COLONIES fast-growing, colourless to flesh (6B3), with aerial mycelium consisting of radially distributed hyphal aggregations, covering the whole area of potato dextrose agar (PDA) of a diameter of 10 cm in the 5th day of growth, becoming flesh (6B3) to light brown (6D5), more compact and uniform in the structure of aerial mycelium when 10-day old and grown at a room temperature; reverse greyish orange (6B4) to brownish grey (6F8) in 10-day old PDA culture.
HYPHAE (h) subhyaline to apricot yellow (5B6), thick-walled, divided by cells ca. 100-250 µm long and (5-)7-12(17) µm wide, frequently constricted at the septa and branches slanted at ca. 90o relative to their parent hyphae, with no clamp connections, frequently anastomosing.
SCLEROTIA irregular, ca. 1 mm diam, soon turning brown, composed of tightly crowded moniliform hyphae.
PLANT HOST AND DISTRIBUTION. Rhizoctonia solani is a facultative, polyphagous plant pathogen, attacking different parts of plants, especially those located near the soil surface (Agrios 1988; Domsch et al. 1980).
Rhizoctonia solani has a worldwide distribution (Domsch et al. 1980).
NOTES. The teleomorph of R. solani is Thanathephorus cucumeris (Frank) Donk (Holliday 1989). Thanathephorus cucumeris appears rarely, usually under high humidity. The teleomorph forms a white to grey membranous layer of mycelium with barrel-shaped basidia, each with four sterigmata, from which ovoid basidiospores origin.
The genus Rhizoctonia DC. ex Mérat comprises seven species, of which R. solani is the best known one (Domsch et al. 1980).
Rhizoctonia solani can affect most annual plants, including almost all vegetables and flowers, many field crops, perennial ornamentals, shrubs, and trees.
The most frequently disease symptoms caused by R. solani are damping off of seedlings and root rot, stem rot or stem canker of young and mature plants.
In potato, R. solani causes so called "black scurf" and stem canker. The black scurf is produced by small, hard, black sclerotia occurring on the tuber surface. When the tubers are planted, mycelium develops from the sclerotia and its hyphae may invade and kill young shoots of the growing plant. The mycelium may also destroy the roots, then the stems at the soil level and cause stem canker, which is sometimes associated with a leaf curl and the formation of small, green, red, or purple processes on the shoots.
Rhizoctonia solani is considered a "collective" species consisting of at least four, more or less unrelated strains, called AG 1-AG 4 (Smith et al. 1988). AG 1 contains isolates causing seed and hypocotyl rots, aerial blight, and web blight. AG 2 isolates cause canker of root crops and root rots of conifers. AG 3 isolates are pathogens of potato and barley, as well as can cause seed rot. AG 4 isolates cause seed and hypocotyl rots of many angiosperm species. The strains are distinguished based on the ability to form anastomosis (fusion of touching hyphae). Anastomosis occurs only between isolates of the same anastomosis group. Each anastomosis group represents a genetically isolated population of R. solani. Although different anastomosis groups are not host specific, they may show well-defined tendencies in causing specific disease symptoms or in preferring some climatic conditions.
Rhizoctonia solani overwinters as mycelium or sclerotia in the soil, in/on infected plants, or propagation material, such as potato tubers. In many plants, e. g., tomato (Lycopersicon esculentum Mill.), rape (Brassica napus L.), lettuce (Lactuca sativa L.), pea (Pisum sativum L.), red clover (Trifolium pratense L.) and broad bean (Vicia faba L.), the fungus can be carried in the seed (Neergaard 1977).
The optimal conditions for infection by R. solani are 15-18oC, although some races prefer a temperature of up to 35oC and moderately wet soils.
Agrios G. N. 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.
Domsch K. H., Gams W., Anderson T. 1980. Compendium of soil fungi. Acad. Press. London-New York-Toronto-Sydney-San Francisco.
Holliday P. 1989. A dictionary of plant pathology. Cambridge University Press. Cambridge, New York, New Rochelle, Melbourne, Sydney.
Neergaard P. 1977. Seed pathology. The Macmillan Press Ltd. London.
Smith I. M., Dunez J., Lelliott R. A., Phillips D. H., Archer S. A. 1988. European handbook of plant diseases. Blackwell Scientific Publications.