Thielaviopsis is a small genus of fungi in the order Microascales, and family Ceratocystidaceae. The genus includes several important agricultural based pathogens. The most widespread is T. basicola, the causal agent in several root rot diseases of economically important crop species including cotton and a variety of vegetables. In cotton, Thielaviopsis causes root rot, also known as black root rot, which causes necrosis of the roots and stunting of the crop plants.