Carrot purple leaf disease was first reported in 2006 in the state of Washington and was associated with Spiroplasma citri. The disease also was reported in California in 2008. The objectives of this work were to fulfill Koch's postulates and to determine 1) whether the beet leafhopper, Circulifer tenellus (Baker) (Hemiptera: Cicadellidae), transmits carrot [Daucus carota L. subsp. Sativus (Hoffm.) Arcang] isolates of S. citri; and 2) whether carrot and citrus [Citrus sinensis (L.) Osb.]derived spiroplasmas are pathogenic to both plant species. C. tenellus adults received a 24-h acquisition access period to a diet containing carrot-derived S. citri. After 30 d, insects were transferred to healthy carrot seedlings (five per plant). Negative controls were carrot and periwinkle [Catharanthus roseus (L.) G. Don] plants exposed to diet-only-fed insects, and positive controls were periwinkle plants exposed to insects fed on spiroplasma-supplemented diet. Purple carrot leaves and small, chlorotic periwinkle leaves were evident 10–45 d after exposure. Spiroplasmas were reisolated only from symptomatic plants, and polymerase chain reaction (PCR) confirmed their identity as S. citri. No symptoms occurred, and no spiroplasma-specific PCR amplifications or spiroplasma cultures were obtained from plants exposed to diet only-fed insects. Carrot-derived S. citri was transmitted to 15 and 50% of carrot and periwinkle plants exposed, respectively. Insects exposed to S. citri isolates from carrot or citrus transmitted the pathogen to both their host of origin and to the other plant host (carrot or citrus), showing no isolate-host specificity. Our findings confirm that carrot is a host of S. citri, Although carrot is not a preferred host of C. tenellus, it is likely that inoculative leafhoppers feed on carrot during seasonal migration.
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Vol. 102 • No. 4