Magnaporthe poae (summer patch on turfgrass)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Sordariomycetidae
Family	Magnaporthaceae
Genus	Magnaporthe

Scientific Name

Magnaporthe poae

Common Name

summer patch

Author: Lee Miller, University of Missouri

Reviewed by:Name, Organization

Pathogen

Magnaporthiopsis poae is a sordariomycete in the order Magnaporthales. Single, black, globose, immersed, 252 – 556 μm diameter perithecia have been observed in the laboratory but not in field conditions. Asci are cylindrical, straight or slightly curved, apex rounded but tapering, and numerous. Mature ascospores are fusoid and triseptate, with end cells that are lighter than center cells (3). M. poae form a conspicuous network of dark brown or black, septate, ectotrophic runner hyphae, lobed hyphopodia, and growth cessation structures on infected roots, crowns, and rhizomes (2).

Symptoms and Signs

Foliar symptoms of summer patch are most commonly observed during high summer temperatures after a previous spring infection period. Initial symptoms appear as small, 2-6 inch diameter patches of turf that begin reddish brown and progress to a straw color. In advanced stages, patches may enlarge to > 3 feet in diameter, and form circles or rings with a distinct “frog-eye” center of healthy turf. In large infestations, patches may fuse together and take over large areas of turf. Infested roots, stems, and crowns may appear black and severely rotted. Dense dark-brown to black networks of hyphae are observed on infected plant parts. Lobed hyphopodia and growth cessation structures are also commonly observed on infected plants (1).

Ecology and Spread

Magnaporthiopsis poae are known as ectotrophic root-infecting fungi, indicating superficial growth and colonization of these pathogens along the surface of below-ground plant tissues before infection. Root infection of M. poae begins in the spring at soil temperatures of 65 – 70° F, but foliar symptoms aren’t expressed until air temperatures reach > 85°F. Infected roots are dysfunctional, and unable to translocate the necessary water and nutrients to maintain plant health under summer stress conditions. Summer patch is most common in stands that are two years or older. The role of ascospores in pathogen dissemination is not fully understood. Long distance dissemination may be facilitated by ascospore transport in wind or water. Pathogen dissemination over shorter distances may occur as healthy roots come into contact with infected plant tissues, pathogen growth over small distances in the soil, or mechanical transport on mowers or cultivators. The pathogen may survive in the absence of living host tissue as a resistant structure or as a saprophyte on soil organic matter (1).

Geographic Distribution

Susceptible hosts include Poa spp. (annual bluegrass and Kentucky bluegrass), fine-leaf Festuca spp. and Agrostis stolonifera (creeping bentgrass) (2-4). Disease occurrence is widespread in the transition zone and northern regions of the United States where susceptible turfgrasses are grown.

Management

Like other soilborne diseases, summer patch is difficult to control. In high amenity situations, fungicides are often applied preventively when soil temperatures reach 65°F and watered-in to the soil profile. Chemical control may not be completely effective and should be used in conjunction with cultural control methods..

Cultural Control

Reduce soil pH to 5.5–6.0 through the use of ammonium-based fertilizers. Applications of manganese sulfate at 2 pounds per acre in the spring may also suppress this disease. Avoid excessive nitrogen in spring, particularly with nitrate-based fertilizers. Plan for 75% or more of total nitrogen applications in the fall, and utilize slow release N sources in spring. Reduce thatch buildup through aggressive aerification, vertical mowing and topdressing. When disease symptoms are present, minimize stress through frequent light irrigation and raising mowing heights. A light application of ammonium sulfate (0.2 pounds of nitrogen per 1,000 square feet) may encourage recovery, but be careful to rinse off leaves to minimize burn potential during high temperatures (2).

Host Resistance

Although no Kentucky bluegrass cultivar has been found totally resistant, several have displayed partial tolerance or resistance. Some of these partially resistant cultivars include ‘Mystere’, ‘Midnight’, ‘Barvette HGT’, ‘NuDestiny’ and ‘NuChicago’ (www.ntep.org). In areas with historically high summer patch pressure, a cultivar blend including a partially resistant cultivar, or a mixture of Kentucky bluegrass and tall fescue should be used.

Diagnostic procedures

Symptomology of the disease may be confused with necrotic ring spot, yellow patch, or Microdochium patch, but unlike these diseases, summer patch damage is observed during the summer months. Summer patch is most often diagnosed based on symptom timing. combined with the observation of the dark network of hyphae on roots with a dissecting scope or hand lens. Observation of growth cessation structures or hyphopodia with a compound microscope in infested roots can also aid diagnosis. Although other soil contaminants can be a problem, M. poae can be directly isolated onto half-strength potato dextrose agar (PDA). Mycelium is first hyaline, and then turns gray or olive-brown with dark, thick strands. Peripheral hyphae are wavy and curl back towards the center of the colony (1, 3).

Resources and References

• Landschoot, P. J., Gould, A. B., and Clarke, B. B. 1993. Ecology and epidemiology of ectotrophic root-infecting fungi associated with patch diseases of turfgrasses. Pages 73-105 in: Turfgrass Patch Diseases: Caused by Ectotrophic Root-Infecting Fungi, B. B. Clarke and A. B. Gould, eds. APS Press, St. Paul, MN.
• Smiley, R. M., Dernoeden, P. H., and Clarke, B. B. 2005. Compendium of Turfgrass Diseases. APS Press, St. Paul, MN.
• Smith, J. D., Jackson, N., and Woolhouse, A. R. 1989. Fungal Diseases of Amenity Turfgrasses. E & F.N. Spon Ltd., London.
• Tredway, L. P. 2005. First report of summer patch of creeping bentgrass caused by Magnaporthe poae in North Carolina. Plant Dis 89 (2):204.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30818 as part of "Diagnostic Image Series Development for Supporting IPM in the North Central Region" (USDA-NIFA-RIPM-003349)