See:
Greenhouse Plants, Ornamental - Black Root Rot
Note This fungal root disease is widely distributed being reported from at least 30 plant families, including many ornamental plants. The pathogen forms thick-walled chlamydospores, which can survive several years in soil. Aboveground symptoms include stunted growth, chlorosis, rolling of upper leaves, and reduction in size and number of flowers. Initially, roots or portions of roots are brown, then develop flat, black root lesions with the formation of chlamydospores on the root surface and in the root cortex. Ruthless sanitation and proper environmental controls are necessary for successful disease management. Fungicides will not compensate for poor sanitation or environmental controls.
References Keller, J. R. and Potter, H. S. 1954. Thielaviopsis associated with root rot of some ornamental plants. Plant Disease Reporter, 38:354-355.