SUMMARY: Pseudomonas syringae pv. actinidiae is the causal agent of bacterial canker of green‐fleshed kiwifruit ( Actinidia deliciosa ) and yellow‐fleshed kiwifruit ( A. chinensis ). A recent, sudden, re‐emerging wave of this disease has occurred, almost contemporaneously, in all of the main areas of kiwifruit production in the world, suggesting that it can be considered as a pandemic disease. Recent in‐depth genetic studies performed on P. syringae pv. actinidiae strains have revealed that this pathovar is composed of four genetically different populations which, to different extents, can infect crops of the genus Actinidia worldwide. Genome comparisons of these strains have revealed that this pathovar can gain and lose the phaseolotoxin gene cluster, as well as mobile genetic elements, such as plasmids and putative prophages, and that it can modify the repertoire of the effector gene arrays. In addition, the strains currently causing worldwide severe economic losses display an extensive set of genes related to the ecological fitness of the bacterium in planta , such as copper and antibiotic resistance genes, multiple siderophore genes and genes involved in the degradation of lignin derivatives and other phenolics. This pathogen can therefore easily colonize hosts throughout the year. Taxonomy: Bacteria; Proteobacteria, gamma subdivision; Order Pseudomonadales ; Family Pseudomonadaceae ; Genus Pseudomonas ; Pseudomonas syringae species complex, genomospecies 8; Pathovar actinidiae . Microbiological properties: Gram‐negative, aerobic, motile, rod‐shaped, polar flagella, oxidase‐negative, arginine dihydrolase‐negative, DNA 58.5–58.8 mol.% GC, elicits the hypersensitive response on tobacco leaves. Host range: Primarily studied as the causal agent of bacterial canker of green‐fleshed kiwifruit ( Actinidia deliciosa ), it has also been isolated from yellow‐fleshed kiwifruit ( A. chinensis ). In both species, it causes severe economic losses worldwide. It has also been isolated from wild A. arguta and A. kolomikta . Disease symptoms: In green‐fleshed and yellow‐fleshed kiwifruits, the symptoms include brown–black leaf spots often surrounded by a chlorotic margin, blossom necrosis, extensive twig die‐back, reddening of the lenticels, extensive cankers along the main trunk and leader, and bleeding cankers on the trunk and the leader with a whitish to orange ooze. Epidemiology: Pseudomonas syringae pv. actinidiae can effectively colonize its host plants throughout the year. Bacterial exudates can disperse a large amount of inoculum within and between orchards. In the spring, temperatures ranging from 12 to 18 °C, together with humid conditions, can greatly favour the multiplication of the bacterium, allowing it to systemically move from the leaf to the young shoots. During the summer, very high temperatures can reduce the multiplication and dispersal of the bacterium. Some agronomical techniques, as well as frost, wind, rain and hail storms, can contribute to further spreading. Disease control: An integrated approach that takes into consideration precise scheduled spray treatments with effective and environmentally friendly bactericides and equilibrated plant nutrition, coupled with preventive measures aimed at drastically reducing the bacterial inoculum, currently seems to be the possible best solution for coexistence with the disease. The development of resistant cultivars and pollinators, effective biocontrol agents, including bacteriophages, and compounds that induce the systemic activation of plant defence mechanisms is in progress. Useful websites: Up‐to‐date information on bacterial canker research progress and on the spread of the disease in New Zealand can be found at: http://www.kvh.org.nz . Daily information on the spread of the disease and on the research being performed worldwide can be found at: http://www.freshplaza.it .

The phylogenetic relationships of 17 phytopathogenic mycoplasmalike organisms (MLOs) representing seven major taxonomic groups established on the basis of MLO 16S ribosomal DNA (rDNA) restriction patterns were examined by performing a sequence analysis of the 16S rDNA gene. The sequence data showed that the MLOs which we examined are members of a relatively homogeneous group that evolved monophyl-etically from a common ancestor. In agreement with results obtained previously with other MLOs, our results also revealed that the organisms are more closely related to Acholeplasma laidlawii and other members of the anaeroplasma clade than to any other mollicutes. A phylogenetic tree based on 16S rDNAs showed that the MLOs which we examined can be divided into the following five primary clusters: (i) the aster yellows strain cluster; (ii) the apple proliferation strain cluster; (iii) the western-X disease strain cluster; (iv) the sugarcane white leaf strain cluster; and (v) the elm yellows strain cluster. The aster yellows, western-X disease, and elm yellows strain clusters were divided into two subgroups each. MLOs whose 16S rDNA sequences have been determined previously by other workers can be placed in one of the five groups. In addition to the overall division based on 16S rDNA sequence homology data, the primary clusters and subgroups could be further defined by a number of positions in the 16S rDNAs that exhibited characteristic compositions, especially in the variable regions of the gene.

The taxonomic identity of the hereditary prokaryotic symbiont of the olive flyBactrocera oleae(Diptera: Tephritidae) was investigated. In order to avoid superficial microbial contaminants and loosely associated saprophytic biota, flies were surface-sterilized at the larval stage and reared under aseptic conditions until adult emergence.B. oleaeflies originating from different geographical locations and collected at different times of the year were tested. Bacterial isolation was undertaken from the cephalic oesophageal bulb, which is known to be a specific site of accumulation for the hosted microsymbionts in the adult insect. Despite evidence of multiplication cycles taking place within the insect, attempts at cultivation of the isolated bacteriaex situwere not productive at any stage, leading to the choice of unculturable status definition. PCR amplification and nucleotide sequencing of the entire 16S rRNA gene consistently yielded a single sequence that displayed marked similarity with enterobacterial lineages, with closest matches (97 %) toErwinia persicinaandErwinia rhapontici. The novel taxon differs from common intestinal bacterial species of fruit flies and from instances of culturable bacteria previously described inB. oleaeraised without sterility precautions, which we also observed as minority occupants or occasional contaminants. The symbiont's identity is also distinct fromPseudomonas savastanoi. In all observations, the numerically dominant inhabitant of the olive fly oesophageal organ was the same unculturable organism, whose presence at later stages was also regularly observed in the midgut. A novel species is proposed, by virtue of its unique properties, under the designation ‘CandidatusErwinia dacicola’.

A recent re-emerging bacterial canker disease incited by Pseudomonas syringae pv. actinidiae (Psa) is causing severe economic losses to Actinidia chinensis and A. deliciosa cultivations in southern Europe, New Zealand, Chile and South Korea. Little is known about the genetic features of this pathovar. We generated genome-wide Illumina sequence data from two Psa strains causing outbreaks of bacterial canker on the A. deliciosa cv. Hayward in Japan (J-Psa, type-strain of the pathovar) and in Italy (I-Psa) in 1984 and 1992, respectively as well as from a Psa strain (I2-Psa) isolated at the beginning of the recent epidemic on A. chinensis cv. Hort16A in Italy. All strains were isolated from typical leaf spot symptoms. The phylogenetic relationships revealed that Psa is more closely related to P. s. pv. theae than to P. avellanae within genomospecies 8. Comparative genomic analyses revealed both relevant intrapathovar variations and putative pathovar-specific genomic regions in Psa. The genomic sequences of J-Psa and I-Psa were very similar. Conversely, the I2-Psa genome encodes four additional effector protein genes, lacks a 50 kb plasmid and the phaseolotoxin gene cluster, argK-tox but has acquired a 160 kb plasmid and putative prophage sequences. Several lines of evidence from the analysis of the genome sequences support the hypothesis that this strain did not evolve from the Psa population that caused the epidemics in 1984–1992 in Japan and Italy but rather is the product of a recent independent evolution of the pathovar actinidiae for infecting Actinidia spp. All Psa strains share the genetic potential for copper resistance, antibiotic detoxification, high affinity iron acquisition and detoxification of nitric oxide of plant origin. Similar to other sequenced phytopathogenic pseudomonads associated with woody plant species, the Psa strains isolated from leaves also display a set of genes involved in the catabolism of plant-derived aromatic compounds.

Phytoplasmas cannot be cultivated in vitro, and remain the most poorly understood plant pathogens. Despite this limitation, the investigation of their nature with the aid of modern tools has produced noteworthy results during the last 20 years. Using biochemical and molecular approaches, the phylogeny of the phytoplasmas has been described, their chromosomal and extrachromosomal components are being studied, and information on the localization, movement, and metabolic interference occurring in their insect and plant hosts accumulated. At the same time, the application of the new findings in phylogeny and genetics has aided the development of powerful diagnostic tools that have improved the ability to manage diseases which are induced by phytoplasmas.