Black rot of cabbage-family crops

About

Black rot is a serious bacterial disease of cabbage, broccoli, cauliflower, kale and their relatives. It usually arrives on seed or transplants, and one infected seed in 10,000 can start a severe outbreak.

Also called black rot

How to recognize it

Symptoms

Yellow, V-shaped patches start at the leaf edge and point toward the midrib, and the veins inside them turn black. Patches turn brown and dry, leaves drop, and the infection can move into the stem, blackening its inner tissue and wilting the plant.

Often confused with

Fusarium yellows also yellows cabbage leaves, often on one side of the plant, but does not blacken the leaf veins in V-shaped patches.

Worse when

Warm (80 to 86°F), humid, wet weather and anything that wounds leaves.

How it spreads

Carried on seed and transplants and spread by splashing water, wind, insects, hands and tools; bacteria survive up to two years in plant debris and in mustard-family weeds.

When

Warm, wet weather in summer.

What to do, gentlest first

Prevention works best; most sprays only protect healthy leaves and can't cure infected ones. Always read and follow the product label.

1. Clean seed and transplants Prevent

   Use certified disease-free seed, or hot-water-treated seed (UMass: 15 to 30 minutes at 122°F/50°C, then dry and test germination). Buy clean transplants.

   Hot-water treatment must be exact or it can kill seed.

2. Resistant varieties Prevent

   WSU lists resistant cabbages such as 'Defender' and 'Guardian' and tolerant ones such as 'King Cole' and 'Superdane'.

3. Rotate and weed Prevent

   Keep cabbage-family crops out of the same spot for three years and control mustard-family weeds.

4. Drip and dry leaves Prevent

   Use drip irrigation, avoid working in wet plants, and clean tools with 70% alcohol or 10% bleach.

5. Remove infected plants and debris By hand

   Pull infected plants and destroy them rather than composting, and clear crop debris after harvest.

6. Copper, before infection only Organic spray

   UMass says copper may help prevent infection if applied before plants are infected, every 7 to 10 days with good coverage of leaf undersides.

   Copper does not cure infected plants, and WSU recommends no chemical control for home gardens. Copper only protects leaves it coats before infection; it will not heal spots already there. It can burn young leaves, especially in weather above 85°F, and some plants are sensitive (Purdue lists celery, strawberry, apple, pear and plum among them). PNW 591 rates copper hydroxide toxic to bees, so spray when bees are not foraging. Follow the label, including the days to wait before harvest.

Plants it affects

Arugula, Bok choy, Broccoli, Brussels sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Mustard greens, Napa cabbage, Radish, Rutabaga, Tatsoi, Turnip