Ring rot is caused by the quarantine bacterium, Clavibacter michiganensis subsp. sepedonicus (Cms). It is a serious plant health threat to potato production in Northern Europe. Ring rot has never been found in Scottish potatoes.
Infected seed lots are the main source of disease spread and Cms can persist for long periods (> 2 years) on surfaces, which raises the prospect of contaminated machinery and storage containers serving as a means of infection.
Cms causes wilting and chlorosis, which starts at the leaf margins. As the disease progresses the plant can collapse. Tubers develop characteristic symptoms, similar to brown rot, where the vascular tissue becomes discolured eventually leading to collapse and rotting of the tuber.
Although in Europe, the economic damage caused by the disease directly, is low. The costs associated with rejection of the infected crop, clean-up measures on farm, loss of reputation etc. for the grower concerned can be very high.