(Basidiomycota: Class Basidiomycetes) [Rhizoctonia, damping off, black scurf]
Life cycle
Rhizoctonia solani was long considered a ‘sterile fungus’, as it did not appear to produce any spores. The fungus does produce basidiospores, however, and therefore belongs to the Basidiomycetes. The teleomorph name is Thanatephorus cucumeris. R. solani is a ubiquitous fungus with a wide range of host plants. The fungus persists in infected soil for a long time. It usually only produces mycelium. The fungus overwinters as sclerotia or mycelium in soil, on crop residue and on seed. The mycelium grows on the plant surface, forming infection cushions from which the plant is infected. The shape of the hyphae is characteristic: the lateral hyphae emerge at right angles. The fungus sometimes produces sexual basidiospores, which germinate and infect the plant through the stomata. Mycelium infection also occurs via stomata and wounds. The fungus affects the roots and base of the plant, but also the stems, leaves and fruits if they are close to the ground. It is dispersed by rain and water and via machines and other materials and indeed anything with infected soil on it. The fungus also grows from plant to plant. Vigorously growing plants are less susceptible than plants with irregular growth or weaker plants. Black scurf develops on potato tubers, which is actually the sclerotia of R. solani.
Twelve ‘anastomosis groups’ (AG’s) are identified within the R. solani genus. Anastomosis is the process by which two hyphae meet and fuse. This only happens between R. solani isolates from the same AG. When isolates from different AGs meet, the cells surrounding that point die. The AGs are reasonably specific for different hosts. Subgroups are recognised within the AGs. Saprophytic R. solani isolates also exist.
Host plants
The optimum temperature range for infection is 15-18 °C. The fungus thrives best in wet soil.
