Necrotic ringspot is a destructive disease of Kentucky bluegrass, red fescue and annual bluegrass. Necrotic ringspot is particularly damaging to bluegrass because it is a perennial problem and the fungus attacks and kills the roots and crowns. Recovery from a disease outbreak is often very slow.
Necrotic ringspot is caused by a soil-borne fungus called Ophiosphaerella korrae. The fungus survives from year to year on dead, colonized bluegrass roots and crowns or on the surface of living roots. The fungus actively colonizes the outside of roots at soil temperatures between 65° and 80° F.
The fungus eventually penetrates roots and colonizes the root cortex. This either debilitates or kills roots and leads to a decrease in water and nutrient uptake by the plant. Plants with extensively damaged root systems are more prone to injury or death, particularly during periods of high temperature or drought stress. Thus, Necrotic Ringspot symptoms often don’t show up until July or August even though root colonization begins in May and continues throughout the summer.
Necrotic Ring Spot continues to be a very difficult disease to manage because of its perennial nature and inconsistency in control with fungicides. Nevertheless a combination of genetic, cultural and chemical treatments can reduce Necrotic Ring Spot severity.
Symptoms first appear in summer as small (6" - 8" in diameter), clustered patches of gray-tan turf. Because necrotic ring spot is a root disease, initial above ground symptoms include dieback from the leaf tips, followed by collapse of the leaf and decline of the entire plant.
There appear to be a few Kentucky bluegrass varieties that have resistance to necrotic ring spot, which include Adelphi, Midnight, and Wabash.
The primary stresses that influence disease development include excesses of thatch, fertilizer, and temperature, as well as incorrect timing of fertilizer applications, low mowing height and low soil pH.
Correct excess soil acidity by liming annually to maintain a pH above 6.2. Do not apply even small amounts of fertilizer during the June-August stress period, this will tend to stimulate the disease. Fertilize only in autumn (September through November) and in late spring
Deep watering is essential for proper root growth. Water the soil in disease-prone areas to a depth of 4" - 5" every 7-10 days during dry periods in the summer. The harmful effects of excessive temperature can be reduced by a light sprinkling of the surface at mid-day.
Cut lawns at 2.5" - 3.5" height, and do so often enough that less than 1/3 of the leaf blade is removed during each mowing.
Thatch (the layer of organic matter between the mineral soil and the green grass) should be no more than 0.5" thick.
Kentucky bluegrass cultivars such as Adelphi, America, Aspen, Columbia, Eclipse, Glade, Midnight, Nassau, Parade, Ram I, Sydsport, Touchdown, Vantage, Windsor, and Victa are less susceptible to Necrotic Ring Spot than others. Blend seed of a resistant cultivar with that of one or more otherwise desirable cultivars. Blending 10-15% (by weight) of perennial ryegrass seed into bluegrass seed will prevent this disease from occurring. Ryegrass can also be seeded into existing lawns.