Blackleg

Blackleg is a fungal disease of the cabbage family.


​This disease used to wreck havoc across the country. Once it was determined that fungal spores were the cause, crucifer seeds started getting treated with hot water. This reduced the chances of disease, but, once in place, it can still be difficult to get rid of. The spores can sit dormant for three or four years! Blackleg is more likely when planting is done during cool, wet weather (spring or fall), and seedlings emerge during hot weather.

Blackleg on stem (UCANR)

Symptoms of blackleg

Since cabbages don’t actually have legs, you may wonder how this name came about. The word ‘blackleg’ refers to dark lesions that first appear on stems at ground level. Each part of the infected plant has its own set of symptoms:

• Seedlings - pale grey lesions cover and kill young seedlings
• Stems - long, brown, sunken lesions, with purple or pinkish margins
• Roots - dark cankers form on root crops, and all root systems are slowly destroyed
• Leaves - tan, circular leaf spots turn ashy as tissue dies; chlorosis
• Heads - heads may look healthy at harvest, but tissue turns black in storage

Leaf spot on cabbage from blackleg (Illinois Extension)

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Infected stems are prone to snap. If you cut into the vein of an infected plant, you can see the blackened xylem - just be sure to disinfect your tools afterward! 


Blackleg host plants

Blackleg fungi love California’s cool weather crops. This includes cabbage, broccoli, Brussels sprouts, cauliflower, Chinese cabbage, mustards, kale, kohlrabi, collards, radish, even turnips and rutabagas! Horseradish, though a member of the cruciferous family, seems to be resistant to blackleg. Blackleg can also infect potatoes and canola.


Blackleg lifecycle

The fungal spores of blackleg (Phoma lingam) are often carried into your garden on seeds. Once they arrive, they can be distributed on tools, by splashing water, or infected plant material that has been cold composted. One spore type can also be carried on the wind. These fungi can also gain entry into otherwise healthy plants through cabbage maggot, cutworm, cabbageworm, and other pest feeding holes. It only takes a few fungi, with the right amount of warmth and moisture, to create an epidemic in a seed bed, garden row, or agricultural field. Blackleg can also refers to these other pathogens (and their host plants): Erwinia carotovora (delphinium) and Pythium spp. (geraniums).


Blackleg controls

Like many other garden variety diseases, prevention is far easier than cure. Protect your plants and your soil from blackleg with thee good practices:

•     only plant certified disease free seeds
•     plan a 4-year crop rotation
•     avoid overhead watering
•     do not work around plants when they are wet
•     monitor plants every 7 to 10 days for signs of blackleg
•     remove cruciferous weeds: shepherd’s purse, wild mustards, and yellow rocket
•     regularly disinfect tools with a 1:9 bleach and water solution
•     place new plants in quarantine until you are certain that they are disease free

Any time you see signs of blackleg, remove and destroy the plant and make a note of the location. You should not plant any cruciferous plants in that spot for at least three years.