Why Are My Radishes Rotting Black Inside?

Cutting open a radish that looks perfectly healthy from the outside to find black or dark brown discolouration inside the flesh is one of the more alarming radish problems. There are two distinct causes — bacterial black rot, which is a disease, and black heart, which is a physiological disorder from boron deficiency — and they look similar internally but have different causes and different management approaches.

Bacterial black rot (Xanthomonas campestris)

Black rot is a bacterial disease of the brassica family that causes V-shaped yellow lesions on leaf margins and dark brown to black discolouration of the vascular tissue inside the stem and root. If you cut the root transversely you will see a ring of brown-black discolouration following the vascular bundles. The disease spreads via infected seed, infected plant debris, water splash, tools and insects. There is no cure for infected plants — remove and destroy them. Prevent by using certified disease-free seed, practising a three-year rotation away from all brassicas, and avoiding overhead irrigation.

Black heart (boron deficiency)

Black heart causes dark brown to black, dry, corky areas in the centre of the root — the vascular ring is not specifically affected, unlike black rot. The leaves may also show curling, bronzing or distortion. Boron is a micronutrient needed for cell wall development and growth of meristematic tissues. Boron deficiency is more common in alkaline, light or recently limed soils, and in dry conditions where soil moisture limits boron mobility to the roots. Apply borax (sodium tetraborate) at 30g per 10 sq m as a soil drench to correct deficiency. Avoid over-liming — keep pH below 7.5 for brassicas.

Secondary rot from slug or maggot damage

Entry wounds from slug feeding or root maggot tunnelling allow secondary bacterial and fungal pathogens to enter the root, causing localised black rot from the wound inward. This type of rotting starts at the wound and spreads outward in a roughly circular pattern. The fix is to prevent the initial pest damage rather than treating the rot directly.

Rotation and hygiene

For bacterial black rot, a three to four year rotation away from all brassicas (cabbage, broccoli, turnip, radish, mustard) is the primary long-term prevention. Remove all crop debris at the end of the season. Avoid walking soil from infected beds to clean beds on boots or tools.