Bacterial Ring Rot

Bacterial ring rot is caused by a rod-shaped, gram-positive bacterium Clavibacter michiganense pv sepedonicum (Corynebacterium sepedonicum). It caused serious crop losses in Canada until the early seventies when intensive seed inspection procedures and disease control programs were adopted. In recent years, these measures have kept disease outbreaks to a minimum. There is a zero tolerance for this disease in seed potatoes. Canada's objective is to eradicate this tuber borne disease.

The bacteria overwinter in slight to moderately infected tubers which may not exhibit symptoms. In Atlantic Canada, the bacteria do not overwinter in field soil but may survive in volunteers. Bacteria can persist for years in dried slime on potato bags, bins, machinery and storage walls and floors. The disease is spread easily by potato cutting knives and equipment used in potato planting, harvesting, handling, storing and transporting.

Ring rot symptoms may first appear 60-70 days after planting but disease expression is dependent on climate and potato cultivar. Under warm growing conditions all of the plant symptoms may be detected while under cooler conditions, few or none of the symptoms may be seen. The first symptoms appear as a pale yellow mottling in the lower leaves which usually show signs of wilting. This may be limited to a single stem at first but affected leaf margins often curl upward, roll inward and eventually, dead brown areas develop. These symptoms usually begin in the lower leaves and proceed upward as the disease progresses. Leaf yellowing may intensify, producing a bar of bright yellow tissue between the veins. With severe infection, the whole plant may wilt and die.

Tuber symptoms may be visible at harvest or during storage but for some cultivars and growing conditions infected tubers may not show symptoms and serological detection methods are needed to find the disease. Tuber infections usually begin at the stem end and when cut, creamy yellow to tan areas of infection are found in the vascular tissue. Often when the cut tuber is squeezed a sticky, cheesy bacterial ooze can be forced from the vascular ring causing tissue separation. As the disease progresses, the tissue surrounding the ring becomes infected and often the entire center of the tuber disintegrates, leaving the outer shell of the potato. Externally, the tubers from diseased plants may appear normal. If badly affected, the skin may have pale reddish brown patches which gradually darken and become slightly sunken. Often irregular shaped cracks appear in these areas. At this stage the tuber may become infected with the common soft rot bacteria and become slimy and give off an offensive odor.

Potato cultivars vary in the degree of symptom expression. Norchip and Red Pontiac usually have easily seen symptoms, Russet Burbank has moderate symptom expression and Désirée and Belrus have less easily seen disease symptoms, e.g. tuber rotting. Cultivars such as Teton and Urgenta rarely exhibit any symptoms of bacterial ring rot when infected, but ring rot bacteria can readily be recovered from infected symptomless tubers of both varieties using laboratory or field techniques. Depending on growing conditions, many cultivars have the potential to remain symptomless.

Disease spread is most frequent at planting time when seed tubers are cut or when pick-type planters are used. Bacteria from infected tubers are transferred onto fresh cut seed surfaces. Only potatoes (S. tuberosum) are naturally infected but the organism can be used to experimentally infect sugarbeet, tomato and eggplant and a considerable number of other Solanum spp.