Soft Rot Disease Severity Is Affected by Potato Physiology and Pectobacterium taxa

Marquez-Villavicencio, M. D. P., Groves, R. L., and Charkowski, A. O. 2011. Soft rot disease severity is affected by potato physiology and Pectobacterium taxa. Plant Dis. 95:232-241. Pectobacterium species cause disease worldwide in many crop and ornamental plants, including potato. A new Pectobacterium subspecies, P. carotovorum subsp. brasiliensis was recently described in Brazil and later found in the United States, Israel, and South Africa. Its virulence traits and host range remain unknown. A comparison of three taxa commonly found on potato showed that both P. carotovorum subsp. carotovorum and subsp. brasiliensis are more aggressive in causing tuber and stem soft rot than P. atrosepticum. Also, despite bacterial growth inhibition in vitro of P. carotovorum subsp. carotovorum and P. atrosepticum strains by P. carotovorum subsp. brasiliensis, this new subspecies and P. carotovorum subsp. carotovorum are able to co-colonize in the same infected tissue. Both subspecies were motile in lesions. Pathogenesis assays showed that host ranges of all three overlap, but are not identical. The host ranges of individual strains of P. carotovorum subsp. carotovorum and subsp. brasiliensis are limited, whereas P. atrosepticum can macerate many plant species in addition to potato. There was high variability in virulence assays with potato tuber; thus physiological factors were investigated. Tuber size, maturity, and field location had significant effects on susceptibility to soft rot, with larger, more mature tubers being more susceptible. The enterobacterial plant pathogen Pectobacterium (formerly Erwinia carotovora) causes soft rot diseases in monocot and dicot host plants in at least 35% of angiosperms (28). In potato, Pectobacterium causes wilt, soft rot, and blackleg and affects plant health during field production and storage (39,40). Tuber soft rot and aerial stem rot often occur after plants are wounded, and tuber soft rot is promoted by low oxygen conditions (6,29). In contrast, blackleg is considered a tuber-borne disease, with the bacterial pathogen causing an inky black decay on the lower part of the potato stem (40). Copper sprays may be used to prevent infection of wounded plant stems and leaves, but once the plant is colonized, there is no chemical control available for this pathogen (11). Resistance genes active against Pectobacterium have been found in multiple host species, but their sequences and mechanisms remain unknown (23–25,33,43,45,50,53,57). No currently grown commercial potato variety has an effective level of resistance to soft rot, stem rot, or blackleg. Pectobacterium pathogenesis has been studied for over a century (19). To promote rot, soft rot pathogens employ a wide range of plant cell wall degrading enzymes to disrupt and metabolize plant cells (1,48). Additional virulence determinants also have been described as contributing to bacterial invasion, establishment, multiplication, and host resistance evasion. These include the flagellar system (36), putative phytotoxins (3), quorum-sensing system (26,41,44), efflux pumps (51), the type III secretion system (16,54), and plant antimicrobial resistance systems (27). Conducive environmental factors are also critical for the infection process, such as water availability, low oxygen levels, and optimal temperatures for bacterial growth (40). The Pectobacterium genus has been divided into four species: P. atrosepticum, P. carotovorum, P. betavasculorum, and P. wasabiae (12). A novel Pectobacterium taxon associated with monocot hosts is likely to be a separate Pectobacterium species or subspecies (56). Of these, P. atrosepticum, P. carotovorum, and P. wasabiae are commonly found on potato. The species P. carotovorum has been further divided into subspecies, with P. carotovorum subsp. carotovorum and P. carotovorum subsp. brasiliensis most commonly found on potato (9,21,28,52). P. carotovorum subsp. carotovorum is typically associated with stem and tuber soft rot (40), although a subtype also causes blackleg (8); P. carotovorum subsp. brasiliensis causes stem rot, tuber soft rot, and blackleg (9); and P. atrosepticum causes blackleg (40). P. carotovorum is considered a broad-host-range pathogen, and this species has been isolated from a wide range of plant species. In contrast, P. atrosepticum appears to be mainly a potato pathogen, although it has been isolated from other hosts, such as sunflower (2) and pepper (46). Pectobacterium taxa may be distinguished and classified with multi-locus sequence analysis (MLSA) (21,28), but no simple assay is available to quickly and accurately identify or differentiate Pectobacterium species. The genes that contribute to differences in symptom development and host range among Pectobacterium species remain unknown. A recent comparative genomic study among P. carotovorum subsp. carotovorum, P. carotovorum subsp. brasiliensis, and P. atrosepticum strains found that these taxa share 77 to 81% of their genes (13). Many lineage-specific genes were identified and account for 18% (P. atrosepticum), 11% (P. carotovorum subsp. carotovorum), and 13% (P. carotovorum subsp. brasiliensis) of these genomes. The lineage-specific genes include those homologous to regulatory, plant cell wall degrading enzymes, toxin, and hemagglutinin-encoding genes, and some of these genes may explain differences in host range and virulence among these strains. Corresponding author: Amy O. Charkowski E-mail: [email protected] Accepted for publication 2 November 2010. doi:10.1094 / PDIS-07-10-0526 © 2011 The American Phytopathological Society