Sometimes it’s amazing that anything manages to grow in the vegetable garden because there are so many things that can go wrong. Fungal and bacterial disease can spell trouble and are difficult to control. These diseases are further complicated when multiple diseases share a common name, causing confusion over treatment. Blackleg disease in vegetables can refer to a fungal pathogen that affects cole crops or bacteria that attacks potatoes. We’ll discuss both in this article so you can manage whichever blackleg plant disease happens to be troubling you.
What is Blackleg Disease?
Blackleg disease in cole crops is caused by the fungus Phoma lingam, which overwinters in soil, on crop debris, and in infected seed. It’s easy to transmit from plant to plant and difficult to control without excellent sanitation practices. Blackleg can strike at any stage of development, but usually starts on seedlings two to three weeks from transplantation.
Potato blackleg, on the other hand, is caused by the bacteria Erwinia carotovora subspecies atroseptica. Bacteria remain dormant in seed potatoes and become active when conditions are right, making it both unpredictable and brutal. Like with cole crop blackleg, there are no sprays or chemicals that can stop this blackleg, only cultural controls will destroy the disease.
What Does Blackleg Look Like?
Cole crop blackleg appears first on young plants as small brown lesions that expand into circular areas with gray centers covered in black dots. As these areas grow, young plants may die quickly. Older plants can sometimes tolerate a low-level infection, causing lesions with reddish margins. If these spots appear low on stems though, the plants can be girdled and will die. Roots can also become infected, causing wilt symptoms including yellow leaves that don’t fall off the plant.
Blackleg symptoms in potatoes are very different from cole crops. They typically involve very inky black lesions that form on infected stems and tubers. Leaves above these spots will yellow and tend to roll upwards. If the weather is very wet, affected potatoes may be slimy; in dry weather, infected tissue may simply shrivel and die.
Treatment for Blackleg Disease
There is no effective treatment for either type of blackleg once it has taken hold, so it’s important to prevent it from getting into your garden in the first place. A four year crop rotation will help kill off both forms of the disease, along with planting only certified, disease-free seeds and seed potatoes. Starting cole crops in a seedbed so you can carefully inspect them for signs of blackleg is recommended; toss out anything that even remotely looks infected.
Good sanitation, including removing infected plants, cleaning up fallen plant debris, and destroying spent plants promptly, will help slow or stop blackleg. Keeping your garden as dry as possible is also a good way to create an unhealthy environment for bacteria and fungus. Good circulation after harvest can keep blackleg from ruining potato harvests.