Rhizoctonia Root Rot is caused by the soil-borne fungus Rhizoctonia solani. It can survive for years in soil and crop debris as fungal growth and hardened survival structures called sclerotia. Infection of mature beets begins as small brown scars that enlarge into dark brown or black lesions extending into the root. Cracking, cavities and secondary rots may follow. Infection can also spread into the crown, causing foliage to wilt and die. Warm soil combined with high soil moisture strongly favours disease development.

How to Fix It
Improve drainage: Grow beets in well-drained soil and avoid keeping the root zone excessively wet.
Crop rotation: Rotate away from beets for at least three years. Because Rhizoctonia has a very broad host range, rotation with non-host crops such as small grains is more effective than simply switching to another vegetable.
Remove infected roots: Pull badly affected beets and remove them from the garden rather than leaving infected tissue in the soil.
Clean up crop debris: Remove infected plant material after harvest to reduce the amount of fungus surviving between crops.
Avoid spreading contaminated soil: Soil containing Rhizoctonia can be carried on tools, footwear and equipment. Clean them before moving from an affected bed to another growing area.
Choose resistant varieties: Where Rhizoctonia is a recurring problem, choose beet varieties reported to have resistance or reduced susceptibility.