Thomas, S.L., L.H. Rhodes, and M.J. Boehm. 2004. Following the disease progression of an ectotrophic root-infecting fungus.The Plant Health Instructor. DOI: 10.1094/PHI-I-2004-1215-01
Samantha L. Thomas, Landon H. Rhodes, and Michael J. Boehm,Department of Plant Pathology, The Ohio State University, Columbus, OH
Familiarize students with the infection and colonization process of root-infecting fungi.
Soilborne root-infecting fungi can be damaging to susceptible hosts. Control of diseases caused by root-infecting fungi is difficult when symptom expression within the host is delayed until periods of unfavorable environmental conditions, such as heat or drought stress. By the time symptoms develop, up to 50% or more of the host plant's roots may be infected. Some disease examples caused by root-infecting fungi include take-all of wheat, Pythium root rot, Rhizoctonia root rot of beans, and black root rot of pansies (caused by Thielaviopsis basicola).
Gaeumannomyces graminis is an ascomycetous fungus that infects and colonizes roots of graminaceous plants. Three morphological variants of this fungus have been described, but all three varieties produce specialized appressoria, called hyphopodia, and darkly-pigmented, ectotrophic, runner hyphae . Hyphopodia function as organs which attach the fungus to host roots. Darkly-pigmented, ectotrophic, runner hyphae grow along the root exterior and disseminate the pathogen by growing from root to root.
Take-all of wheat (Triticum aestivum), caused by Gaeumannomyces graminis var. tritici, is a devastating disease on wheat grown in the Pacific Northwest region of the United States. The disease cycle of the fungus is fairly simple as the fungus survives, disseminates, and infects through mycelial growth (Figure 1). The fungus may survive between wheat crops in infested wheat residues or on the roots of weedy grass species that may grow in or near wheat fields. When a new wheat crop is planted, the fungus grows out from infested crop or weed residue and infects newly seeded plants (Figure 2). The fungus will grow ectotrophically producing hyphopodia and infection hyphae which then penetrate the root and begin the colonization process. Fungal mycelium grows throughout the host roots, around plant cells, and eventually into the stele. Once G. graminis var. tritici begins colonizing the stele, vascular dysfunction occurs, and water and nutrient uptake from the soil is reduced. Vascular discoloration (Figure 3), a key diagnostic symptom, is produced by fungal colonization of the stele. Fungal spread from plant to plant occurs as dark ectotrophic, runner hyphae grow from root to root. Once a host is colonized, perithecia are typically produced on the plant crown or embedded in host tissues (Figures 4, 5). Perithecia contain many asci (Figure 6) and each ascus contains eight ascospores (Figure 7). Ascospores are not considered to play an important role in disseminating this pathogen.
Similar disease cycles occur for other G. graminis-incited diseases, such as those caused by G. graminis var. avenae on creeping bentgrass (Agrostis palustris) and oats (Avena sativa) and G. graminis var. graminis on rice (Oryza sativa). The objective of this exercise is to familiarize students with the root-infection process utilizing test tube assays which permit the observation of the disease progression. This assay is also a good way to produce perithecia of G. graminis. (Laboratory Instructor's Note #1)
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For Instructor/Laboratory preparers: (Laboratory Instructor's Note #2)
Preparation of materials for students (by instructor):
Table 1. Be sure to note pathogen and host growth, the date root infection is first observed on each host, and any signs or symptoms as they are produced. (Laboratory Instructor's Note #9)
Bockus, W.W. and N.A. Tisserat. 2000. Take-all root rot. The Plant Health Instructor. DOI:10.1094/PHI-I-2000-1020-01.
Clarke, B.B. and A.B.Gould, eds. 1993. Turfgrass Patch Diseases Caused by Ectotrophic Root-infecting Fungi. American Phytopathological Society Press, St. Paul, MN.
Smiley, R.W., P.H. Dernoeden, and B.B. Clarke. 1992. Compendium of Turfgrass Diseases, 2nd Ed., American Phytopathological Society Press, St. Paul, MN.