( Clavibacter michiganensis subsp. sepedonicus )
Ring Rot is caused by the bacterium Clavibacter michiganensis subsp. sepedonicus. Until this year (November 2003) it has not been reported in the UK, but it has now been found on a few farms in England and Wales. The symptoms are similar to Potato Brown Rot, the aerial parts of the plant can be yellowish and stunted. The area between the veins of the leaves turn yellow and the edges curl upward. Usually the plants wilt due to blocking or destruction of the vascular bundles in the stem, but the most prominent feature is a dark stain in the vascular area of the tubers. As the infection develops a creamy-white ooze appears in the ring, the area may rot away completely and the skin of the tuber may crack.
The bacteria are not thought to survive in the soil, but 'groundkeepers' (tubers missed at harvest) can carry an infection over to following seasons. The infection is introduced in affected seed potatoes or waste material, but is not passed from plant to plant - infection occurs through damaged areas on the tubers or stems from contaminated water or tools. It is known to survive for several years on tools, bags and barn walls which have come into contact with the ooze from infected tubers.
Good hygiene is the best way to prevent infection, and the use of only certified seed which are all tested for the presence of the disease. If cutting large seed tubers, the knife should be disinfected with spirit or a flame between cuts. If the crop is affected dig up and destroy the whole plant. When harvesting try to remove all of the crop to prevent groundkeepers which can harbour many diseases, any which sprout during following seasons should be dug up - it is tempting to allow them to grow! Harvested tubers may not show the presence of an infection which can develop in storage. After destroying the affected tubers, the containers must be disinfected if they are to be re-used - household bleach should do.
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