See:
Greenhouse Plants, Ornamental - Black Root Rot
Note Aboveground symptoms include poor vigor with senescent, yellowing leaves. 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.
References Keller, J. R. and Potter, H. S. 1954. Thielaviopsis associated with root rot of some ornamental plants. Plant Disease Reporter, 38:354-355.