A disease that severely affects processing potatoes (Solanum tuberosum L.), termed zebra chip (ZC), has been identified in several locations in the United States (Texas, Nebraska, Colorado, Kansas, New Mexico, Arizona, and Nevada), Mexico, and Central America (4). The disease name comes from the rapid oxidative darkening of freshly cut tubers and the dark stripes and blotches that occur in chips processed from infected tubers. Recently, the disorder has been associated with a new ‘Candidatus Liberibacter’ species in New Zealand (3). Also, a bacterium designated ‘Candidatus Liberibacter psyllaurous’ has been identified recently in potato plants with “psyllid yellows” symptoms that resemble foliar symptoms of ZC (2). In the fall of 2008, 10 tubers were received at the Prosser laboratory from a commercial potato grower and five had symptoms characteristic of ZC. The tubers, cv. Dakota Pearl, were grown near Lancaster in southern California. The tubers showed rapid oxidation upon slicing and the sunken stolon attachment characteristic of ZC (4). Nucleic acid was extracted from symptomatic tubers (1) and tested by PCR for ‘Ca. Liberibacter’ species with primer pairs OA2/OI2c (5′-GCGCTTATTTTTAATAGGAGCGGCA-3′ and 5′-GCCTCGCGACTTCGCAACCCAT-3′) and CL514F/R (5′-CTCTAAGATTTCGGTTGGTT-3′ and 5′-TATATCTATCGTTGCACCAG-3′), which amplify from the 16S rDNA and rplJ and rplL ribosomal protein genes, respectively (3). Four of the five tubers with distinct ZC symptoms yielded the expected amplicons with both primer pairs. Two tubers with mild internal discoloration yielded correctly sized amplicons but in lesser amounts than from the severely affected tubers. Nucleic acid from healthy potato tubers yielded no product with these primers. One clone of the 1,168-bp OA2/OI2c amplicon and two clones of the 669-bp CL514F/R amplicon from a strongly positive sample were sequenced in both directions (ACGT, Inc., Wheeling, IL). BLAST alignments of the consensus sequences of the OA2/OI2c and CL514F/R amplicons (GenBank Accessions Nos. FJ498802 and FJ498803, respectively) revealed 100% identity with analogous ‘Ca. Liberibacter’ sequences reported from ZC-symptomatic potatoes in New Zealand (GenBank Accession Nos. EU849020 and EU919514). The OA2/OI2c amplicon was also identical to a sequence of ‘Ca. Liberibacter psyllaurous’ (GenBank Accession No. EU812559) from psyllid yellows-affected potatoes in the United States (2) and also showed a 99% identity with sequences from a ‘Ca. Liberibacter’ species reported in ZC tubers from Kansas (GenBank Accession No. EU921626). Potato crops with symptoms of ZC have been observed previously in California (4), but this is the first identification of ‘Ca. Liberibacter psyllaurous’ from diseased potatoes grown in California. Since ZC was first reported in the mid- to late-1990s, information from potato growers and processors suggests that ZC is becoming more important. The disease has caused millions of dollars in losses, particularly in south Texas (4). The identification of ‘Ca. Liberibacter psyllaurous’ in California provides additional evidence that the disease is increasing in importance in other potato-growing regions.
References: (1) J. M. Crosslin et al. Plant Dis. 90:663, 2006. (2) A. K. Hansen et al. Appl. Environ. Microbiol. 74:5862, 2008. (3) L. W. Liefting et al. Plant Dis. 92:1474, 2008. (4) J. E. Munyaneza et al. Subtrop. Plant Sci. 59:30, 2007.
