Pectobacterium parmentieri
(black leg disease of potato)

Pectobacterium parmentieri is a bacterial pathogen of potato present in Europe since the 1960s. The bacterium was earlier classified as Pectobacterium carotovorum. After reclassification of P. carotovorum subsp. carotovorum SCC3193 to P. wasabiae and later on to P. parmentieri, several studies devoted to identification of pectinolytic bacteria in international collections and identification of the strains isolated from infected potato plants have indicated that this bacteria commonly occurs in several regions of Europe, Canada, USA, New Zealand and South Africa. P. parmentieri can cause symptoms of blackleg and soft rot on potato tubers. These diseases are usually a consequence of latent infection of seed potatoes. In the majority of countries pre-basic and basic seed tuber potatoes intended for the production of seed tuber crops should be free of Pectobacterium spp. and Dickeya spp. P. parmentieri is not present on any international or national alert lists.

Increased international trade has played a major role in the spread of disease caused by P. parmentieri, while the distribution of seed potato tubers may be the main cause of its spread. There is no information about other plant hosts (particularly ornamentals) for P. parmentieri. There are also risks associated with unidentified pathways of accidental introduction of the organism to new areas.

Genetics

The genome of Pectobacterium parmentieri SCC3193 consists of a single, circular chromosome of 5.16 Mbp, with an overall G+C content of 50%, without any plasmids. The chromosomal genome contains 4,705 predicted protein-coding sequences, 76 tRNA genes, 7 rRNA operons, and 2 CRISPR loci. On the basis of the orthologous grouping, 772 (16%) of the SCC3193 CDSs have no detectable homologs in any of the complete Pectobacterium or Dickeya proteomes published to date (Koskinen et al., 2012). There is a high variability in genome sequence among P. parmentieri strains when genomic fingerprinting using repetitive sequences-based PCR are applied (Versalovic et al., 1994). They exhibited in at least 5 REP-PCR genomic profiles (Zoledowska et al., 2018).  In Poland, profile I (also characteristic for P. parmentieri SCC3193 isolated in Finland) is the most abundant (approx. 44% of isolated strains). The recA gene-based phylogenetic analysis divided P. parmentieri strains isolated in different countries into two distinct clades. Evaluation of the phenotypic traits such as: pectinase, cellulase and protease activities, siderophore production in addition to potato tissue maceration, swimming and swarming motility also indicated significant differences among the characterized strains (Zoledowska et al., 2018). Genomic overview of type strain of P. parmentieri 08.42.1A (CFBP 8475^T) indicated horizontal acquisition of quorum sensing genes (Khayi et al., 2015).

Physiology and phenology

P. parmentieri strains produce acids from (+)-raffinose, α-d(+)-α-lactose, d(+)-galactose and (+)-melibiose but not from methyl α-d-glycopyranoside, (+)-maltose or malonic acid. They are pectinolytic, form cavities on selective CVP medium and can rapidly macerate potato tissue.

Incidence

P. parmentieri has been present on potato in Europe since the 1960s. Data presented by Waleron et al. (2013) and Zoledowska et al. (2018) indicate that it is quite common in Polish seed potato plantations (about 15% of the population of pectinolytic bacteria).

P. parmentieri has been reported in the USA, Canada, Africa and New Zealand (Pitman et al., 2010; de Boer et al., 2012; Ngadze et al., 2012; Moleleki et al., 2013) but no epidemiological data are available.

Effect on seed quality

Plants affected by P. parmentieri show wilting, stunting and chlorosis, also black soft rot at the stem base, extending upwards from the mother tuber. When disease occurs just after emergence, blanking is observed in the field. In low-temperature growing regions, infections often develop  with dark colouring, chlorosis and wilting. As the disease progresses, stems wilt and blackleg symptoms appear. Contamination of foliage can result in aerial stem rot. Infected plants rapidly develop internal stem rotting, but externally the stem base appears healthy. P. parmentieri commonly causes rotting of developing progeny tubers in the field and/or soft rot in storage.

Seed potatoes with soft rot symptoms are not accepted for trading. Latent infections are monitored via isolation of bacteria on selective CVP media (Hélias et al., 2012), Real-Time PCR (Humphris et al., 2015, van der Wolf et al., 2017), PCR with the use of plant homogenates and species specific primers (de Boer et al., 2012; Potrykus et al., 2014). Recently, Cigna et al. (2017) reported the application of the gapA gene sequencing assay for identification of different Dickeya and Pectobacterium species including P. parmentieri.

If pectinolytic bacteria (P. parmentieri) are detected, pre-basic and basic seed potatoes are rejected.

Pathogen transmission

The rise in international trade of seed potatoes has increased the risk of introduction and spread of P. parmentieri. According to Toth et al. (2011) higher risks of introduction arise from more intensive exchange of seed potatoes and the implementation of environmentally friendly methods of biodegradable waste disposal, such as composting and anaerobic digestion. These methods do not involve sterilization, and pectinolytic bacteria such as P. parmentieri can survive these treatments, thus spreading is intensified.

Seed treatments

No effective seed treatments have been described.

Seed health tests

In the majority of countries, seed potato fields are visually inspected for the presence of blackleg. However, the best method for the detection and identification of P. parmentieri is Real-Time PCR with specific primers (Humphris et al., 2015; van der Wolf et al., 2017) or Multiplex PCR with three pairs of specific primers (simultaneous identification of Pectobacterium atrosepticum, Dickeya spp. and P. carotovorum together with P. parmentieri (Potrykus et al., 2014)), followed by a PCR reaction with primers specific to P. parmentieri (de Boer et al., 2012) performed on stolon ends of potato seed tubers.

General

• Laboratory use
• Research model

Fields should be inspected by qualified inspectors to select infected plants. Detection and identification of P. parmentieri is performed using molecular diagnostic methods such as Real-Time PCR with specific primers (Humphris et al., 2015; van der Wolf et al., 2017) or Multiplex PCR with three pairs of specific primers (simultaneous identification of P. atrosepticum, Dickeya spp. and P. carotovorum together with P. parmentieri (Potrykus et al., 2014)), followed by a PCR reaction with primers specific to P. parmentieri (de Boer et al., 2012). Cigna et al. (2017) reported application of the gapA gene sequencing assay for identification of P. parmentieri. For a review of detection and identification methods for pectinolytic bacteria, see Czajkowski et al. (2015).

P. parmentieri is a pathogen of potato. It can cause symptoms of blackleg and soft rot that are very similar to those caused by Pectobacterium carotovorum subsp. carotovorum, P. carotovorum subsp. brasiliense,P. atrosepticum, Dickeya solani and D. dianthicola. The best method for identification of P. parmentieri is Real-Time PCR with specific primers (Kim et al., 2012; Humphris et al., 2015; van der Wolf et al., 2017) or Multiplex PCR with three pairs of specific primers (simultaneous identification of P. atrosepticum, Dickeya spp. and P. carotovorum together with P. parmentieri (Potrykus et al., 2014) followed by a PCR reaction with primers specific to P. parmentieri (de Boer et al., 2012).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Within the European Community (EC) P. parmentieri is important but is not a quarantine pest. It is controlled in the majority of Member States by their respective potato seed certification schemes. When potato seed production is initiated from pathogen-free microplants/microtubers and field production is limited to a restricted number of generations, the occurrence of P. parmentieri is limited (Toth et al., 2011). Control is based on visual inspection of field crops and tubers. However, latent infections are quite common and cannot be detected using this method. The post-harvest testing of seed potatoes is necessary to monitor seed stocks for the presence of bacteria from genus Pectobacterium and Dickeya (Czajkowski et al., 2011b). Certified pathogen-free seed potatoes should be cultivated. Latently infected seed potatoes should be rejected.

Eradication

There are no commercial methods available for eradication of P. parmentieri.

Recent studies have indicated that P. parmentieri is susceptible to attack from soil saprophytic bacteria and specific bacteriophages (Krzyzanowska et al., 2012; Czajkowski et al., 2011a, b; Smolarska et al., 2018).

Movement control

There are some measures that can reduce the impact and risk of spread of P. parmentieri from latently infected seed potatoes. Cleaning and disinfection of machinery, equipment and grading lines is very important and a range of disinfectants have shown efficacy in suppressing Dickeya solani (Czajkowski et al., 2011b). P. parmentieri has not been reported in waterways. A 3-year survey of Polish waterways did not detect P. parmentieri, however, P. carotovorum was detected.

Biological control

Biological control may be beneficial in reducing the impact of P. parmentieri. Smolarska et al. (2018) described bacteriophages of family Podoviride and order Caudovirales that infect some strains of P. parmentieri but not strains of other species of Pectobacterium and Dickeya. Antagonistic bacteria such as Ochrobactrum A44 and some strains of Pseudomonas sp. can be used to limit the spread of infection by Pectobacetrium and Dickeya (Czajkowski et al., 2011a; Krzyzanowska et al., 2012).

Further information is required on the geographical distribution of P. parmentieri. Research is also needed to investigate the survival of P. parmentieri in soil, water and air, its sensitivity to approved pesticides, and effective methods of eradication.

Netherlands: EUPHRESCO ERA- NET, Dr. Jan van der Wolf, PO Box 16 6700AA WAGENINGEN

27/07/17 Original text by:

Ewa Lojkowska, Intercollegiate Faculty of Biotechnology, Univesity of Gdansk & Medical University of Gdansk, 80-307 Gdansk, Abrahama 58, Poland