When it was realized that blackleg is not a soilborne disease, the blackleg-affected plant was thought to be the main source of the pathogen. Therefore, disease control would be achieved by an indexing (certification) system of seed obtained from disease-free crops. In Europe, plant inspection services under national jurisdiction are responsible for certification of seed potatoes. The European Plant Protection Organization (http://www.eppo.org) provides standardized protocols and guidance for certification of plant material. Inside the European Union, the Phytosanitary Directive (2000/29/EG) describes general regulation on crop production requirements and guidance for member states in respect of good cultivation practices (for example, in the Netherlands, NAK provides guidance for certification).
For more than half a century, certification of seed potatoes has been the traditional approach to ensure that this objective is achieved. However, the degree of control achieved is erratic and heavily dependent on the weather prevailing during growth of the seed crop. Greatly improved understanding of blackleg epidemiology and aetiology can now explain why this approach has met with little consistent success. These measures cannot detect widespread latent infection of progeny tubers (the next generation of seed) from symptomless plants, as discussed above. Moreover, depending on weather conditions, heavily contaminated seed can give rise to little or no disease, and the converse is also true. Despite this, the measures can do some good. For example, roguing at an early stage of crop growth, which entails the removal of diseased plants, including daughter tubers, no doubt contributes to reducing an important source of the pathogen. Rotting progeny tubers are common on plants with symptoms, from which bacteria can spread during mechanical handling at harvest and postharvest.
Seed potato crops are classified into different seed grades according to several criteria, including the level of roguing and blackleg, from nil to a given percentage, depending on national certification scheme criteria. Seed crops are usually subjected to field inspections twice during the growing season in most seed-producing countries in Europe. Infected crops can be downgraded to a lower seed category or rejected from the market. Harvested progeny tubers (future-generation seed) can be tested also with molecular and serological methods to detect latent infections. However, this is not yet an obligatory part of testing programmes. As tubers from disease-free crops are frequently contaminated, laboratory testing can help to detect latently infected tubers. In contrast to other bacterial diseases of potato where there is often zero tolerance, some contamination of seed tubers can be allowed, in particular in low-grade seed. Therefore, the use of a detection procedure which allows estimation of both the density of bacteria and the incidence of contaminated tubers is desirable.
The relationship between seed health status and its contamination level has not been fully evaluated. It involves several steps, namely, collection of representative tuber samples from large quantities of seed lots, preparation of tuber tissue for testing, use of a quantification method and, last but not least, interpretation of the results in terms of blackleg risk assessment (Pérombelon, 2000). An additional compounding factor is the high cost involved in testing for contamination level. One possibility is to restrict testing to the highest seed grades where tolerance level could be zero. Testing of tubers should include the peel to detect lenticel and wound infections, and the stolon end, including the vascular tissue.
When it was demonstrated that the seed (mother) tuber is an important source of the soft rot bacteria, attempts were made to produce pathogen-free progeny (next-generation) tubers. Initially, potato stem cuttings were used (Graham & Harper, 1967), later axenically produced microplants and currently in vitro-produced minitubers, which should yield bacteria-free progeny tubers (Stead, 1999). Minitubers are grown in a controlled pathogen-free environment, using aeroponic and hydroponic cultures or in artificial soil systems in order to prevent contamination with soft rot bacteria (Ranalli et al., 1994; Ali et al., 1995; Rolot & Seutin, 1999; Farran & Mingo-Castel, 2006). Testing of approximately 100 seed lots of minitubers per year during 4 consecutive years showed that minitubers were consistently free of blackleg-causing bacteria (Velvis & van der Wolf, 2009).
In large-scale seed potato production, multiplication of the initial minitubers in the field is necessary for economic reasons. However, this has led, even after only two or three field generations, to c. 30% contamination with Dickeya spp. and 10% with Pa (Velvis & van der Wolf, 2009). Similar results were found in previous studies in Scotland, which showed that an initially bacteria-free potato stock became progressively more contaminated after the third year in the field. Interestingly, contamination occurred at the time that mechanical handling at harvest and grading in store became necessary (Pérombelon et al., 1980). Therefore, it is likely that initial contamination came from machines already contaminated, although contamination by airborne bacteria cannot be ignored. In an attempt to overcome this problem, the number of generations from bacteria-free initial propagative material is restricted to a set number before loss of seed status in order to reduce buildup of contamination during seed-stock multiplication.
Studies carried out in the 1980s on the ecology of the bacteria identified several sources of the bacteria to contaminate seed crops before and after harvest (Pérombelon, 1992). This knowledge has allowed a more focused approach to reducing risks of introducing the bacteria at different stages of seed production. For example, it is desirable that crops should be dedicated as seed or ware, since the tolerance levels for blackleg are different, as well as harvesting time. However, when, for economic reasons, dual-purpose crops are sometimes grown, harvested late to maximize yield, and the seed-size fraction separated later from the ware, it is unavoidable that quality suffers (Van Der Zaag & Horton, 1983). Use of well-drained fields reduces the risk of tubers being surrounded by a water film that can result in anaerobiosis and consequent tuber decay in the field (Pérombelon, 1992). Late harvesting allows bacterial multiplication on leaves and in debris left on the ground following haulm flailing. This may result in contamination of progeny tubers underground during wet weather conditions (Burgess et al., 1994). Monitoring tuber contamination during bulking in crops derived from stem cuttings over 5 years on five seed-producing farms in Scotland showed that farms which regularly applied hygienic measures consistently produced cleaner seed than the others (Pérombelon et al., 1980). Washing and disinfection of machines used when planting, spraying, haulm flailing, harvesting and grading in store no doubt help to reduce risks of introducing soft rot bacteria in a pathogen-free crop (Pérombelon & Kelman, 1980; Pérombelon, 2002). Spreading and smearing of the bacteria in a seed lot can be reduced by removal of rotten tubers during harvesting and grading. Avoidance of wounding by correct machinery adjustment during harvesting and grading is important to reduce the risks of wounding, as bacteria can survive after wound healing (Pérombelon, 1992; van Vuurde & de Vries, 1994). Use of mature tubers with a well-developed periderm will also reduce risks of wounding.
Storage in bulk, or preferably in one-tonne boxes in the case of seed, in well-ventilated stores at low temperatures will avoid condensation on tuber surfaces, which in turn will prevent multiplication of the blackleg pathogen. If the tubers remain wet long enough, tuber decay can ensue, resulting in further spread of the bacteria when tubers are graded, and sometimes massive tuber decay (Pérombelon, 2000). It is critical to dry the tubers rapidly by forced ventilation with warm air to favour wound healing, followed by cooler air to control sprouting and for long-term storage (Wale & Robinson, 1986; Wale et al., 1986). Good storage management is of importance, not only to prevent tuber decay, but also to avoid increasing the tuber inoculum load, which would result in greater subsequent disease risks.
Reduction in the incidence of tuber infections with blackleg and soft rot bacteria can be achieved by using true seeds instead of seed tubers. Such seeds, derived from sexual crosses, are believed to be free from blackleg and soft rot bacteria (Pérombelon & Kelman, 1980). Although soft rot bacteria are not easily transferred to true seeds via vascular tissue, some reports suggest that true seeds may also become externally contaminated with low populations of the bacteria (Colyer & Mount, 1983). However, this externally sited inoculum can be removed by hot-water or chlorine treatments (Colyer & Mount, 1983).
For the majority of developing countries, seed potato classification schemes have failed or are not yet available, while imported seed potatoes are often too expensive (Van Der Zaag & Horton, 1983; Chujoy & Cabello, 2007). The use of true seeds may be an attractive and low-cost alternative to the use of seed tubers in these countries in particular. The main advantage of using true seeds is that they can be easily produced in large numbers (Chujoy & Cabello, 2007). Above all, botanical seeds do not require cold storage facilities. They can be kept in simple stores for a long time, which is of considerable importance in hot regions in developing countries (Wiersema, 1986). However, the major drawback is the genetic diversity of the progeny, which requires that every generation has to be carefully selected for desirable traits to ensure a stable and as uniform a crop as possible (Chujoy & Cabello, 2007).
Effect of nutrition on plant resistance to blackleg and soft rot
Plant nutrition is believed to be an important component of natural disease resistance. Nutrition affects the growth of plants and their interactions with pathogens and other microorganisms, and in general is important for plant fitness status (McGovern et al., 1985). Deficiency of essential elements will often result in an increased susceptibility to diseases.
Calcium fertilization is known to reduce soft rot caused by Pectobacterium spp. in Chinese cabbage (Park, 1969) and in bean (Platero & Tejerina, 1976). McGuire & Kelman (1984) showed, both in in vitro and field experiments, that bacterial soft rot caused by Pa was negatively correlated with calcium concentration in tubers. Resistance to blackleg and tuber soft rot appeared to be related to calcium concentration in seed tubers, but the results varied between the cultivars tested and over the 3 years of field experimentation (Pagel & Heitefuss, 1989). Calcium is not equally distributed inside plant parts and potato tubers in particular often have a low calcium level (Dunn & Rost, 1945; Collier et al., 1980). Soils naturally low in Ca can be amended by adding CaSO4 (gypsum) to increase resistance, not only to blackleg, but also to soft rot of progeny tubers (Bain et al., 1996).
The level of nitrogen seems to be another factor that can affect susceptibility to soft rot pathogens. High levels of nitrogen, between 1050 and 1700 p.p.m., significantly reduced bacterial leaf blight of Philodendron selloum caused by Dickeya spp., but resulted also in a significant deterioration of plant growth (Haygood et al., 1982). The effect of nitrogen levels on blackleg and soft rot in potato has not been explored, apart from a study by Graham & Harper (1966), who showed that blackleg incidence caused by Pa was lower in field plots treated with high than with low levels of N fertilizers.
A balanced fertilization of potato plants and an increase of the calcium content in soils alone will not provide sufficient control of blackleg and soft rot pathogens. It may, however, be a part of an integrated control strategy.
Physical seed tuber treatment
The traditional method to conserve seed tubers in good health has been storage in ventilated stores at low temperatures (c. 5°C). These conditions are easily met in temperate countries when temperatures in stores can be regulated using cold outside air over the winter until planting time in the spring. However, in tropical and subtropical regions, unless expensive refrigerated stores are available, storage is a real problem. At high ambient temperatures, not only is dormancy restricted, but also, infection by pests, fungi and particularly soft rot bacteria can cause serious losses. This is an important factor limiting potato production in these regions.
Disease can develop before cold storage or in transit to the market, and specific treatments become useful. Physical control, mainly by heat, is recognized as competitive to biological and chemical methods as it does not require registration and may be effective against a broad range of pathogens. However, physical procedures affect not only superficially located pathogens but also beneficial microorganisms and may negatively influence tuber emergence and health. Most of the information that is available on physical control of plant pathogens in potato comes from the control of Pectobacterium spp. under postharvest storage conditions. There is only limited information concerning Dickeya spp. The physical factors applied in controlling tuber soft rot infection are those involving hot water, steam (Robinson & Foster, 1987; Shirsat et al., 1991), dry hot air (Bartz & Kelman, 1985) and UV and solar radiation (Ranganna et al., 1997; Bdliya & Haruna, 2007).
Hot water treatment of potato tubers to control soft rot bacteria contamination was first applied in 1983 (Mackay & Shipton, 1983). Pcc and Pa could not be detected in tuber peel after dipping naturally infected potato tubers for 10 min in water at 55°C. In field experiments, no blackleg was observed in plants grown from treated tubers. Similar results were obtained by Wale & Robinson (1986) and Shirsat et al. (1991), who showed that incubation in water at 44·5°C for 30 min or at 56°C for 5 min significantly reduced the periderm and lenticel contamination of seed potatoes and consequently blackleg incidence in the field (Wale & Robinson, 1986; Shirsat et al., 1991). However, failure to dry large quantities of the tubers rapidly could result in multiplication of any surviving bacteria and even rotting. This difficulty was overcome by a continuous hot water treatment in which 50-kg batches were continuously treated for 5 min at 55°C followed by drying under forced ventilation with air knives. Cooling of the water when a large number of tubers is immersed is avoided and any residual moisture evaporated by the latent heat still in the tubers (Pérombelon et al., 1989a). Effective blackleg control was obtained in field experiments with both vacuum-infiltrated and naturally contaminated tubers. Moreover, the treatment led to the control of several fungal pathogens causing gangrene, skin spot, silver scurf and black scurf (Dashwood et al., 1991). The temperature/time combination used is critical, more so in bulk tuber dipping than in the continuous treatment. However, several side effects can adversely affect growth and have to be taken into consideration: depending on the cultivar used, tuber physiology can be altered resulting in delayed sprouting or even tuber death, and as a result, yield can be affected (Robinson & Foster, 1987; Pérombelon et al., 1989a).
Steam was also tested as an alternative to hot water treatment to remove fungi and bacteria, especially Pc and Pa present superficially in the tuber periderm. The use of steam treatment reduced infestation of tuber periderm from 26–59% to 1–3% (Afek & Orenstein, 2002).
Bartz & Kelman (1985) reported that external but not internal populations of Pectobacterium spp. can be eliminated from washed tubers by application of hot dry air at 50°C. Hot dry air also dries the tubers and stimulates wound healing without interfering with tuber sprouting as much as hot water treatment. However, heat transmission by air is generally less effective than by water, necessitating a longer incubation time, which could adversely affect tuber physiology.
Ranganna et al. (1997) tested the efficacy of UV radiation for controlling Pcc in potato tubers. When tubers were inoculated by vacuum infiltration 6 h before radiation, bacteria were totally eliminated by a relatively low UV dose of 15 kJ m−2. Vacuum-infiltrated tubers infected with Pcc and exposed to direct sunlight for at least 180 min did not develop soft rot symptoms, probably more because of an increase in the temperature of tuber superficial tissues than the action of UV energy, which is unable to penetrate tuber tissue to reach the pathogens. However, the practical value of these methods such as steaming, hot dry air and UV radiation is doubtful when applied on a large scale involving several tonnes of tubers.
In conclusion, physical control methods, especially hot water treatment, are environmentally friendly and allow some control of blackleg caused Pectobacterium spp. as well as of several superficial fungal pathogens simultaneously. Their limitations, however, are the inability to kill plant pathogenic bacteria located deep inside the tubers (vascular level) without a negative effect on plant growth, the substantial operational costs and the difficulty of expanding for large-scale use.
Chemical seed treatment
Chemical control strategies used against bacterial diseases are based on the eradication of the pathogen and/or the creation of unfavourable environmental conditions (e.g. low or high pH, etc.) for disease development. Once disease has been initiated, disease control is limited because of rapid bacterial multiplication and spread, and the inability of the chemicals to penetrate the inner tissues (Bartz & Kelman, 1985). Therefore, disease control has focused on latently infected tubers rather than blackleg-affected plants. A wide range of chemical compounds has been tested to reduce infection on or inside tubers by Pectobacterium spp. and Dickeya spp.
Most compounds used contain antibiotics (mainly streptomycin and its derivatives), inorganic and organic salts or combinations of these compounds. For a long time, streptomycin was considered a promising control agent against blackleg and soft rot diseases in potato. Immersion of seed tubers in a mixture of streptomycin and oxytetracycline hypochloride or streptomycin and mercury compounds before planting reduced the incidence of blackleg in the field and tuber decay in storage (Bonde & de Souza, 1954). Similar results were obtained when kasugamycin or virginiamycin was substituted for streptomycin (Wyatt & Lund, 1981; Bartz, 1999). However, although treatments with antibiotics showed promise, larger-scale field studies are no longer allowed because of the risks of introducing resistance to bacterial pathogens of humans or animals.
As an alternative to antibiotics, a wide range of potential bactericides have been tested, more often in small laboratory-scale experiments than in the field. Thus, organic compounds such as hydroxyquinoline and 5-nitro-8-hydroxyquinoline were effective for control of soft rot in wounded potato tubers (Harris, 1979). Similar results were obtained with chlorine-based compounds, bronopol (2-bromo-2-nitropropane-1,3-diol) and the synthetic bactericide, 7-chloro-1-methyl-6-fluoro-1,4-dihydro-4-oxo-3-quinolinic carboxylic acid (Bartz & Kelman, 1986). Immersion of potato tubers in citric, acetic, ascorbic or malonic acids also reduced rotting by Pcc in freshly vacuum-infiltrated potato tubers without affecting sprouting in in vitro conditions (Bartz & Kelman, 1986). Mills et al. (2006) showed that certain inorganic and organic salts, including aluminium acetate, sodium metabisulphate, propyl paraben, sodium benzoate, alum (hydrated potassium aluminium sulphate), potassium sorbate, calcium propionate, sodium hypochloride, sodium bicarbonate, aluminium chloride and copper sulphate, could inhibit the growth of Pcc and Pa in vitro. Some of these salts have already been approved as food preservatives and consequently their use to control soft rot bacteria would need limited additional registration testing. The activity of organic and inorganic salts may be attributed to the presence of cationic ions released from the salts that inhibit bacterial cell membrane protein functions or by modulation of the environmental pH by the anion moiety (Mills et al., 2006).
Synthetic antimicrobial peptides were evaluated as they are a group of antibacterial agents that by interacting with the bacterial cell membrane increase its permeability (Gabay, 1994). Kamysz et al. (2005) reported that the synthetic peptide CAMEL (KWKLFKKIGAVLKVL, a hybrid peptide derived from two naturally present antibacterial peptides, cecropin A and melittin (Oh et al., 2000)) gave greater protection to potato tubers against Pa and Dickeya spp. than streptomycin, protecting tuber tissue from rotting.
These chemical treatments of tubers to control blackleg and tuber soft rot are far from being straightforward. First, there is the problem of reaching the bacteria, which are usually well protected in lenticels, suberized wounds and the vascular system. Even systemic bactericides, if available, would fail if applied postharvest because there is no vascular activity in harvested tubers. A gaseous bactericide might be more successful, but penetration in tubers is likely to be poor and can be phytotoxic, as found by Eckert & Ogawa (1988) in the case of chlorine gas. The apparent success mentioned above can be explained by the fact that freshly harvested tubers with unsuberized lenticels and wounds were used. It may also be that testing was done on cut seed tubers, the use of which is common practice in some countries (Eckert & Ogawa, 1988). In addition, treating large quantities of tubers after harvest with a liquid bactericide would require efficient drying of the tuber surface to prevent multiplication of the bacteria and rotting, depending on storage method. For example, one option would be to treat freshly harvested washed tubers at the last rinse with hypochlorite solution to reduce superficial inoculum load, then dry them by forced ventilation using air knives to minimize the risks of rotting during storage in plastic bags in supermarkets.
Biological control of plant pathogenic bacteria could be an alternative to chemical and physical control and breeding for resistance. Biocontrol strategies comprise the use of antagonists affecting pathogen populations directly, or via antibiosis, competition for nutrients or induction of plant systemic resistance (Howarth, 2003). Although several attempts have been made to control Pectobacterium spp. and Dickeya spp. on potato using biological control agents, most were restricted to in vitro overlay studies, potato slice assays or in vitro-cultured potato plants; few included field experiments to check for consistency of results. Only the more recent work will be discussed here.
It has long been shown that bacteria isolated from the potato rhizosphere or those isolated from potato tuber periderm can be used successfully to protect potato tubers from Dickeya and Pectobacterium infections in laboratory conditions (Kloepper, 1983; Rhodes & Logan, 1986; Jafra et al., 2006). Initial selection of the control agent was based on random occurrence of bacteria that inhibited growth of soft rot bacteria in in vitro overlay studies. Further selection was based on characters likely to be inimical to soft rot bacteria. The agent is usually applied to seed tubers to control blackleg and rarely to control soft rot in stores.
In general, soil fluorescent and non-fluorescent Pseudomonas spp. obtained by in vitro screening have shown to be potential candidates for biological control of blackleg and soft rot diseases (Kastelein et al., 1999). They are able to survive in the potato rhizosphere and in soil (Kloepper, 1983; Azad et al., 1985; Gross, 1988; Loper & Henkels, 1999) and produce a variety of secondary antibacterial metabolites (Weller, 1988) including mainly siderophores, antibiotics and surfactants (Kloepper et al., 1980; Cronin et al., 1997; Compant et al., 2005). Fluorescent Pseudomonas spp. applied to tubers were able to reduce populations of blackleg and soft rot bacteria on potato roots and inside progeny tubers (Kloepper, 1983). They could also apparently control soft rot on potato when applied as a bacterial suspension directly to the tuber periderm (Colyer & Mount, 1984). Cronin et al. (1997) used Pseudomonas fluorescens strain F113 producing 2,4-diacetylphloroglucinol (DAPG) to control Pa in vitro and on potato tubers. The wild-type strain F113 was able to inhibit growth in vitro and colonization of tubers by Pa, whereas a F113 mutant unable to produce DAPG was not effective, indicating that biocontrol occurred via antibiosis.
Kastelein et al. (1999) used strains of Ps. fluorescens to protect wounds and cracks on tubers from colonization by Pa. Application of individual and combinations of strains reduced the contamination of potato tuber peel by 85% and 60–70%, respectively, indicating the potential of Pseudomonas spp. for controlling soft rot caused by Pa.
Lactic acid bacteria are commonly found on fresh fruits, vegetables and milk products and pose no risk to human or animal health. Lactobacillus plantarum, La. acidophilus, La. buchneri, Leuconostoc spp. and Weissella cibaria isolated from fresh fruits and vegetables showed in vitro antagonistic activity towards Pcc in overlay assays which was attributed to the production of hydrogen peroxide and acidification of the medium (Trias et al., 2008). In general, lactic acid bacteria possess different modes of action, mainly the production of organic acids, hydrogen peroxidase and siderophores, which can be effective for biocontrol. Lactic acid bacteria are able to inhibit more than one phytopathogen, thus La. plantarum, W. cibaria and La. acidophilus also inhibit the fungus Botrytis cinerea. They have a wide range of growth temperatures, ranging from 8 to 45°C, providing possibilities for broad applications (Trias et al., 2008).
Gram-positive Ba. subtilis BS 107, which was selected for its broad antibiotic activity towards different plant pathogenic bacteria and fungi, was used as a biocontrol agent against soft-rot- and blackleg-causing bacteria (Sharga & Lyon, 1998). The strain was active in overlay assays against not only human pathogenic or opportunistic Ps. aeruginosa, Klebsiella pneumoniae, Micrococcus luteus, Staphylococcus aureus and Escherichia coli, but also against plant pathogenic Pcc, Pa and Dickeya spp., Ps. syringae and Xanthomonas campestris, which indicates that it is a potentially powerful agent to control different plant diseases. Cladera-Olivera et al. (2006) reported a bacteriocin-like substance produced by Bacillus licheniformis P40 that was bactericidal to Pcc. This substance interacted with cell membrane lipids, provoking lysis of Pcc cells. It was also effective in protecting potato tubers against soft rot under standard storage conditions (Cladera-Olivera et al., 2006).
Jafra et al. (2006) focused on bacteria able to degrade quorum-sensing signal molecules produced by Pectobacterium spp. and Dickeya spp., which is a useful and effective strategy for the control of the bacteria by preventing the secretion of large quantities of pectolytic enzymes to macerate tuber tissue. The result of this work was a selection of several bacterial isolates (e.g. Delftia spp., Ochrobactrum spp., Rhodococcus spp.) able to control pectinolytic bacteria by the quorum-quenching mechanism in which infection of potato plants by target Dickeya and Pectobacterium bacteria was attenuated.
Predatory bacteria are ubiquitous in nature, present in different environments and able to invade and consume other bacteria (Stolp & Starr, 1963). Bdellovibrio bacteriovorans is a motile δ-proteobacterium that preys on Gram-negative bacteria (Rendulic et al., 2004). Epton et al. (1990) tested different strains of B. bacteriovorans to control Pa on potato. However, only limited control of soft rot was obtained in potato slice assays when co-inoculated with P. atrosepticum and B. bacteriovorans. The main difficulty in using B. bacteriovorans as a biocontrol agent is that interaction with the prey bacterial cells is ruled by a specific predator–prey relationship in which the populations of both microorganisms may fluctuate without complete eradication of the target bacteria (Crowley et al., 1980). Thus, Varon & Zeigler (1978) estimated that Bdellovibrio spp. were efficient as predators only when large populations of target bacteria were present. The minimum population density required for biocontrol of the target bacterium is about 105–106 CFU mL−1, which, in the case of Dickeya and Pectobacterium spp., may already be high enough to establish infection in plants under conditions favourable to disease (Varon & Zeigler, 1978). Finally, as B. bacteriovorans feeds on Pectobacterium spp., it is impossible to use Bdellovibrio spp. to prevent contamination in tubers free of soft rot bacteria.
Another possibility for controlling bacterial diseases of plants is the use of bacteriophages. Bacteriophages are viruses that infect and lyse bacterial cells. They are specific to their hosts and do not infect other microorganisms. They are self replicating, persistent in the environment and safe to use, as they cannot infect humans or animals. It has already been shown that bacteriophages possess the potential to control plant pathogenic bacteria (e.g. Erwinia amylovora, Agrobacterium tumefaciens) (Jones et al., 2007). However, their use is limited as they are not motile and the target bacteria tend to become rapidly resistant. Until now, little attention has been paid to the use of bacteriophages to control soft rot and blackleg bacteria in potato, but since Ravensdale et al. (2007) succeeded in reducing soft rot incidence on calla lily tubers inoculated with Pcc by up to 50% in greenhouse trials, there has been greater interest in this approach.
To date, no commercial biocontrol agents active against blackleg and soft rot bacteria have been produced. In fact there are few instances of this approach being successful in other crop–pathogen systems. The main difficulty is the requirement for the antagonist to satisfy several criteria. It has to reach its target, which in the case of potato would be located in lenticels, suberized wounds and the vascular system, sites not readily available at all times. Then, to be active, the agent needs to survive and multiply, preferably becoming established in the tuber and in the rhizosphere microflora. Another requirement is the preparation of a stable formulation. Too often, previous attempts have failed because some of the above criteria were not met. Moreover, transfer from small-scale to large-scale field testing can be difficult because of annual variations in the weather, resulting in lack of consistency in the results. Finally, there is the costly and time-consuming registration of biological control agents which would require expensive large-scale field experiments and ecotoxicological studies (Weller, 1988).
A possible approach which takes into account most of the above requirements is the application of the selected antagonist bacteria, preferably spore-forming, at the initial stage in seed-stock multiplication. Inoculation of microplants producing minitubers or of the microtubers before planting could allow establishment of the agent which could persist in later generations in the field. Protection of the first generations of seed crops is crucial, as control at that stage would reduce the risks of multiplication and spread of the pathogens at a later stage, at least in the high-grade seed lots.