Garczynski, S. F.

In 2017, potato tubers suspected of being infected with the bacterium ‘Candidatus Liberibacter solanacearum’ were received from the Animal and Plant Health Inspection Service in the United States. A total of 368 chipping tubers were observed for internal symptoms of zebra chip disease, which is associated with ‘Ca. L. solanacearum’ infection in the United States, Mexico, Central America, and New Zealand. A single tuber sliced at the stem end showed classic zebra chip symptoms of darkened medullary rays, with streaking and necrotic flecking. The symptomatic tuber was confirmed positive for the bacterium by polymerase chain reaction targeting three different ‘Ca. L. solanacearum’ genes. Sequence analysis of these three genes, and subsequent BLAST analysis, identified the pathogen with 99, 98, and 97% identity to ‘Ca. L. solanacearum’ for the 16S ribosomal RNA gene, 50S ribosomal proteins L10/L12 genes, and the outer membrane protein gene, respectively. Sequence analysis did not identify the sample as one of the six known haplotypes of ‘Ca. L. solanacearum,’ indicating that a seventh haplotype of the pathogen was identified. This new haplotype, designated haplotype F, is now the third haplotype of the bacterium that infects Solanum tuberosum in the United States.

Carrot (Daucus carota) plants with symptoms resembling those of carrot psyllid (Trioza apicalis) damage (3,4) were observed in 14 commercial fields in southern Finland in August 2008; all cultivars grown were affected at approximately 5 to 35% symptomatic plants per field. T. apicalis, a pest of carrots in northern and central Europe, can cause up to 100% crop loss (3,4). Symptoms on affected plants included leaf curling, yellow and purple discoloration of leaves, stunted growth of shoots and roots, and proliferation of secondary roots (3,4). Given recent association of liberibacter with several annual crops affected by psyllids (1,2), an investigation on whether this bacterium is associated with symptoms of psyllid damage on carrots was conducted. Total DNA was extracted from petiole tissue of 20 symptomatic and 18 asymptomatic plants (cv. Maestro, Nanda, Nipomo, Nerac, and Fontana) sampled from 10 psyllid-infested fields in southern Finland, as well as 15 plants (cv. Primecut, Cheyenne, and Triple Play) grown from seed in an insect-free greenhouse, with the cetyltrimethylammoniumbromide (CTAB) method (2). DNA was also extracted from 10 carrot roots (cv. Nantura) of plants continuously exposed to field-collected carrot psyllid colonies in the laboratory. DNA samples were tested by PCR using primer pairs OA2/OI2c and CL514F/R to amplify a portion of 16S rDNA and rplJ/rplL ribosomal protein genes, respectively, of “Candidatus Liberibacter solanacearum” (1,2). A 1,168 bp 16S rDNA fragment was detected in DNA from 1 asymptomatic and 16 symptomatic plants and a 669 bp rplJ/rplL fragment was amplified from DNA from 19 symptomatic and 6 asymptomatic plants, indicating presence of liberibacter. DNA from all 10 root samples yielded similar amplicons with both primer pairs. DNA from all the greenhouse carrot plants yielded no amplicon. Amplicons from DNA from three petioles and three roots with each primer pair were cloned (pCR2.1-TOPO; Invitrogen, Carlsbad, CA) and three clones of each of the 12 amplicons were sequenced (MCLAB, San Francisco, CA). BLAST analysis of the 16S rDNA consensus sequences from petiole and root tissues (GenBank Accession Nos. GU373049 and GU373048, respectively) showed 99.9% identity to those of “Ca. L. solanacearum” amplified from Capsicum annuum (FJ957896) and Solanum lycopersicum (FJ957897) from Mexico, and “Ca. L. psyllaurous” from potato psyllids (EU812559). The rplJ/rplL consensus sequences from petioles and roots (GenBank Accession Nos. GU373051 and GU373050, respectively) were 97.9% identical to the analogous rplJ/rplL “Ca. L. solanacearum” ribosomal protein gene sequence from solanaceous crops in New Zealand (EU834131) and to “Ca. Liberibacter” sp. sequence from zebra chip-affected potatoes in California (FJ498803). To our knowledge, this is the first report of “Ca. L. solanacearum” associated with a nonsolanaceous species and the first report of this pathogen outside of North and Central America and New Zealand (1,2). References: (1) L. W. Liefting et al. Plant Dis. 93:208, 2009. (2) J. E. Munyaneza et al. Plant Dis. 93:552, 2009. (3) G. Nehlin et al. J. Chem. Ecol. 20:771, 1994. (4) A. Nissinen et al. Entomol. Exp. Appl. 125:277, 2007.