Why Do My Brassica Leaves Have Yellow V-Shaped Edges?

A distinctive yellow or pale green wedge-shaped patch spreading inward from the leaf margin, with blackened veins visible through the affected tissue, is one of the most recognisable disease symptoms in the brassica garden. This is black rot, caused by the bacterium Xanthomonas campestris. It is a serious disease that can spread rapidly in warm wet weather and ultimately kill the plant if it reaches the stem and crown.

Why V-shaped patches form

Black rot bacteria enter the plant through the natural pores (hydathodes) at the leaf margins, which secrete water under humid conditions. The bacteria multiply in the leaf's vascular system — the network of veins — which is why the veins turn black or dark brown. As the infection spreads inward from the entry point it creates the characteristic triangular or V-shaped yellow patch. In cross section, the stem of an infected plant shows blackened vascular tissue.

How the disease spreads

Black rot spreads through infected seed, contaminated water (especially overhead irrigation or rain splash), and on tools and hands. It favours warm temperatures (25–30°C) and wet conditions. One infected plant can spread to neighbours rapidly if management is poor. The bacteria can survive in crop debris in the soil for several weeks, making rotation and debris clearance important.

Managing an outbreak

There is no cure for black rot in the home garden — no fungicide or bactericide available to amateur growers is effective against it. Remove infected plants promptly and bin them; never compost them. Disinfect tools with a dilute bleach solution or surgical spirit between plants. Stop overhead watering and switch to drip or base watering to avoid splash spread. Harvest any unaffected plants that are near maturity before the disease reaches them.

Prevention

Buy certified disease-free seed or start with reputable transplants. Maintain a four-year brassica rotation. Remove all brassica debris at the end of the season. Avoid overhead irrigation. Resistant varieties are available for some brassica types — check catalogues for black rot resistance ratings.