NRS most commonly occurs when wet weather is followed by hot, dry periods. Symptoms of the disease on Kentucky bluegrass appear under cool, moist conditions, so it is common to see NRS from mid spring through late fall. The fungus is most active when temperatures are between 50 and 70 degrees F.

Environmental stresses, such as heat and lack of moisture, may weaken the host and make it more susceptible to the disease. Turf grass may be more susceptible to infection when stressed for several years in a row. NR is particularly evident two to three years after sod lawns are established.

The most common problem leading to NRS is laying sod on top of hard, compacted clay soil. The hard soil inhibits root development, enhances thatch buildup, and is water resistant. This leaves the grass drought stressed even though it is watered frequently. Proper site preparation before sodding a lawn may seem expensive at the time but results in a healthier, more pleasing lawn with overall lower maintenance costs.

Control

As with most turf diseases, NRS is principally a disease of stressed turf. Dense turf with heavy thatch tends to be more prone to infection.  Aerate the lawn twice a year to help reduce thatch buildup and improve soil condition. 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 every day with 0.15 inches of water. Avoid watering after 5:00 PM and before 4:00 AM.  During prolonged droughts, when measurable rain does not occur during a two week period, the irrigation should be raised to 0.20 inches per day.  This allows the infected plants with a depleted root system to survive the heat stress of the day.  Use organic fertilizer to increase soil microbial activity.