See:
Greenhouse Plants, Ornamental - Black Root Rot
Note Out of over a hundred samples there have been 5 that were diagnosed with black root rot by the OSU Plant Clinic. 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. The diagnostic flat, black root lesions occur because of 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.