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First Report of Pseudomonas syringae pv. actinidiae, the Causal Agent of Bacterial Canker of Kiwifruit in France

In June 2010, young plants of kiwifruit growing in the French regions of Rhone-Alpes (Actinidia deliciosa ‘Summer’) and Aquitaine (A. chinensis ‘Jintao’) showed small, angular, necrotic leaf spots and cankers on some canes that was sometimes associated with production of a red exudate. Most of the affected canes died, and in a few cases after a few months, the entire plant died. Symptoms were consistent with Pseudomonas syringae pv. actinidiae, a bacterium that affects several species of Actinidia (A. deliciosa and A. chinensis, the two most important commercial species of kiwifruit). A recent outbreak of this disease is devastating the Italian kiwifruit industry. Bacterial colonies were isolated on King's medium B (KB) from leaf spots and infected canes. Three isolates from Aquitaine and 10 from Rhone-Alpes were retained for further characterization. The 13 isolates were gram negative, induced a hypersensitive reaction when infiltrated in tobacco plants, did not have a cytochrome c oxidase, an arginine dehydrolase or urease activity, did not hydrolyze esculin, starch, or gelatine, and did not induce ice nucleation. When plated on KB, these strains did not show strong fluorescence usually associated with strains of P. syringae. Complete lack of fluorescence reported for the pathotype strain ICMP 9817 has not been observed for those strains. They showed the same weak fluorescence as the strains of P. syringae pv. actinidiae recently isolated from Italy. Those characteristics match those of strains of P. syringae pv. actinidiae (3). Using total DNA of the 13 strains, the pathotype strain, and primers PsaF1/R2 (2), a 280-bp fragment was amplified by PCR, supporting the strains as being P. syringae pv. actinidiae. The amplicon from 6 of the 13 strains was sequenced and found to be 100% similar to the corresponding DNA fragment of the pathotype strain ICMP 9617 (GenBank AY342165). Partial sequences of 1,381 bp of the 16S rDNA gene of four of the six isolates, three strains isolated from Rhone-Alpes and one strain isolated from Aquitaine, were obtained by amplification with primers 27_f and 1492_r (1). Except for the sequence of strain 181, which was isolated from Aquitaine and had a 1 bp difference (GenBank JF323026), the other sequences were 100% identical to each other (GenBank JF323027 to JF323029). These four sequences were 99% identical to the 16SrDNA sequences of ICMP 9617, the pathotype strain of P. syringae pv. actinidiae (GenBank AB001431). These four strains and the pathotype strain were sprayed (1 × 10⁹ CFU/ml) on leaves of four 6- to 8-week-old seedlings of A. chinensis each. After 4 days, small, necrotic, angular spots were found on all plants inoculated with those four strains and the pathotype strain. No symptoms were found on plants treated with water only. From those leaf spots, bacteria that had all the characteristics of P. syringae pv. actinidiae (as previously described) were isolated. Recently, two different haplotypes for the housekeeping gene cts were described for P. syringae pv. actinidiae (4), the strains isolated from France belong to the haplotype I. This is the same haplotype to which strains isolated from the recent Italian outbreak also belong. To our knowledge, this is the first report of bacterial canker of kiwifruit in France.

References: (1) V. Gurtler and V. A. Stanisich. Microbiology 142:3 1996. (2) J. Rees-George et al. Plant Pathol. 59:453, 2010. (3) Y. Takikawa et al. Ann. Phytopathol. Soc. Jpn. 55:437, 1989. (4) J. L. Vanneste et al. N.Z. Plant Prot. 63:7, 2010.