Necrotic Ring Spot

Disease

Leptosphaeria korrae

Photos

Necrotic ring spot first appears as small patches 6 in. to 1 ft (15 cm to 0.3 m) in diameter. When the fungus is actively attacking the Kentucky bluegrass plants in the fall of the year, dark red blades of grass can be seen in the patches. The pathogen attacks root systems in the spring and fall, and in the summer, infected plants begin to wilt in patches. Patches eventually turn straw colored, and older patches (2 yrs +) appear as a "frog-eye" with a center of healthy turf surrounded by a ring of dead turf.

Necrotic ring spot occurs in the cool regions of the world where Kentucky bluegrass (a primary host) is grown. The fungus achieves maximum growth at temperatures between 68° and 82° F (20° and 28° C). The pathogen is active in the cool weather of the spring and fall, even though symptoms are seen in the warm weather of the summer. The fungus is thought to move among the turf stand by growing along the surface of roots and rhizomes.

Crops Affected: turf

Damage

The location of this disease is home lawns and golf courses. General symptoms are a patch, or irregular coloring. Foliar symptoms are browning and yellowing. This can occur during the months of April, May, June, July, August, September, and October. Hosts of the disease are Kentucky Bluegrass, Fine Fescue, Annual Bluegrass, and Rough Bluegrass.

Management

Summer symptoms are the result of the inability of the turfgrass to take up adequate amounts of water during the summer stress period because the fungus has destroyed the root system during the cool weather. One way to keep the weakened plants alive during the summer is through the use of light, daily irrigation. Turf should be irrigated daily with between 0.1 and 0.2 in. (0.25 and 0.5 cm) of water, preferably applied between noon and 4:00 P.M. This rate can be doubled during periods of prolonged droughts.
Adequate levels of nitrogen should also should be used in the management of necrotic ring spot. Improved Kentucky bluegrass cultivars require 4-6 lb (2 to 2.9 kg) of actual nitrogen per 1000 ft² (100 m²) per season. Slow release nitrogen fertilizers such as Turf Restore, Nature Safe, Lawn Restore, IBDU, UF etc, are more effective in managing the disease than fast-release fertilizers like urea. Complete fertilizers (N-P-K) with slow release nitrogen carriers are the most effective.

Curative: The thiophanate methyl and azoxystrobin fungicides provide the best curative management. To be effective, these fungicides need to be drenched into the soil before they dry on the foliage, so that they are translocated upward from the roots. Best results are obtained when the turf is irrigated before the fungicides are applied.
Preventive: The DMI fungicides such as fenarimol, myclobutanil, and propiconazole work best preventively. These fungicides are translocated in both directions, and do not have to be drenched into the soil to be effective. Unless adequate nitrogen is applied, these fungicides will not be effective.

MSU Extension Turf