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Marcelletti S.,Cra Research Center For Fruit Trees | Ferrante P.,Cra Research Center For Fruit Trees | Scortichini M.,Cra Research Center For Fruit Trees
Diversity | Year: 2010

Forty-five Xanthomonas arboricola pv. juglandis (Xaj) strains originating from Juglans regia cultivation in different countries were molecularly typed by means of MultiLocus Sequence Typing (MLST), using acnB, gapA, gyrB and rpoD gene fragments.A total of 2.5 kilobases was used to infer the phylogenetic relationship among the strains and possible recombination events. Haplotype diversity, linkage disequilibrium analysis, selection tests, gene flow estimates and codon adaptation index were also assessed. The dendrograms built by maximum likelihood with concatenated nucleotide and amino acid sequences revealed two major and two minor phylotypes. The same haplotype was found in strains originating from different continents, and different haplotypes were found in strains isolated in the same year from the same location. A recombination breakpoint was detected within the rpoD gene fragment. At the pathovar level, the Xaj populations studied here are clonal and under neutral selection. However, four Xaj strains isolated from walnut fruits with apical necrosis are under diversifying selection, suggesting a possible new adaptation. Gene flow estimates do not support the hypothesis of geographic isolation of the strains, even though the genetic diversity between the strains increases as the geographic distance between them increases. A triplet deletion, causing the absence of valine, was found in the rpoD fragment of all 45 Xaj strains when compared with X. axonopodis pv. citri strain 306. The codon adaptation index was high in all four genes studied, indicating a relevant metabolic activity. © 2010 by the authors; licensee MDPI, Basel, Switzerland.

Scortichini M.,Cra Research Center For Fruit Trees | Marcelletti S.,Cra Research Center For Fruit Trees | Ferrante P.,Cra Research Center For Fruit Trees | Petriccione M.,Cra Research Unit For Fruit Trees | Firrao G.,University of Udine
Molecular Plant Pathology | Year: 2012

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 inplanta, 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 58.5-58.8mol.% 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: Daily information on the spread of the disease and on the research being performed worldwide can be found at: http://www.freshplaza.it. © 2012 The Authors. Molecular Plant Pathology © 2012 BSPP And Blackwell Publishing Ltd.

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