Introduction:

Ophiosphaerella (formerly Leptosphaeria) korrae is one of the pathogens that cause spring dead spot, the most important disease of bermudagrass in North America and Australia. Ophiosphaerella korrae also causes necrotic ring spot, a severe disease on Kentucky and annual bluegrass. Symptoms of spring dead spot are large sunken circular patches in the spring when bermudagrass should be breaking dormancy. In cool, wet weather, necrotic ring spot symptoms on bluegrass are similar, but oftentimes turf will regrow into patch centers causing a ring appearance. Both diseases severely disrupt the aesthetics, uniformity, and functionality of lawns, golf courses, and sports fields.

Host Range and Distribution:

Ophiosphaerella korrae is one of three Ophiosphaerella species that cause spring dead spot on bermudagrass. Ophiosphaerella herpotricha is found primarily in the southern Great Plains region of the western United States. Ophiosphaerella namari is the prevalent spring dead spot pathogen in California and Australia, but is not common in most other regions of the United States. Ophiosphaerella korrae is found in the eastern United States and southern California, and is the predominant causal agent of spring dead spot in North Carolina.

All bermudagrasses (Cynodon dactylon), including hybrids, are susceptible to spring dead spot injury. Conversely, newer cultivars of Kentucky bluegrass (Poa pratensis) have been bred for resistance to necrotic ring spot, which has lessened the disease's impact considerably. Ophiosphaerella korrae has also been reported on several other turf genera including Festuca, Axonopus, and Eremochloa, but these hosts tolerate infection much better than bermuda or bluegrasses.

Identification:

Isolation of O. korrae is done via surface sterilization of infected roots or stolons and plating on potato dextrose agar amended with antibiotics. Putative colonies are allowed to grow out for a 5-7 days and selected based on colony morphology (aerial and felt-like in appearance. Definitive identification of the colony, however, is not as simple.

Ophiosphaerella korrae is a member of the ectotrophic root-infecting fungal complex of soil-borne pathogens on turfgrass. These fungi produce dark brown to black septate hyphae that grow along roots and stolons. Some species produce infection structures termed hyphopodia, but otherwise provide few morphological clues that can be used for identification. Inducing the teleomorph stage in culture is also laborious and often unsuccessful. PCR primers based on specific ITS sequences of O. korrae and other ectotrophic root infecting species have been developed, and are the most effective means for identification.

Necrosis of stolons and roots induced by O. korrae
(Rollover) Microscopic view of mycelial mat and infection on a root.

Symptoms and Signs:

Symptoms of spring dead spot appear as circular patches of dead, bleached grass that occur when dormant bermudagrass should be resuming growth. Individual patches can be up to 3 feet (1 m) in diameter and tend to reappear and expand in the same spot for three or more years. Patches often coalesce, causing nonuniform damage that can resemble winterkill. After three years, patch centers may become reinfested with weeds or living bermudagrass and take the form of rings.

Necrotic ring spot symptoms on bluegrass can be similar to spring dead spot symptoms, with all the plants dying in a patch, leaving a sunken or craterlike depression. Oftentimes however, bluegrass survives or recolonizes infection centers, giving the patch a ringlike appearance. Symptoms may also appear as yellow or brown foliage in diffuse patterns that coalesce into larger areas of turf.

Roots and stolons of both bermudagrass and bluegrass plants in symptomatic areas are rotted and severely stunted. Dark brown to black mycelial plaques may cover infected roots and stolons. Occasionally, ascocarps of the fungus (pseudothecia) may be observed on severely infected tissues.

Ecology and Life Cycle:

Ophiosphaerella korrae is thought to survive unfavorable environments as mycelial plaques in plant debris. The fungus is thought to move from plant to plant by growing ectotrophically along the surface of roots and rhizomes, and infecting cells in the root cortex. Infection in bermudagrass occurs in the fall, with symptom development occurring in the spring. Bluegrass infection occurs throughout the growing season during cool, wet periods with symptoms first occurring in late spring.

Ophiosphaerella korrae is an ascomycete. It produces ascospores in black pseudothecia, which are formed on necrotic roots, rhizomes, leaf sheaths, and crowns. The importance of ascospores in pathogen dissemination is unknown.

Links to other sites:

Managing Spring Dead Spot in Hybrid Bermudagrass - NCSU Turfgrass Disease Note 4

Turfgrass Trends article - Spring Dead Spot Control

UC IPM Online: Spring Dead Spot

www.springdeadspot.com

Turfgrass Disease Profiles: Necrotic Ring Spot - Purdue Extension

Necrotic Ring Spot - Colorado State Extension

Necrotic Ring Spot Fact Sheet - Cornell Extension

Bibliography:

Alexopoulos, C. J., Mims, C. W., and Blackwell, M. 1996. Introductory Mycology. 4th ed. New York, NY: John Wiley and Sons, Inc.

Butler, E.L. 2004. Development of novel strategies for control of spring dead spot of bermudagrass. M.S. Thesis

Couch, H. B. 1995. Diseases of Turfgrasses. 3rd ed. Malabar, FL: Krieger Publishing.

Smiley, R. M., Dernoeden, P. H., and Clarke, B. B. 2005. Compendium of Turfgrass Diseases. 3rd ed. St. Paul, MN: APS Press.

Smith, J. D., Jackson, N., and Woolhouse, A. R. 1989. Fungal Diseases of Amenity Turfgrasses. London: E & F.N. Spon Ltd.

Vargas, J. J. M. 1994. Management of Turfgrass Diseases 2nd ed. Boca Raton, FL: Lewis Publishers.

Acknowledgements:

If not otherwise cited, images and figures are courtesy of E.L. Butler, Turfgrass Diagnostician at North Carolina State University

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