Blackleg is caused by the bacterium Pectobacterium atrosepticum (previously known as Erwinia carotovora pv. atroseptica). It is one of the few important plant diseases caused by bacteria in the UK.
This disease normally comes into gardens (or allotments) via infected seed potatoes. Infected plants are removed from commercial seed crops, but a small number of infected tubers escape detection and seed lots will usually contain a few of these. These infections in the seed tubers are usually symptomless and impossible for gardeners to detect.
If the infected plants that grow from these are not removed from the crop promptly, the disease will spread through the soil to infect new plants. The bacteria survive over winter principally in infected volunteer tubers and potato plant debris. They also survive at low levels in the root zone of some weeds. The bacteria may also be carried through the crop by insects or on contaminated soil.
However, in gardens it is infected seed potatoes that are the most important source. If affected plants are not removed they may form some tubers, but a large proportion of these will be infected via the stolons, and they will rot (usually from the end to which the stolon was attached) in the soil or during storage.