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First Report of Pseudomonas syringae pv. actinidiae the Causal Agent of Bacterial Canker of Kiwifruit on Actinidia arguta Vines in New Zealand

Actinidia arguta is commercially grown in New Zealand and few other countries; the fruit are sometimes sold as kiwiberry or hardy kiwi. In New Zealand, two biovars of Pseudomonas syringae pv. actinidiae have recently been found to cause bacterial canker on both A. chinensis and A. deliciosa, which produce the yellow and green fleshed kiwifruit, respectively (4). In November 2011, in a commercial orchard in the Bay of Plenty, New Zealand, A. arguta ‘Tahi’ and ‘Rua’ showed small angular necrotic leaf spots. About 50% of the vines randomly located throughout the orchard showed symptoms. Canker or shoot dieback were not detected on any of the infected plants. Four strains, labeled 13093 to 13096, were isolated onto King's B medium (KB) from leaves selected from four different plants showing symptoms. These four strains were gram-negative, induced a hypersensitive reaction when infiltrated in tobacco plants, lacked cytochrome c oxidase, arginine dehydrolase, and urease activity, and were unable to hydrolyze esculin, starch, and gelatine, and to induce ice nucleation. When plated on KB, these strains showed the same weak fluorescence associated with some strains of P. syringae pv. actinidiae (4). All these characteristics support identification of the strains as P. syringae pv. actinidiae. Using P. syringae pv. actinidiae-specific primers PsaF1/R2 (2), the expected 280-bp fragment was amplified by PCR from genomic DNA extracted from the four strains. The four amplicons were sequenced (GenBank Accession Nos. KF206138 to 41) and found to be 100% identical to each other and to the corresponding DNA fragment of the pathotype strain, ICMP 9617 (AY342165). A similar conclusion was reached using the duplex PCR targeting the ompP1 and the avrD genes (1); two amplicons of 492 and 226 bp were obtained with each of the four strains as expected for P. syringae pv. actinidiae. The DNA sequence of the 492-bp amplicon (KF206134 to 37) was 100% identical to that of strains of P. syringae pv. actinidiae, such as Psa 10627 (JQ934475.1). Strain 13094 isolated from A. arguta and pathotype strain ICMP 9617 were sprayed at a concentration of 3 × 10⁹ cfu/ml on to the undersides of leaves of three 6- to 8-week-old seedlings of A. chinensis‘Hort16A’ and three similar seedlings of A. deliciosa ‘Bruno.’ Those are the conditions under which the pathogenicity of strains of P. syringae pv. actinidiae is usually evaluated (4). After 2 weeks of incubation, small necrotic angular spots were observed on all plants inoculated with 13094 or ICMP 9617 but not on the water-treated control plants. The bacteria isolated from those necrotic spots had the same morphological characteristics on KB as P. syringae pv. actinidiae and gave a 280-bp amplicon after PCR with the PsaF1/R2 primers. Leaves of two rooted cuttings of A. arguta‘Tahi’ were spray inoculated with strain 13094 at a concentration of 2.7 × 10⁹ cfu/ml or with water. Necrotic spots developed on leaves 1 week after inoculation. No spots developed on the water-treated plants. The bacteria isolated from those necrotic spots had the same morphological characteristics on KB as P. syringae pv. actinidiae and gave a 280-bp amplicon after PCR with the PsaF1/R2 primers. Isolation of P. syringae pv. actinidiae from A. arguta has been reported only once before (3). This is this is the first report of P. syringae pv. actinidiae being isolated from A. arguta vines in New Zealand. This limited outbreak did not lead to any loss of production and since then only very few symptoms have been observed in this particular orchard.

References: (1) A. Gallelli et al. J. Plant Pathol 93:425, 2011. (2) J. Rees-Gorge et al. Plant Pathol. 59:453, 2010. (3) K. Ushiyama et al. Ann. Phytopath. Soc. Japan 58:476, 1992. (4) J. L. Vanneste et al. Plant Dis. 97:708, 2013.