Bacterial Ring Rot of Potato

Ned Tisserat   Extension Specialist, Plant Pathology

Bacterial ring rot, caused by the bacterium Corynebacterium sepedonicum, is one of the most devastating diseases of potato. It can cause losses in the field and during storage. The pathogen normally is introduced into a field or storage facility on contaminated seed pieces or tubers. Therefore, it is critical that only certified seed (blue tag) be used for commercial production. It is also strongly recommended that homeowners purchase certified seed for planting. The extra expense is well worth it! SYMPTOMS Symptoms of ring rot may not be noticed until late in the growing season. Affected plants show a yellowing and wilting of the lower leaves. A pale yellow discoloration may eventually be noticed between the veins of affected leaves. The most diagnostic symptoms of the disease occur on the tubers. Some tubers may be badly decayed while others show no outward appearance of a disease. However, splitting the tuber in half reveals a yellow to brown discoloration of the vascular system (which is located approximately 1/4 inch below the surface of the tuber). The discolored area often has a cheesy appearance, and when the tuber is squeezed, a milky ooze leaks from the discolored area. Infected tubers may also show surface cracks. These cracks allow entry of other soft rot bacteria which impart a foul odor to the rotting potato. In some cases, tubers and plants remain symptomless, especially under cool, wet growing conditions. The disease may recur during storage and cause extensive losses. DISEASE CYCLE The bacteria can survive in infected tubers and in dried slime on machinery, sacks, and other equipment. While the organism does not survive in soil, it can persist in tubers left in the field. The bacteria enter tubers through wounds. This is especially critical when seed pieces are cut, since a contaminated knife may result in inoculation of a large number of potatoes. Disease development is favored by temperatures ranging from 64 to 72 F. CONTROL The most important means of controlling ring rot is to buy seed certified to be free of the pathogen. Thoroughly disinfect all equipment on a regular basis. This includes planters, harvesters, grading machinery, knives, etc. If ring rot has been a problem the previous year, do not replant potatoes on the same ground. Destroy all culled tubers. Do not leave these tubers in the field.