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Experiments were conducted to determine whether the beet leafhopper, Circulifer tenellus (Baker) (Hemiptera: Cicadellidae), transmits the purple top phytoplasma to potato, Solanum tuberosum L.; beets, Beta vulgaris L.; and selected weed hosts. The beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma was identified as the causal agent of the potato purple top disease outbreaks that recently occurred in the Columbia Basin of Washington and Oregon. The phytoplasma previously was found to be associated almost exclusively with the beet leafhopper, suggesting that this insect is the probable vector of BLTVA in this important potato-growing region. Eight potato cultivars, including 'Russet Burbank', 'Ranger Russet', 'Shepody', 'Umatilla Russet', 'Atlantic', 'FL-1879', 'FL-1867', and 'FL-1833', were exposed for a week to BLTVA-infected beet leafhoppers. After exposure, the plants were maintained outdoors in large cages and then tested for BLTVA by using polymerase chain reaction after 6 to 7 wk. The leafhoppers transmitted BLTVA to seven of the eight exposed potato cultivars. Sixty-four percent of the exposed plants tested positive for the phytoplasma. In addition, 81% of the BLTVA-infected potato plants developed distinct potato purple top disease symptoms. Beet leafhoppers also transmitted BLTVA to beets and several weeds, including groundsel, Senecio vulgaris L.; shepherd's purse, Capsella bursa-pastoris (L.) Medik); kochia, Kochia scoparia (L.) Schrad; and Russian thistle, Salsola kali L. This is the first report of transmission of BLTVA to potatoes, beets, and the above-mentioned four weed species. Results of the current study prove that the beet leafhopper is a vector of the potato purple top disease.

At least 16 taxa of cicadellids and delphacids were tested by polymerase chain reaction (PCR) for the presence of a phytoplasma in the clover proliferation group, designated 16SrVI. Nucleic acid extracts from individual insects or groups of 5-10 were tested using PCR primers designed from the DNA sequence of 16S-23S rRNA or ribosomal protein genes of the pathogen. The beet leafhopper, Circulifer tenellus (Baker), was most often associated with the phytoplasma, with approximately 16% of the insects testing positive. The phytoplasma was occasionally found associated with Ceratagallia spp. Leafhopper species that were not associated with the phytoplasma included Macrosteles spp., Dikraneura spp., Colladonus montanus (Van Duzee), Circulifer geminatus (Van Duzee), Ballana spp., Amplysellus spp., Paraphlepsius spp., Texananus spp., Balclutha spp., Latalus spp., Erythroneura spp., Exitianus exitiosus (Uhler), and unidentified delphacids. The detected phytoplasma was similar to, or synonymous with, the beet leafhopper-transmitted virescence agent that is associated with the potato purple top disease in the Columbia Basin region of Washington and Oregon. This is in contrast to the phytoplasma associated with potato purple top disease in Mexico that is related to aster yellows (group 16SrI). The association of the group 16SrVI phytoplasma almost exclusively with the beet leafhopper suggests that this insect is the major vector of the phytoplasma in this region.