Thanatephorus cucumeris is a basidiomyceteous fungus, the anamorph Rhizoctonia solani belongs to the Agonomycetales which do not produce conidia. R. solani is considered to be an aggregate species including different (host-specific) entities which may be differentiated by anastomosis grouping (AG); R. solani AG-4 and AG-13 cause diseases on cotton.
As part of a complex of pathogens including also species of the genera Pythium, Thielaviopsis and Fusarium, T. cucumeris can attack the seed before or at germination. The pathogen(s) causing seedling disease result(s) in pre- and post-emergence damping off. Early infection causes seed decay and pre- and post-emergence damping-off. Symptoms include seed decay and seedling decay before emergence, girdling of the emerged seedling stems, and root rot. The disease is associated with a soft, watery rot.
Later infection affects the stem cortex and may result in stem canker, wirestem and eyespot. Infected seedlings that emerge are pale, stunted, slower growing, and may die within a few days. Infections due to T. cucumeris are characterized by reddish brown, sunken lesions at or below ground level. Lesions enlarge, girdle the stem, and cause shrivelling (wirestems). Seedling disease results in uneven, slow-growing stands; in some years, replanting is necessary.
The fungus is present in most soils, surviving as actively growing mycelium, resting mycelium or sclerotia, and has been also detected on cotton seeds.
T. cucumeris forms hyaline, multinucleate hyphae (Ø 8 - 12 µm) with a prominent dolipore septum. Young hyphae typically branch at 90 ° angles, with constrictions at the point of origin of hyphal branches. The formation of sclerotia, often on the plant surface, is stimulated by a sudden change in temperature or by brief flooding. They are irregularly hemispherical, 1 - 6 mm in diameter, but may coalesce to form larger structures. The sclerotia are extremely resistant and may remain infective for up to 21 months in dry soil. Survival in soil is influenced by soil factors and by soil micro-organisms. The fungus spreads by soil movement with water, and on contaminated tools and plant parts. Basidiospores are only rarely produced and are considered to be of minor importance in epidemiology.