Thielaviopsis is a small genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of fungi in the order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Microascales

Microascales

The Microascales are an order of fungi in the Sordariomycetes class, subclass Hypocreomycetidae. This is a relatively small order of mostly saprobic fungi that live in soil, rotting vegetation and dung. Some species are plant pathogens, such as Ceratocystis fimbriata, transmitted by beetles to...

. The genus includes several important agricultural pathogens

Phytopathology

Plant pathology is the scientific study of plant diseases caused by pathogens and environmental conditions . Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants...

. The most widespread is T. basicola, the causal agent in several root rot diseases of economically important crop species including cotton

Cotton

Cotton is a soft, fluffy staple fiber that grows in a boll, or protective capsule, around the seeds of cotton plants of the genus Gossypium. The fiber is almost pure cellulose. The botanical purpose of cotton fiber is to aid in seed dispersal....

 and a variety of vegetables. In cotton, Thielaviopsis root rot, also known as black root rot causes necrosis of the roots and stunting of the crop plants.