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Clavibacter michiganensis subsp. sepedonicus

EPPO code



Corynebacterium sepedonicum

Corynebacterium michiganense pv. sepedonicum

Common names

English names: Bacterial ring rot
Nordic names: Kartoffelringbakteriose (DK), Rengasmätä (FI), Ringbakteriose(NO), Ljus ringröta(SE)
Estonian name: Kartuli-ringmädanik

Major host plants

Natural infections have been found on potatoes only. In inoculation tests, many members of the Solanaceae, including tomatoes and aubergines, were found to be susceptible.


The first symptoms of wilting develop in the lower leaves. The margins of the leaves roll inwards and upwards, and the surface loses its light shiny appearance. As the disease progresses, leaves become dull light-green, subsequently grey-green with occasional mottling, then yellow and finally brown and necrotic. When infected stems are cut across, discoloration of vascular tissue is not obvious. Symptom formation is enhanced by hot, dry weather conditions.

In tubers, symptoms may occur before harvest or in storage. Externally, the tubers from diseased plants may appear normal. Tuber infections usually begin at the stem end, and when cut, creamy yellow to tan areas of infection are found in the vascular tissue. When squeezing a cut tuber, sticky, cheesy, bacterial ooze can often be forced out from the vascular ring. As the disease progresses, the tissue outside the vascular ring can easily be separated from the inner tissues. Finally the entire center of the tuber disintegrates, leaving the outer shell of the potato only. Irregular shaped cracks may appear in the skin. At this stage the tuber may become infected with the common soft rot bacteria and become slimy and give off an offensive odor. Tuber symptoms may be confused with those caused by the brown rot bacterium Ralstonia solanacearum.

Symptomless foliage and tubers may be latently infected.

Courtesy of EPPO - Central Science Laboratory, Harpenden (GB). British Crown.


Clavibacter michiganensis subsp. sepedonicus is most widespread in Asia, North America and the EPPO region.

A map can be downloaded from EPPO’s website. See instructions here.


After a diseased potato is planted, the bacteria multiply very rapidly and follow the vascular tissue into the stems and petioles. The bacteria reach the roots and maturing daughter tubers, sometimes within 8 weeks after planting. The bacteria do not overwinter in field soil but persist for years in dried slime on potato bags, boxes, machinery and other equipment. Self-sown plants (volunteers) after an infected crop maintain the disease in the field. The optimum temperature for the bacteria is rather low (21oC) which results in favorable climate in north, north-west and central Europe.

Major pathway(s)

The disease is spread by planting infected seed potatoes as well as by using contaminated potato cutting knives and other equipment for potato planting, harvesting, handling, storing and transporting.

Detection and inspection

Because symptoms of Bacterial Ring Rot are variable and sometimes latent or masked by other diseases, the disease be confirmed only by laboratory tests, including a pathogenicity test on aubergine and a serological or molecular test. Until now analysis by host test is essential for confirming latent infections.

The symptoms in field may be obscured by or mistaken for potato blight (Phytophthora infestans), wilt (Verticillium albo-atrum), stem canker (Thanatephorus cucumeris) or drought.

Peststatus and importance

Bacterial Ring Rot is an important disease of potatoes. This disease is particularly serious because it has the potential to spread quickly throughout a farm and may lead to severe losses if left unchecked. Economic losses are due to wilt and tuber rotting in the field and in store. Indirectly, expenses of disinfecting sacks, machinery, stores, etc., prohibition of potato cultivation, and restriction or prohibition of export trade may increase economic loss.

Source of information

See further information here:

Author: Göran Kroeker

Editor: Elise T. Yamamoto Buch