This photo shows a progression of plants that have been infected with Thielviopsis basicola, root rot, often called Black rot. The plant on the left has healthy roots (although plant is nutrient deficient and pot bound), the middle plant showing blackened roots and plant on the right with roots completely dead.
Practice good sanitation, starting with clean growing media, pots, flats, and plant material. Avoid overwatering and overfertilizing. Thielaviopsis is favored by alkaline pH and can be managed by keeping the pH of the media at 5.5. Fungicide applications will not cure affected plants, but will prevent infection of healthy plants. Discard infected plants and remove plant debris and infested soil. Drench the soil with Cleary's 3336,Fungo Flo, Contrast, Medallion, or Terraguard. When stem rot is present, fungicide must contact the stems for effective control. Cleary's 3336 and Fungo Flo consist of the same active ingredient, thiophanate methyl, and should be rotated with Contrast, Medallion, or Terraguard to prevent resistance development.