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Symptoms and signs

Foliage

Blackleg disease sometimes develops early in the growing season soon after the plants emerge. This is referred to as early blackleg and is characterized by stunted, yellowish foliage that has a stiff, upright habit (Figure 1). The lower part of the belowground stem of such plants is dark brown to black in color and extensively decayed (Figure 2). The pith region of the stem is particularly susceptible to decay and in blackleg-infected plants the decay may extend upward in the stem far beyond the tissue with externally visible symptoms. The typical blackening and decay of the lower stem portion is the origin of the "blackleg" designation for this disease. Young plants affected by blackleg fail to develop further and typically die.

Figure 1

Figure 2

In addition to early blackleg, the disease may also develop later during the potato growing season. In more mature plants, blackleg appears as a black discoloration of previously healthy stems, accompanied by a rapid wilting, and sometimes yellowing, of the leaves (Figure 3). Black discoloration of the stems always starts below ground and moves up the stem, often until the entire stem is black and wilted. At the early stages of disease development in mature stems, the leaves may turn yellow and wilt, even before the black decay is evident. However, after the entire stem becomes diseased, it decays, becomes desiccated, and is often lost from view in the potato canopy.

Figure 3

Blackleg disease inevitably originates at the seed tuber from which the plant is grown. Bacterial decay that originates in broken or damaged stems is not to be confused with blackleg although the symptoms have some similarity. This aerial stem rot is usually caused by Erwinia carotovora subsp. carotovora, a close relative of the blackleg bacterium. Aerial stem rot is usually a lighter brown in color than blackleg and although the decay moves up the stem, it does not start below ground (Figure 4). A stem wet rot is much like blackleg in many respects, but is caused by yet another bacterium, Erwinia chrysanthemi. Although stem wet rot is a significant disease in Europe, particularly in the Netherlands where it often cannot be distinguished from blackleg, it is not known to occur in North America.

Figure 4

Tubers

There are two ways by which the blackleg bacterium may reach the progeny tubers produced on the potato plant. One important route of tuber infection is via the stolon by which the tuber is attached to the plant. Tubers with blackleg disease generally first become decayed at the stolon attachment site where the tuber tissue becomes blackened and soft (Figure 5). As the disease progresses, the entire tuber may decay or the rot may remain partially restricted to the inner perimedullary (or parenchymal) tissue, that is, the tissue inside the vascular ring (Figure 6).

Figure 5

Figure 6

An alternate route for the pathogen to attack progeny tubers is via the soil. As the blackleg disease causes the belowground stem and seed tuber to decay, the causal bacterium spreads from infected tissue into soil water and becomes distributed throughout the root zone in which the progeny tubers are growing. Bacterial cells enter lenticels of the progeny tubers and either become inactive, or when conditions are favorable, initiate decay.

In a poorly managed potato storage environment, blackleg bacteria present on the surface of tubers can cause extensive decay (Figure 7). Sometimes when storage conditions are improved, decay lesions around tuber lenticels or mechanically damaged areas become arrested, resulting in a condition known as "hard rot." Hard rot is typified by slightly sunken, brownish-black, dry, necrotic lesions surrounding individual lenticels or damaged areas.

Figure 7

Once decay of potato tubers is incited by the blackleg bacterium, growth of secondary bacteria often contributes to the decay process and certainly modifies symptomatology of the disease. Hence a general bacterial soft rot develops from the initial blackleg infection in tubers. Bacterial soft rot is characterized by total maceration of tuber tissue and seepage of a putrid, dark-colored liquid.

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