The bacterial canker of kiwifruit caused by Pseudomonas syringae pv. actinidiae is a severe threat to kiwifruit production worldwide. Many aspects of P. syringae pv. actinidiae biology and epidemiology still require in-depth investigation. The infection by and spread of P. syringae pv. actinidiae in xylem and phloem was investigated by carrying out artificial inoculation experiments with histological and dendrochronological analyses of naturally diseased plants in Italy. We found that the bacterium can infect host plants by entering natural openings and lesions. In naturally infected kiwifruit plants, P. syringae pv. actinidiae is present in the lenticels as well as in the dead phloem tissue beneath the lenticels, surrounded by a lesion in the periderm which appears to indicate the importance of lenticels to kiwifruit infection. Biofilm formation was observed outside and inside plants. In cases of advanced stages of P. syringae pv. actinidiae infection, neuroses of the phloem occur, which are followed by cambial dieback and most likely by infection of the xylem. Anatomical changes in wood such as reduced ring width, a drastic reduction in vessel size, and the presence of tyloses were observed within several infected sites. In the field, these changes occur only a year after the first leaf symptoms are observed suggesting a significant time lapse between primary and secondary symptoms. It was possible to study the temporal development of P. syringae pv. actinidiae-induced cambial dieback by applying dendrochronology methods which revealed that cambial dieback occurs only during the growing season.

The bacterial canker of kiwifruit by Pseudomonas syringae pv. actinidiae is an emblematic example of a catastrophic disease of fruit crops. In 2008 a new, extremely virulent form of the pathogen emerged and rapidly devastated many Actinidia spp. orchards all over the world. In order to understand differences in populations within this pathovar and to elucidate their diffusion and movements on world scale, it is necessary to be able to quickly and on a routine basis compare new isolates with previous records. In this report a worldwide collection of 142 strains was analyzed by MLVA, chosen as investigative technique for its efficacy, reproducibility, simplicity and low cost. A panel of 13 Variable Number of Tandem Repeats (VNTR) loci was identified and used to describe the pathogen population. The MLVA clustering is highly congruent with the population structure as previously established by other molecular approaches including whole genome sequencing and correlates with geographic origin, time of isolation and virulence. For convenience, we divided the VNTR loci in two panels. Panel 1 assay, using six loci, recognizes 23 different haplotypes, clustered into ten complexes with highest congruence with previous classifications. Panel 2, with seven VNTR loci, provides discriminatory power. Using the total set of 13 VNTR loci, 58 haplotypes can be distinguished. The recent hypervirulent type shows very limited diversity and includes, beside the strains from Europe, New Zealand and Chile, a few strains from Shaanxi, China. A broad genetic variability is observed in China, but different types are also retrievable in Japan and Korea. The low virulent strains cluster together and are very different from the other MLVA genotypes. Data were used to generate a public database in MLVAbank. MLVA represents a very promising first-line assay for large-scale routine genotyping, prior to whole genome sequencing of only the most relevant samples.

Pseudomonas syringae pv. actinidiae (Psa) causes kiwifruit bacterial canker (KBC), with a severe infection of the kiwifruit plant resulting in heavy economic losses. Little is known regarding the biodiversity and genetic variation of populations of Psa in China. A collection of 269 strains of Psa were identified from 300 isolates obtained from eight sampling sites in five provinces in China. The profiles of 50 strains of Psa and one strain of P. syringae pv. actinidifoliorum (Psaf) were characterized by Rep-, IS50-P, and RAPD-PCR. Discriminant analysis of principal coordinates (DAPC), principal component analysis (PCA), and hierarchical cluster analysis were used to analyze the combined fingerprints of the different PCR assays. The results revealed that all isolates belonged to the Psa3 group, that strains of Psa from China have broad genetic variability that was related to source geographic region, and that Chinese strains can be readily differentiated from strains from France, but are very similar to those from Italy. Multilocus sequence typing (MLST) of 24 representative isolates using the concatenated sequences of five housekeeping genes (cts, gapA, gyrB, pfk, and rpoD) demonstrated that strain Jzhy2 from China formed an independent clade compared to the other biovars, which possessed the hopH1 effector gene, but lacked the hopA1 effector gene. A constellation analysis based on the presence or absence of the four loci coding for phytotoxins and a cluster analysis based on the eleven effector genes showed that strains from China formed two distinct clades. All of the strains, including K3 isolated in 1997 from Jeju, Korea, lacked the cfl gene coding for coronatine. In contrast, the tox-argK gene cluster coding for phaseolotoxin was detected in K3 and in the Biovar1 strains (K3, Kw30, and Psa92), and produced a false positive amplicon for the hopAM1-like gene in this study. To date, only one biovar (Biovar3) is represented by the strains of Psa from China, despite China being the center of origin for the kiwifruit.

The increasing spread of kiwifruit bacterial canker caused by Pseudomonas syringae pv actinidiae (Psa) prompted a modelling effort to assess the global and local potential risk of this species The current potential distribution of Psa was modelled with two wellused models (CLIMEX and MaxEnt) based on available presence records and environmental data Most discrepancies in model projections occurred for New Zealand data that was used for validation Model projections can provide information to alert decisionmakers in kiwifruitgrowing regions to prepare for possible incursions of Psa However in this study because model findings did not agree on the New Zealand validation data more research is necessary to achieve greater confidence on projections for novel areas Despite that result this study provides useful information for some kiwifruit growing countries that have not yet been affected by Psa such as USA Iran Greece Belgium Denmark and especially South Africa where commercial kiwifruit orchards have been planted recently

No abstract