Thielaviopsis root and stem rot, often called Black rot, is caused by Thielaviopsis basicola. Roots and lower stems may be shrivelled, brown to black and under-developed. The black roots can be seen by washing roots free of growing media and then viewing them carefully with microscope as seen in this photo. Confirm your diagnosis through your University plant diagnostic lab as proper diagnosis is needed to determine an effective management strategy.
Practice good sanitation beginning 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 infected plants, but will prevent infection of healthy plants. Discard infected plants and remove plant debris and infested soil. See the current greenhouse recommendation guide for information on fungicides.