Pythium (Pythium blight)
Author: Lee Miller, University of Missouri
Reviewed by:Name, Organization
Pathogen
All turfgrass-infecting Pythium spp. are homothallic, produce oospores and sporangia, and coenocytic (aseptate) mycelia. Although they may appear so in their vegetative state, Pythium spp. are not true fungi. Pythium is more closely related to diatoms and algae, and differs from fungi by reproducing asexually through biflagellate zoospores, oogamous (oospore) sexual reproduction, producing cellulose in their cell walls, a diploid thallus, and several other characters. Morphological speciation is mainly based on oospore size and characters, and the structure and arrangement of the oogonium and antheridia. Oospores observed in tissue may vary in size from 15-30 μm diameter depending on the species (4).
Symptoms and Signs
Pythium blight is initially noticed as irregularly shaped, small (1-4 inch diameter), patches, often with a purple to reddish hue and frogeye appearance. As the disease progresses, patches may merge and streak along areas of surface water flow. Patches may also eventually turn a straw or bleached color, with individual leaves that become stuck and matted to each other. Individual plants initially exhibit water soaked lesions that eventually turn a tan color. In an active outbreak, cottony mycelium may be observed in early morning, which may cause a grayish appearance to patch margins. As colonization of the fungus continues, the leaves become soft and slimy causing the blades to mat together (3). Due to these sign and symptom characters, Pythium blight also goes by the common names “cottony blight”, “water mold”, and “greasy spot” (4).
The collection number is the taken from the set of images you selected in the Bugwood Image Database.
Ecology and Spread
Pythium blight is a warm-weather disease that primarily affects cool-season grasses but can also occur on warm-season turfgrasses growing slowly in cool, wet periods. The disease is most destructive during humid weather when high temperatures are > 90o F. Pythium spp. can also cause seedling dampling off or a root rot symptom, but the foliar blight symptoms are most often observed on Kentucky bluegrass or tall fescue, and not on mature creeping bentgrass putting greens. The pathogen is often present in the thatch and/or soil, but also survives well in waterlogged conditions or on pond debris. The oospore, the sexual structure of Pythium spp., is highly resistant to environmental stresses, and enables the pathogen to overwinter within infested leaf tissue or organic matter. Pythium blight spreads from plant to plant via mycelial growth, but also spreads easily in water films with motile zoospores (5).
Geographic Distribution
Pythium species affecting turfgrasses have been identified in Europe, Japan, and the United States (2).
Management
Unfortunately, Pythium blight is one of the most quickly developing and devastating fungal diseases, so preventive management practices are paramount. Cultural management of Pythium requires good irrigation practices, increasing soil drainage, and avoiding excess nitrogen levels.
Cultural Control
Pythium blight ~ 14 hours of leaf wetness duration to cause a severe epidemic. Therefore, maintaining adequate soil drainage and reducing leaf wetness duration are essential cultural control practices. Irrigating, mowing, or other practices aimed at removing dew/guttation fluid early in the morning will reduce Pythium severity. Pruning trees or utilizing fan on high amenity areas to encourage air movement will also help mitigate this disease. Pythium blight is also much more severe on highly fertilized, lush turfgrasses, so applying fertilizer sparingly in spring and early summer can reduce disease occurrence.
Host Resistance
No cultivars are known to be resistant to Pythium diseases, but there are some species differences. Kentucky bluegrass is more susceptible to attack than tall fescue. Pythium diseases are rarely observed on higher cut, warm season turfgrasses used for lawns and sports fields.
Diagnostic procedures
White, cottony mycelium is often observed on affected leaves and sheaths in early morning dew or after 12-24 h incubation in a humidity chamber. Mycelium should be observed under the microscope to confirm it is not septate, and therefore not Sclerotinia homoeocarpa (dollar spot) or Rhizoctonia solani, (brown patch) which may also produce hyphae in humid environments (1). Under the microscope, thick, double walled oospores are also a conspicuous indicator of Pythium presence in infected plant tissue.
Resources and References
1. Allen, T. W., Martinez, A., and Burpee, L. L. 2004. Pythium blight of turfgrass. The Plant Health Instructor.
2. Beard, J. B., and Tani, T. 1997. Color Atlas of Turfgrass Diseases. Ann Arbor Press, Chelsea, Michigan.
3. Couch, H. B. 1995. Diseases of Turfgrasses. Krieger Publishing, Malabar, FL.
4. Smiley, R. M., Dernoeden, P. H., and Clarke, B. B. 2005. Compendium of Turfgrass Diseases. APS Press, St. Paul, MN.
5. Vargas, J. M. 1994. Management of Turfgrass Diseases Lewis Publishers, Boca Raton, FL.
