1. IDENTIFYING CHARACTERISTICS
Saprophytic fungi.
Reproductive organs and spores:
Sexual: Short subcylindrical to barrel or ovoid shaped basidia. The spores are ellipsoid with one side flattened, hyaline to fawn colored.
Asexual: Sclerotia
There are a number of strains or biotypes of the fungus Rhizoctonia, one such example is Rhizoctonia solani. The fungus survives in soil as mycelium in decomposing plant tissue. Mycelium on plant tissue basically consists of a network of hypha. R. solani is divided up into anastomosis groups (AG). These (AG) groups affect a wide variety of crops. Some (AG) examples include AG-4, which is a severe seed rot pathogen. AG-1, AG-2-2, and AG-4 cause severe postemergence damping-off of seedlings. R. solani AG-2-1, AG-3, AG-5, unidentified R. solani and binucleate Rhizoctonia-like fungi are slightly to moderately pathogenic to seedlings. On older plants, R solani AG 2-2 is a severe crown rot pathogen.
3. ISOLATION AND ECOLOGY
Figure J. Infection of a hypha of Rhizoctonia solani from germinating spores of Verticillum biguttatum.
Figure K. This is the strain of Rhizoctonia solani that causes black scurf of potatoes, growing on sterile filter paper in laboratory culture. Most of the fungal growth is inconspicuous, but after the filter paper was colonised the nutrients in the hyphae were mobilised to sites where the fungus produced large sclerotia, which is the survival structures that cause the black scurf symptoms on potato tubers.
Rhizoctonia can be either a soil-borne or seed-borne pathogen. Besides surviving in plant tissue, it can survive as sclerotia on tuber surfaces or in the soil for extended periods of time. Populations of R. solani decline in the absence of a susceptible host although the rate of decline is affected by soil type, rotation of crops and possibly the amount of organic matter present in the soil.
The disease progresses rapidly when air temperatures are between 75-85 degrees Celsius and free moisture is present. Growth stops at temperatures greater than 90 degrees Celsius. The disease can be controlled by limiting moisture on the grass and/or allowing to dry quickly after watering. The fungicide PCNB works well in controlling the disease brown patch, which is caused by the fungus Rhizoctonia.
Image J taken from a videotaped interaction on a thin film of water agar (van den Boogert & Deacon, 1994).
4. ADDITIONAL SOURCES OF INFORMATION
Rhizoctonia species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control. Editors: B. Sneh, S. Jabaji-Hare, S. Neate and G. Dijist. Kluwer Academic Publishers, The Netherlands. 1996
Holt, J.G., N.R. Krieg, P.H.A. Sneath, J.T. Staley, and S.T. Williams. 1994. Bergey's Manual of Determinative Bacteriology. Ninth Edition. Williams and Wilkins. Baltimore, Maryland. 787 pp.
Identification of Rhizoctonia Species. Editors: B. Sneh, Lee Burpee, Akira Ogosh. Publisher: St. Paul, Minn., USA: APS Press c. 1991
5. LINKS TO OTHER SITES ON RHIZOCTONIA
http://www.ndsu.nodak.edu/instruct/gudmesta/lateblight/basic_frame1.htm
http://helios.bto.ed.ac.uk/bto/microbes/apical.htm
http://www.pacificcoast.net/~mycolog/chapter17.htm
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