Blackleg is a fungal disease of cole crops and other brassicas, caused by Leptosphaeria maculans (also known by its asexual stage, Phoma lingam). It can infect seedlings, leaves and stems. Leaf spots are often pale or greyish with tiny black dots inside them, while stem infections develop into dark, sunken lesions or cankers.

As a stem lesion enlarges, it can girdle the plant and interfere with the movement of water and nutrients. Plants may become stunted, wilt or topple over. The fungus survives in infected crop debris and can also be carried on infected seed. Spores spread by rain splash and wind.

How to Fix It

Remove infected plants and brassica debris and dispose of them rather than composting them. At the end of the season, clean up remaining stems and leaves carefully because infected debris is an important source of future infections.

Start with certified disease-free seed or healthy transplants and rotate brassicas with unrelated crops for several years. Avoid planting new brassicas beside old infected crop debris. Give plants adequate spacing, keep weeds in the brassica family under control, and water at soil level to reduce leaf wetness and splash.

Some biological products containing Bacillus or Trichoderma species may provide preventive suppression, but they will not cure an infected plant. For the home garden, clean seed, sanitation and crop rotation are the most important controls.