Black rot of brassicas

Xanthomonas campestris pv. campestris

The main bacterial disease of cabbage and kale: yellow V-shaped wedges eating in from the leaf edge with the veins inside them turned black.

Bacterium Serious Seed-borneRain splashSpread on tools & handsSpread in water Spreads to neighbors

What you'll see

A yellow V or wedge biting in from the leaf margin, with its point aimed at the midrib.
Veins inside the wedge blackened, visible as a dark net when the leaf is held to the light.
The blackening running down the leaf stalk into the stem, and a ring of black dots when a stem is cut across.
Older leaves collapsing and dropping, leaving a bare-legged plant with a small heart.
Heads and curds rotting soft and foul once secondary soft rot follows the bacteria in.

Why it happens

A bacterium that arrives chiefly on seed and on bought transplants, then moves in water — rain splash, overhead irrigation, wet leaves brushing each other, and your hands and knife working down a wet row. It enters through the little water pores at the leaf margin, which is why the damage always starts at the edge rather than in the middle.

Warm and wet is what it wants: temperatures above 25 °C with rain or heavy dew. A humid summer, a bed watered from overhead, or a crowded row of transplants set out in a wet spell will all bring it on.

Treating it

There is no cure for an infected plant. Pull and bag the worst, and take out every plant if the row is going down.
Never work among wet brassicas; wait until the leaves have dried before harvesting, weeding or thinning.
Strip the marked outer leaves off a lightly hit cabbage and use the heart, which is often sound.
Do not compost the debris in a heap that will not get hot; bin or burn it instead.
Sterilize knives and pruners between plants with alcohol when cutting brassicas in an affected bed.

Keeping it away

Buy hot-water-treated seed, or treat your own at 50 °C for 20 minutes — this is the single most effective measure there is.
Raise your own transplants rather than buying in, and inspect any bought plants at the leaf margins.
Rotate brassicas on a three- or four-year cycle and clear every stump and root at the end of the season.
Water at the base and space plants so the leaves dry quickly.
Control cruciferous weeds — shepherd purse, wild mustard — which carry it between crops.

Who gets it

Cabbage, kale, broccoli, cauliflower, Brussels sprouts, collards, turnips and radishes.