Black Root is caused by Aphanomyces raphani, a soil-borne, fungus-like organism. It survives in soil and infected crop debris as resting spores and infects roots, often where side roots emerge or where the root has been damaged. Lesions begin brown to bluish-black and enlarge into dark, constricted areas that may completely girdle the root. Infection can stunt seedlings, distort developing radishes and cause black discoloration extending into the root. Wet soil encourages infection, and disease can become severe under warm, moist conditions.

How to Fix It
Improve drainage: Plant radishes in well-drained soil and avoid overwatering. Aphanomyces needs abundant soil moisture to infect roots.
Crop rotation: Avoid radishes and other susceptible brassicas in the affected area for at least 3–4 years. Rotate with non-brassica crops.
Control brassica weeds: Remove wild radish, mustards and other cruciferous weeds that can maintain the pathogen between crops.
Clean up crop debris: Remove or thoroughly incorporate infected plant material after harvest.
Choose resistant varieties: Where Black Root is a recurring problem, grow resistant or tolerant radishes. French Breakfast, Fuego and Belle Glade are among varieties reported as resistant or relatively resistant.
Avoid spreading contaminated soil: Soil containing the pathogen can be moved on tools, machinery and infected plant material, so clean soil from equipment before moving to another garden area.