IPM Identification and Mangement Of Turfgrass Diseases

Author(s)

Necrotic Ring Spot

Pathogen: Ophiosphaerella korrae
Hosts: Primarily Kentucky bluegrass but also red fescue and annual bluegrass

Symptoms and signs

The first symptoms usually appear in early fall or late spring. Initially, light green to straw-colored patches from several inches to several feet in diameter develop in the turf (Figure 1). As the disease progresses, patches develop more of a doughnut or frogeye pattern. The ring becomes matted and crater like, whereas the grass inside the ring remains healthy and green (Figure 2). The patches may be localized to one part of the turf area (Figure 3) or widely scattered throughout. Rings may eventually coalesce to create larger arcs of dead turf (Figure 4). Symptoms may persist throughout the growing season and increase in size and severity in successive years.

Bluegrass infected by Ophiosphaerella korrae exhibits extensive root and crown discoloration and rot (Figure 5). This can be observed by cutting a small sod piece from the lawn and carefully washing off soil and organic matter from roots. Darkly pigmented fungal hyphae may be visible on the root surface with the aid of a hand lens. The presence of large frogeye patches and root discoloration are usually diagnostic; however, necrotic ring spot can be confused with summer patch. Summer patch symptoms usually do not appear until late June or July and then symptoms tend to disappear with cooler weather in the fall. Also, frogeye patches associated with summer patch tend to be smaller than those associated with necrotic ring spot. Plant disease diagnostic labs can verify the presence of necrotic ring spot.
Small pear-shaped pseudothecia (fruiting bodies) may form on decaying roots and crowns, but this rarely occurs in the field (Figure 6). Darkly pigmented multicelled ascopores are produced in asci (sac-like structures) within the pseudothecia (Figure 6, inset).

Conditions

O. korrae 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 degrees and 80 degrees F. In the Midwest, these temperatures occur in the spring and fall, but along the Front Range of the Rockies they occur from mid-May through mid-September, with some variation on the west slope and at higher elevations. O. korrae 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, symptoms often do not show up until late summer or early fall even though root colonization occurs earlier. Symptoms may persist through the winter.

Symptoms generally appear two to three years after turf establishment, although in some cases the disease may not develop for a decade or more. The disease tends to be more severe in areas established from sod and on sites with compacted soils. Excessive irrigation and fertilization may also exacerbate the severity of the disease.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.