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A New ‘Candidatus Liberibacter’ Species in Solanum tuberosum in New Zealand

Symptoms resembling “zebra chip” disease (3) were observed in potato (Solanum tuberosum) tubers harvested from a breeding trial in South Auckland, New Zealand in May 2008. The tubers had necrotic flecking and streaking that became marked when the potatoes were fried. Affected plants generally senesced early, at the beginning of April. The mean yield was approximately 60% less than expected and harvested tubers had less dry matter (13%) than normal (19%). Large numbers of the psyllid Bactericera cockerelli were observed on the crop during the summer. Total DNA was extracted from the vascular tissue of five symptomatic tubers and seven volunteers collected from the affected field with a DNeasy Plant Mini Kit (Qiagen, Valencia, CA). Samples were tested by PCR using primers OA2 (GenBank Accession No. EU834130) and OI2c (2). These primers amplify a 1,160-bp fragment of the 16S rRNA sequence of a ‘Candidatus Liberibacter’ species identified in tomato and capsicum in New Zealand. No fragment was amplified from healthy plants, but amplicons of the expected size were obtained from all symptomatic tubers and one plant. A 650-bp fragment of the β operon was also amplified from symptomatic tubers. The amplicons were directly sequenced (GenBank Accession Nos. EU849020 and EU919514). BLAST analysis showed 100% identity to the tomato/capsicum liberibacter (GenBank Accession Nos. EU834130 and EU834131). From a commercial potato field adjoining the breeding trial, groundkeeper tubers were collected and separated into those that were asymptomatic and those that exhibited a range of symptoms. Total DNA was extracted and tested by PCR using the OA2/OI2c primers. In the first category, 6 of 10 tubers tested positive, whereas the 10 tubers in the second category tested negative. Two phytoplasmas seem to be involved in the “zebra chip” disease complex (4) but were not detected in the samples in this study. To our knowledge, this is the first report of a liberibacter associated with disease in potato. From transmission electron microscope observations, previous researchers have hypothesized that a bacterium-like organism may cause “zebra chip” (1) and B. cockerelli is associated with the disease (3). “Zebra chip” was first reported in Mexico in 1994, since then it has caused significant economic damage in Guatemala, Mexico, and the southwestern United States. The economic impact of the disease in New Zealand is yet to be determined.

References: (1) S. H. De Boer et al. Page 30 in: New and Old Pathogens of Potato in Changing Climate. A. Hannukkala and M. Segerstedt, eds. Online publication. Agrifood Research Working Paper 142, 2007. (2) S. Jagoueix et al. Mol. Cell. Probes 10:43, 1996. (3) J. E. Munyaneza et al. J. Econ. Entomol. 100:656, 2007. (4) G. A. Secor et al. Plant Dis. 90:377, 2006.