Blackleg

Blackleg refers to two entirely different diseases. One is a fungal disease of the cabbage family, and the other is a bacterial disease of potatoes. I know, I know. It gets confusing sometimes. But understanding how these conditions start makes them easier to prevent.

Leaf symptoms of blackleg on rutabaga (Eleanor Brock) CC BY 4.0

Blackleg of cabbages

All plants in the cabbage family, except horseradish, are susceptible to blackleg. This disease used to wreak havoc across the country. Once scientists determined that fungal spores (Leptosphaeria maculans) were the cause, seed growers started treating cruciferous seeds with hot water. This treatment reduces the chances of disease but does not eliminate it.

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

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

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!

The fungal spores of blackleg often find their way into your garden on seeds. Once they arrive, they hitchhike on tools, splashing water, wind, or infected plant material that has been cold composted. These fungi can also gain entry into otherwise healthy plants through cabbage maggots, cutworms, 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 of potato stem (Amy Charkowski) Public Domain

Blackleg of potatoes

Potato plants infected with Pectobacterium carotovorum develop blackleg. This condition causes chlorosis, stunting, upright growth, wilting, cankers, and plant death. Those cankers develop on the lower stem or “leg” of the plant. Those cankers are dark brown or black.

Blackleg is more likely when seed planting occurs during cool, wet weather (spring or fall), and seedlings emerge during hot weather.

Blackleg controls

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

• Only use 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, such as shepherd’s purse, wild mustards, and yellow rocket. 
• Regularly disinfect tools with a household cleaner.
• Place new plants in quarantine until you are sure 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 install similar plants in that spot for at least three years. Once in place, blackleg can be very difficult to eradicate.

Both cases of blackleg are arguments against planting food from the grocery store to start garden plants. Food may be certified safe to eat, but it is not guaranteed to be disease-free.