Ralstonia solanacearum is the causative agent of bacterial wilt, one of the most destructive plant diseases. While chemical control has an environmental impact, biological control strategies can allow sustainable agrosystems. Three lytic bacteriophages (phages) of R. solanacearum with biocontrol capacity in environmental water and plants were isolated from river water in Europe but not fully analysed, their genomic characterization being fundamental to understand their biology. In this work, the phage genomes were sequenced and subjected to bioinformatic analysis. The morphology was also observed by electron microscopy. Phylogenetic analyses were performed with a selection of phages able to infect R. solanacearum and the closely related phytopathogenic species R. pseudosolanacearum. The results indicated that the genomes of vRsoP-WF2, vRsoP-WM2 and vRsoP-WR2 range from 40,688 to 41,158 bp with almost 59% GC-contents, 52 ORFs in vRsoP-WF2 and vRsoP-WM2, and 53 in vRsoP-WR2 but, with only 22 or 23 predicted proteins with functional homologs in databases. Among them, two lysins and one exopolysaccharide (EPS) depolymerase, this type of depolymerase being identified in R. solanacearum phages for the first time. These three European phages belong to the same novel species within the Gyeongsanvirus, Autographiviridae family (formerly Podoviridae). These genomic data will contribute to a better understanding of the abilities of these phages to damage host cells and, consequently, to an improvement in the biological control of R. solanacearum.

Ralstonia solanacearum is a bacterial phytopathogen affecting staple crops, originally from tropical and subtropical areas, whose ability to survive in temperate environments is of concern under global warming. In this study, two R. solanacearum strains from either cold or warm habitats were stressed by simultaneous exposure to natural oligotrophy at low (4 °C), temperate (14 °C), or warm (24 °C) temperatures in environmental water. At 4 °C, the effect of temperature was higher than that of oligotrophy, since R. solanacearum went into a viable but non-culturable (VBNC) state, which proved to be dependent on water nutrient contents. Resuscitation was demonstrated in vitro and in planta. At 14 °C and 24 °C, the effect of oligotrophy was higher than that of temperature on R. solanacearum populations, displaying starvation-survival responses and morphological changes which were stronger at 24 °C. In tomato plants, starved, cold-induced VBNC, and/or resuscitated cells maintained virulence. The strains behaved similarly regardless of their cold or warm areas of origin. This work firstly describes the natural nutrient availability of environmental water favoring R. solanacearum survival, adaptations, and resuscitation in conditions that can be found in natural settings. These findings will contribute to anticipate the ability of R. solanacearum to spread, establish, and induce disease in new geographical and climatic areas.

Currently, kiwifruit cultivation arouses great economic interest in the agricultural sector in several countries of the European Union due to high consumer demand and good results achieved in terms of production potential and fruit quality. One of the main bacterial species that cause yield losses in kiwifruit plants is Pseudomonas syringae. Diseases such as bacterial canker, caused by pathovar (pv.) actinidiae; floral bud necrosis caused by pv. syringae and leaf spots caused by pv. actinidifoliorum (Pfm) are clear examples. Between 2014 and 2017, in the main kiwifruit producing areas in the north and east of Spain, several surveys were carried out in search of these pathogens. Analyses realized from symptomatic and asymptomatic plants of Actinidiadeliciosa revealed the existence of new bacterial isolates close to Pfm. These new isolates werelow virulence pathogens similar to Pfm but belonging to a new group of P. syringae that affected the leaves of A. chinensis var. deliciosa. This study focused on the characterization and classification of these new isolates by a polyphasic approach in order to provide more information for understanding how the different populations of P.syringae affecting kiwifruit. They had the phenotypic characteristics of Pfm but by molecular approaches, they constituted a supported genetic lineage closely-related to Pfm independent of the five lineages described so far. This work revealed the great diversity found in P. syringae species affecting kiwifruit plants and supports the hypothesis that Pfm is a low virulence pathogen which is long established in Europe.