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Disease Cycle and Epidemiology

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The blackleg bacterium survives poorly in soil. Although other members of the pectolyic erwinia survive in surface water and in the soil environment, all evidence suggests that the blackleg bacterium does not survive very well outside of association with host plant tissue. Hence, the seed tuber is the most important source of inoculum in the blackleg disease cycle. When a contaminated or infected seed potato is planted, one of three things may occur. (1) The blackleg bacteria may move via the vascular bundles directly into the growing plant and result in blackleg disease. If tuber contamination is confined to the lenticels, decay of the seed tuber occurs first and when bacterial populations become great enough, invasion of the growing stem occurs. Both the process of seed tuber decay and the spread of the pathogen into the stem is highly dependent on environmental conditions. Moist, cool conditions favor the disease. (2) When conditions are favorable for growth of the potato plant, no disease may occur even when the blackleg bacterium is present. (3) Decay of the seed piece may occur prior to the establishment of a plant, and this, too, is an important manifestation of blackleg although other members of the pectolytic erwinia can also cause seed piece decay.

The most frequently occurring situation that follows the planting of contaminated seed is that the seed piece decays after a plant has been established, and no blackleg disease develops at all. In this case, the blackleg bacteria seeping from the decaying seed piece contaminate the entire root zone including developing progeny tubers. Surfaces of the progeny tubers become contaminated with the bacteria surviving particularly well in lenticels. In storage, the contaminated tubers may decay, develop hard rot symptoms, or remain symptomless. When symptomless, but contaminated, tubers are used for planting, which they often are, the cycle is repeated.

Contamination of potato tubers is exacerbated by harvesting and storage operations. A single tuber with blackleg decay may contaminate many additional tubers as they pass over conveyer belts on harvesters and bin pilers. During storage, decay and rotting of contaminated tubers is a common problem. Damaged tubers are particularly vulnerable to decay by the pectolytic erwinia. The presence of moisture on stored tubers is also conducive to development of decay, since a film of water surrounding tubers causes them to become anaerobic. The lack of oxygen inhibits tuber metabolic activity and prevents them from staging a normal resistance reaction.