De Boer, S. H. 2004. Blackleg of potato. The Plant Health Instructor. DOI:10.1094/PHI-I-2004-0712-01
DISEASE: Blackleg of potato
PATHOGEN: Erwinia carotovora subsp. atroseptica (Synonym: Pectobacterium atrosepticum)
HOSTS: potato (Solanum tuberosum)
AuthorSolke H. De Boer,Centre for Animal and Plant Health, Charlottetown, PE, Canada
The blackened stem and wilted leaves are typicalof the potato blackleg disease. (Courtesy S.H. De Boer)
Potatoes are grown world-wide and the crop is usually considered to be the fourth most important staple food source after wheat, rice, and corn. It is one of the few staple food crops that are vegetatively propagated. Vegetative propagation means that the potato crop is not grown from true seed but rather from asexually produced propagules or "seed potatoes." Potatoes are underground storage organs known as tubers and are attached to the mother plant by stolons. Potato tubers are not only harvested as a food source for fresh market and processed products, but are also used for planting a new crop. Seed potatoes only differ from eating and processing potatoes in that they are produced as a highly regulated crop to keep them free of potential pathogens and pests. True botanical seed tends to exclude many disease-causing microorganisms even if they are present in the parent plant. Vegetative propagules such as tubers, on the other hand, are often infected or contaminated by the pathogens associated with the parent plant. The bacterium that causes the blackleg disease of potato is one of the pathogens that is tuber-borne. The blackleg disease can cause severe economic losses to the potato crop. However, the occurrence of blackleg depends very much on the growing conditions, particularly temperature and rainfall after planting.
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.
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.
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.
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).
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.
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.
The causal agent of blackleg is Erwinia carotovora subsp. atroseptica, a Gram-negative, rod-shaped bacterium closely related to enteric bacteria of importance as human and animal pathogens. Bacteria in the genus Erwinia, however, are not known to be harmful to humans or animals. When grown on a medium containing sodium polypectate, the blackleg bacterium develops pits or craters in the medium due to the excretion of pectolytic enzymes that liquefy the pectate (Figure 8). The pectolytic enzymes are, in fact, an important component of the pathogenicity factors of this and related bacteria. In recognition of the unique pectolytic activity of these bacteria it has been proposed to place them in a separate genus, Pectobacterium. Furthermore, it has recently been suggested that on the basis of its genetic composition, the blackleg bacterium be considered a unique species, Pectobacterium atrosepticum.
Several other subspecies of E. carotovora that cause disease in other crops have been described. These subspecies can cause decay of potato tuber slices but do not cause the blackleg disease. A new strain of E. carotovora has been described recently which causes a blackleg-like disease of potato in Brazil. Preliminary results suggest that the atroseptica subspecies does not occur on potato where the newly-described subspecies, tentatively named brasiliensis, occurs.
The most distinguishing feature of the blackleg bacterium is its pectolytic enzyme activity but in contrast to many of the other pectolytic bacteria it does not grow above 36°C/97°F. It is facultatively anaerobic meaning that it can grow both with and without the presence of oxygen. It is motile with numerous peritrichous flagella. Most strains of E. carotovora subsp. atroseptica belong to serogroup I, although several other serogroups occur. Because of the relatively uniform serological type, serological methods such as enzyme linked immunoassay (ELISA) and immunofluorescence can be used for its detection. Molecular methods including the polymerase chain reaction (PCR) are also available for detection and identification of this bacterium.
The blackleg bacterium can be isolated on a selective medium such as the crystal violet pectate medium (CVP, Figure 9) most efficiently at about 22°C/72°F. Colonies of pectolytic erwinia can be readily identified on this medium with a dissecting microscope using oblique illumination, that is, by shining a light through the bottom of the Petri plate from an angle. Colonies transferred from CVP grow on general bacteriological media such as nutrient agar. Pathogenicity of isolates can be easily determined on young (10-15 cm/4-6 in. high) potato plants by stabbing into the stem a toothpick smeared with bacterial cells. Symptoms of blackleg develop within two weeks (Figure 9).
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.
There was a time when almost all potato tubers were contaminated with the blackleg bacterium. That is no longer true today. The use of healthy tissue culture plantlets to initiate seed potato stocks has broken the cycle of carrying tuber contamination forward from year to year. Also by limiting the number of field generations to 5 to 7 years for production of individual seed lots after tissue culture, the buildup of tuber contamination is curtailed. Hence the incidence of blackleg is significantly lower than it was before the incorporation of tissue culture into seed potato production programs. Although disease reduction has been very significant in some geographic areas, the disease remains important in others where similar practices are used. The reason for the difference in disease incidence is unclear but is probably related to the rapidity by which new seed stocks are exposed to blackleg inoculum. The risk of exposing new seed stocks to inoculum will depend on specific agronomic practices and the ability of the bacterium to persist outside of potatoes in the prevailing climatic conditions of the different geographic locations.
Planting limited generation seed in well-drained soil after soil temperature has increased above 10°C/50° F is recommended for avoiding the development of blackleg.
Roguing out blackleg-diseased plants including belowground portions reduces soil inoculum but is only a useful practice if precautionary measures are taken to prevent contact of diseased tissue with other plants in the field.
Avoiding injury to potato tubers during harvest is important to minimize decay in storage. Removal of decayed potatoes before they spread their contents over grading lines and bin pilers avoids spreading the bacterium to other tubers. Wound healing is important in the early phase of potato storage to prevent development and spread of rots. During storage, however, the potatoes should be kept at a low temperature with adequate aeration to provide a dry environment and to prevent condensation of moisture on tuber surfaces.
The different manifestations of potato blackleg as a disease of potato plants, seed piece decay, and storage rot all contribute to economic losses. Although the disease is now considered to be of minor importance in some potato growing regions, it continues to be a major production factor in others. Control of the disease relies wholly on crop management practices as there are no chemical control measures. Although cultivars vary in disease susceptibility, none is immune. Continued use of tissue culture-derived plantlets and minitubers (grown from plantlets in a protected environment) to initiate seed stocks coupled with limited generations of field planting are essential for minimizing the contamination of seed stocks and maintaining the level of control that has been achieved. In those areas where the disease is not adequately controlled by these measures, further research is required to determine the source from which erwinia-free planting material becomes contaminated. Whether the bacterium survives in soil or irrigation water below the detection level threshold warrants investigation. Environmental spread of bacteria from late generation crops to new seed stocks via water, wind-driven rain, or insects also needs to be studied.
Molecular research on the pectolytic erwinia, including the blackleg bacterium, has revealed many fascinating aspects concerning the genetics of pathogenicity in plant pathogenic bacteria. The complex genetic control mechanisms that modulate expression and excretion of pectolytic enzymes are now just beginning to be understood. The importance of biofilm formation and associated signaling mechanisms among bacterial cells and between bacteria and host are currently being investigated. Sequencing of the genome of the blackleg bacterium is underway and is expected to reveal even more about its ability to cause disease in potato and the mechanisms by which it does so.
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