Necrotic ring spot caused by Ophiosphaerella korrae is a widespread and serious disease of Kentucky bluegrass (Poa pratensis), annual bluegrass (Poa annua), rough bluegrass (Poa trivialis), and creeping red fescue (Festuca rubra). The pathogen is active in the cool, wet periods of spring and fall, although disease symptoms are mainly expressed in the warm weather of the summer. The summer symptoms are the result of the inability of the turfgrass plant to take up water due to earlier root destruction by the fungus. The first symptoms are small (2-4 in), light green spots which enlarge into patches that can range from 1 to 3 feet. The turf in infected patches turns reddish brown to bronze and eventually fades to a light, straw color. All of the plants in an infected area may die leaving a sunken depression, but more often plants survive or recolonize infection centers resulting in a ring-like appearance. Symptoms can sometimes occur as a diffuse yellowing or browning that coalesces into large areas of blighted turf; leaf lesions are not seen with this disease. Necrotic ring spot can appear throughout the growing season. The disease is characterized by a blackening of roots and rhizomes and dark brown, ectotrophic hyphae on affected roots, rhizomes, and crowns. Roots and crowns may become extensively rotted.
O. korrae is thought to survive as mycelium in plant debris; little is known about its behavior in the soil. The pathogen grows along the surface of host roots and rhizomes and when conditions favor disease development, infection hyphae penetrate to interior portions of the roots. While the growth of O. korrae is favored by cool, wet conditions, heat and drought stress intensify symptom expression. The disease can appear over a wide range of pH, is most severe in compacted soils, and most prevalent on 2-4 year old stands that were established with sod. Left untreated, it may decline in severity after several years due to the build-up of antagonistic microflora.
- Avoid drought stress. Generally, deep and infrequent irrigation is recommended, but when this disease is severe in hot weather, light daily applications of water will reduce heat stress and stimulate recovery. Syringing during the hottest part of the day cools the turf and allows the weakened plants with dysfunctional root systems survive heat stress.
- Maintain adequate levels of potassium and phosphorous fertility as well as optimal nitrogen levels. Slow release or organic forms of nitrogen are recommended.
- Avoid high nitrogen levels, especially during the spring and summer.
- Aerify to reduce compaction, manage thatch build-up, and improve drainage.
- Overseed with perennial ryegrass or reseed with resistant varieties of Kentucky bluegrass or tall fescue.
Management with Fungicides
Fungicide applications should begin preventively in April/May and may need to be repeated in September/October. Systemic fungicides must be watered in before they dry on the foliage to be effective against necrotic ring spot. Best results are obtained when the turf is irrigated prior to application. Applications of chlorothalonil may increase disease pressure.
For a listing of fungicides currently labeled to manage this disease, refer to the Disease Management chapter of UMass Extension's Professional Guide for IPM in Turf for Massachusetts.
Written by: M. Bess Dicklow