Black root rot is caused by the fungus Thielaviopsis basicola (Berk. & Broome) Ferraris. It infects over 137 species of plants from a wide range of families including cotton (Gossypium hirsutum Linn.), tobacco (Nicotiana tabacum Linn.) and many legumes (Otani, 1962; cited in Meyer, Shew and Shoemaker, 1989). Infected plants are usually stunted during the seedling stages, leading to delayed maturity and loss of production (Young and Allen, 1991 ). Seedlings and older plants pulled from the soil often have shriveled and blackened root tissues. Survival in the soil is by thick-walled resting spores (called chlamydospores) which may remain viable in the soil for many years (Hillocks, 1992).