Clavibacter michiganensis subsp. sepedonicus

(Ring rot)

• Causal agent and transmission

The bacterium Clavibacter michiganensis subsp. sepedonicus (synonym Corynebacterium sepedonicum) is the causal agent for bacterial wilt in growing crops and for ring rot on tubers.

This bacterium is typical of areas with a cool climate: Scandinavia, North Europe, North Asia, North America and temperate zones in South America. However, it has been discovered more recently in countries with a warmer climate such as Greece and Spain. The natural host of the bacterium is the potato, although it can also be transmitted to the aubergine or the tomato by artificial contamination.

The bacterium penetrates the plant’s vascular system through wounds. It multiplies actively in the plant and the accumulation of the bacteria and the products of degradation of the plant tissues cause occlusion of the xylem vessels. Then the circulation of the sap is disturbed and consequently the plant wilts.

• Significance

Ring rot is a quarantine disease of the potato that is listed in the European Plant Health Directive and which may be very destructive to the potato crop (wilting, yield losses up to 50%, tuber rot) when cutting seed tubers at the time of planting. As the bacterium is highly contagious and persistent in plant and soil debris, this disease is submitted to strict regulation to avoid its dissemination by plant transport.