Why Are My Cabbage Leaves Yellowing From the Edges Inward?

Cabbage leaves developing a characteristic V-shaped or wedge-shaped yellow-to-brown lesion starting at the leaf margin or tip and advancing toward the midrib — often with clearly blackened veins visible within and beyond the lesion — have black rot, caused by the bacterium Xanthomonas campestris pv. campestris. This is one of the most serious bacterial diseases of brassicas worldwide, and the distinctive pattern of the lesion (always V-shaped from the leaf edge, always with black veins) makes it relatively easy to identify once you know what to look for.

The diagnostic pattern

The V-shaped lesion is the key identifier. It originates at a natural leaf pore (hydathode) at the leaf margin — bacteria enter through these pores and travel into the vascular system, causing the characteristic black vein discolouration. Leaves outside the obvious yellowing zone may show blackened veins when held up to the light. In severe infections, the bacteria spread into the stem and other leaves via the vascular system, and cross-sections of the stem show a black ring — the blackened vascular tissue. A plant with systemically infected stem vascular tissue cannot be saved.

How black rot spreads

Black rot is seed-borne — it can be present on seeds from infected plants and germinates with the seedling. It also spreads through infected plant debris left in the soil and through irrigation water splashing soil onto leaves. Insects, particularly flea beetles that create leaf entry wounds, facilitate spread. Warm, wet conditions favour rapid spread within a planting. Infected plants should be removed immediately and disposed of — not composted.

Prevention and management

Strict rotation (do not grow any brassicas in the same ground more often than every three to four years) prevents soil-resident bacteria from building up. Use seed from reputable suppliers — high-quality commercial seed is tested for black rot. Remove and destroy infected plants promptly. Improve plant spacing and airflow. There are no approved chemical treatments for black rot in the home garden; management is entirely preventive.