Necrotic ring spot is caused by the pathogen Ophiosphaerella korrae. Although it primarily affects Poa pratensis and Poa annua turf, it has also been reported in Festuca rubra, Agrostis stolonifera, and Poa trivialis. Additionally, O. korrae is one of the main causes of spring dead spot in Cynodon and Zoysia spp. in North America.

Causal agents

Ophiosphaerella korrae

Primary Hosts

Poa pratensis and P. annua (primary), and Agrostis stolonifera, Festuca rubra, and P. trivialis (secondary)

Disease Cycle

Ophiosphaerella korrae is believed to survive unfavorable periods as septate mats or mycelium in plant debris. It spreads from plant to plant through the growth along the surfaces of roots and rhizomes. Ectotrophic hyphae initially colonize the root surfaces, followed by the infection hyphae that penetrate the root cortex. Infection occurs in roots, rhizomes, leaf sheaths, and crowns during spring and autumn. Infested sod and cultivation equipment are likely sources of pathogen spread, while the role of ascospores in dissemination is not well understood.

Epidemiology

Ophiosphaerella korrae infects and colonizes roots and rhizomes during cool, wet weather in the spring and fall. Above-ground symptoms are most commonly observed during these periods. In moderate climates, symptoms may subside during warmer and drier summer conditions. In transition zone environments, symptoms tend to persist and worsen due to heat and drought stress. The disease can occur in soils with pH ranging from 5 to 8 but is most severe in compacted soils. It is frequently observed in 2- to 4-year-old lawns established with sod, although seeded areas and young turf can also be affected. Over time, untreated cases of necrotic ring spot often show a decline in severity.

Symptoms

Symptoms of necrotic ring spot begin as small, light-green spots in circular patches during cool, wet weather. As the disease progresses, the affected grass turns reddish brown, bronze, and eventually straw-colored. Severe cases may result in the death of plants within the patch, creating sunken depressions. Surviving plants can recolonize the center, giving the patch a ring-like appearance. The disease can also manifest as yellow or brown foliage in diffuse patterns. Differentiating it from similar diseases is important for accurate diagnosis.

Credit: John Kaminski/Penn State University

Signs

The signs of the pathogen causing necrotic ring spot include the presence of blackening roots and rhizomes, as well as dark brown ectotrophic hyphae on affected roots, rhizomes, and crowns. The roots may exhibit severe rotting, and black pseudothecia (fruiting bodies) of the pathogen, similar to those causing spring dead spot, can occasionally be found on these tissues. These signs distinguish necrotic ring spot from other diseases, as the symptoms can occur throughout the growing season. The presence of these signs helps in the identification and diagnosis of necrotic ring spot in turfgrass.

Management

Necrotic ring spot is not a common disease of turfgrass systems, and chemical control is usually the most effective option. In chronic situations, cultural practices should be adjusted to create conditions unfavorable for growth of the pathogen.

Cultural

Cultural control options include managing thatch buildup and reducing compaction through regular aerification. Avoiding high nitrogen fertility, especially during spring and summer, is essential. Proper irrigation is crucial to prevent drought stress. Although deep and infrequent irrigation is recommended for most turf diseases, a strategy of light and frequent irrigation can promote survival after an outbreak of necrotic ring spot, as the disease leads to a shallow and more drought-sensitive root system. This irrigation strategy may be particularly beneficial when implemented during the hottest part of the day. Maintaining a mowing height no lower than 2 inches is also recommended for lawn areas.

Chemical

For chemical control, fungicides should be applied in April or May when soil temperatures are between 55 and 65°F (10-18°C). It is essential to water the fungicides into the thatch and soil before they dry on the leaves. When addressing affected areas, overseeding with perennial ryegrass or renovating with resistant varieties of Kentucky bluegrass or tall fescue can be effective. Effective fungicides include the DMIs, thiophanate-methyl, and iprodione. However, it’s important to note that applications of chlorothalonil may actually enhance disease pressure.