Black rot

a yellow, V-shaped patch eating in from the edge of a leaf, with the point of the V aimed inward toward a vein — like a wedge or a slice cut out of the leaf margin. It usually shows on the older, outer leaves first.

hold the leaf up to the sunlight and look inside that yellow V — the veins have turned black, like a net of dark ink lines. As the patch grows it turns brown, dry and papery, and the black vein-work reaches deeper into the leaf.

the whole plant turns stunted and lopsided — yellowing and wilting worse on one side. Cut across the stem near the base and you will see a black or brown ring in the middle where the sap-tubes have rotted. This means the germ has spread through the whole plant (gone "systemic") — that plant cannot recover.

with: downy mildew (yellow patches with a grey-white fuzz on the leaf underside, and no black veins); fertiliser or wind scorch (brown, dried leaf edges but no yellow V and no blackened veins); or plain nitrogen hunger (older leaves go yellow evenly all over, not in a wedge from the edge). Black rot is the one with the yellow V from the margin and the blackened veins inside it.

Check seedlings every few days from the moment they come up, again in the first weeks after transplanting, and right through heading — the disease does its worst when it gets in early, while plants are young.

The bacteria love heat (about 25-30°C) and heavy, damp air, and they travel in rain-splash and overhead-irrigation splash — so step up your checks after every rain, after every warm humid spell, and after any overhead watering.

so you cross all four corners and the middle, not just the row nearest the path. Make about 10 stops, and at each stop check 10 plants — look at the outer leaf edges, hold a leaf up to the light for black veins, and watch for lopsided, stunted plants.

In the nursery, even a single seedling with a yellow V or a blackened vein means pull it and its close neighbours at once. In the field, the first V-shaped lesion you find is your signal to act that same day — because one infected plant becomes twenty after the next rain.

Sow only certified seed (seed that has been tested free of the disease), or treat your own seed with the hot-water method (soaking the seed in carefully warmed water to kill the bacteria carried on it). Raise your transplants on a clean nursery bed well away from old crucifer ground, and never lift seedlings from a bed that has any sick plants in it.

Do not plant cabbage, kale, broccoli or cauliflower on the same land for 2-3 years — the bacteria survive on old crucifer debris and on weeds of the same family (wild mustard, wild rape), so pull those weeds out too. After harvest, remove and destroy every old stalk and leaf; do not leave them lying or compost them near the field.

Stop overhead irrigation — water at the base of the plant so the leaves stay dry, and water in the morning so anything wet dries fast. Never weed, tie or handle the crop while it is wet with rain or dew: your hands and tools carry the bacteria from plant to plant. Give plants room to breathe rather than crowding them, so the leaves dry quickly.

where one is available and suited to your area. Tolerant cabbage types do not get sick as badly and lose far less to the systemic rot — ask your local seed supplier or extension officer which variety is recommended for you.

Remove any seedling or plant showing a yellow V or black veins, together with its nearest neighbours, and carry them right out of the field in a bag — do not drop infected leaves along the rows. Do this when the plants are dry, and wash your hands and tools afterwards so you don't spread it yourself.

Copper gives only partial protection and works only as a shield on healthy leaves before infection — it cannot cure a plant the bacteria are already inside. If you use it, spray it preventively at the first sign of risk, and stay safe: cover your skin, eyes, nose and mouth, spray in the cool of the day with the wind at your back, keep it off food, off open water, and away from children and bees, wait the full pre-harvest interval (the set number of days you must leave between spraying and picking) before you harvest, and wash well afterwards. Don't lean on copper alone every season — rotate it with the clean-up steps above so the bacteria don't grow used to it.

stop all overhead watering, and harvest any healthy heads early rather than risk losing them. Once a plant is stunted, lopsided, or shows the black ring in the stem, it is finished — pull it and destroy it, don't try to save it. If a whole block is lost, cut your losses: clear and burn or deep-bury every plant, keep that land out of cabbage, kale, broccoli and cauliflower for 2-3 years, and start your next crop with clean certified seed on a fresh, clean nursery site.