Blackleg of potato
Pectobacterium atrosepticum, Pectobacterium carotovorum subsp. brasiliensis, Pectobacterium wasabiae, Dickeya dianthicola, and Dickeya solani
potato (Solanum tuberosum)
The blackened stem and wilted leaves are typical
of the potato blackleg disease. (Courtesy S.H. De Boer)
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 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, leading to premature senescence 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. Aerial stem rot is usually caused by Pectobacterium carotovorum subsp. carotovorum, 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). When present, it is not uncommon to find these pathogens together in the same host or field, thus it is sometimes difficult to distinguish between blackleg and aerial stem rot on the basis of symptoms alone.
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 and irrigation water. 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 remain dormant, or in favorable conditions, initiate disease development and decay.
In a poorly managed potato storage environment that is inadequately aerated with high humidity, the causal bacterium present in lenticels or 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 modifies the 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.
Pathogen Biology
The main causal agent of blackleg in Canada and the United States is Pectobacterium atrosepticum but other species of bacteria also cause the disease and may be the chief cause of blackleg in other countries. In Brazil and South Africa, for example, blackleg is caused by Pectobacterium carotovorum subsp. brasiliensis. In New Zealand, a causal agent of blackleg disease in potato was identified as Pectobacterium wasabiae. P. wasabiae also caused blackleg in Canada and is associated with decay in potato tubers in the U.S. While in Europe, P. atrosepticum is still a major cause of blackleg, increasingly the disease is being caused by species of the genus Dickeya. The major species are D. dianthicola and D. solani. The latter species, in particular, has caused much concern in the European potato industry.
Figure 8 |
Although the disease symptoms caused by the various blackleg-causing bacteria are, for the most part, indistinguishable, the bacteria differ in their biochemical and physiological characteristics. However, they are all Gram-negative, necrotrophic, nonsporing, rod-shaped bacteria. They have peritrichous flagella, which allows motility and aids in finding a host. They all belong to the family Enterobacteriaceae, which includes other well-known human and plant pathogens. P. atrosepticum has a narrow host range and predominantly infects potato, whereas related species such as Pectobacterium carotovorum and Dickeya spp. have a broad host range. Their survival in temperate regions varies. Whereas some species such as P. atrosepticum survives poorly in soil, water, and plant debris, others such as the Dickeya spp. survive much better. The main mode of infection is through the secretion of pectolytic and other plant cell wall degrading enzymes via the type II secretion system. When grown on a medium containing sodium polypectate, the pectobacteria and dickeya develop pits or craters in the medium, due to the excretion of pectolytic enzymes that liquefy the pectate (Figure 8). It is due to these pectolytic enzymes and genetic composition that the genus, Pectobacterium, was designated. Later, some of the pectolytic Enterobacteriaceae were separated into a separate genus because of significant differences in their genomic makeup, and named Dickeya after the American phytobacteriologist, Robert Dickey. Since P. atrosepticum was initially described as the sole cause of blackleg, it is the most extensively studied of the blackleg-causing bacteria. Characteristic of Enterobacteriaceae, the P. atrosepticum genome is a single circular chromosome of approximately 4.8Mb. Genome sequencing has determined that P. atrosepticum and related bacteria have secretions systems important for pathogenesis.
Various biochemical and physiological assays have been developed to aid in the identification and differentiation of the blackleg-causing bacteria. A good distinguishing feature of P. atrosepticum and P. wasabiae, unlike related bacteria, is their inability to grow above 36°C/97°F. Each related bacteria has its own biochemical assay profile, and some biochemical tests such as the production of reducing substances from sucrose, utilization of α-methylglucoside, phosphatase production, and sensitivity to erythromycin have been particularly useful for species differentiation. Molecular methods using polymerase chain reaction (PCR) with species-specific primers and nucleic acid sequencing also aid in identification and detection. Many serogroups of Pectobacterium have been described, but almost all P. atrosepticum isolates belong to serogroup I. Due to the relatively uniform serological type, serological methods such as enzyme linked immunoassay (ELISA) and immunofluorescence can be used for P. atrosepticum detection. However, these methods are less useful for the other blackleg-causing species because of their serological variability.
Blackleg-causing bacteria can be isolated from infected stems. They can be selected for on CVP (crystal violet pectate) growth medium. CVP selectivity is based on the presence of crystal violet which inhibits growth of most Gram-positive bacterial species, and the use of polypectate as the sole carbon source. After isolation on CVP, pure cultures can be maintained on a general bacteriological medium such as nutrient agar or Luria broth agar.
Pathogenicity assays of isolates can be easily done by inoculating young potato plants. Inoculations can be done either by stabbing a toothpick smeared with bacterial cells or injecting 10 µl of bacterial cells (usually 108 cfu/ml) into the stem of a potato plant. Maintaining high humidity by covering the inoculated plants with plastic bags enhances symptom development, which occurs within two weeks.
Figure 9 |
Disease Cycle and Epidemiology
Disease Cycle
Seed tubers are 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 are 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 blackleg-causing bacteria are 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.
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 bacteria. 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.
Disease Management
Pre-planting
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 probably related to the diversity of blackleg-causing bacteria and 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.
At planting
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. In addition, a field that contains low nitrogen levels with higher calcium and magnesium levels is recommended. Although there are no true resistant commercial varieties, they do vary in tolerance. Thus choosing a more tolerant variety helps against the development of blackleg.
During the growing season
Once a plant is infected, there is no salvaging it and must be removed. Thus, 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. Practicing good sanitation with equipment decreases the spread of the pathogen from different seed lots. Copper compounds may be used to prevent the spread of the pathogen; however, there is a risk of such compounds being damaging to the environment and human health.
At harvest and during storage
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.
Significance
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 chemical control measures are limited and expensive. 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 pectinolytic-free planting material becomes contaminated.
Molecular research on the pectolytic bacteria, including the blackleg bacterium, has revealed many fascinating aspects concerning the genetics of pathogenicity in plant pathogenic bacteria. Portions of the complex genetic control mechanisms that modulate expression and excretion of pectolytic enzymes are now understood. It is now understood how biofilm formation and associated signaling mechanisms among bacterial cells and between bacteria and host are important for the pathogens colonization, progression and survival. The use of current molecular methods and sequenced genomes is expected to reveal even more about this group of plant pathogenic bacteria.
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