|Yokomi, Raymond - Ray|
Submitted to: Journal of Economic Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/20/2009
Publication Date: 8/10/2009
Citation: Mello, A.F., Wayadande, A., Yokomi, R.K., Fletcher, J. 2009. Transmission of different strains of Spiroplasma citri to carrot and citrus by Circulifer tenellus Baker (Hemiptera:Cicadellidae). Journal of Economic Entomology. 102(4):1417-1422. Interpretive Summary: Carrot purple leaf disease (CPLD) was associated with the presence of Spiroplasma citri in 2006 in Washington State. This report fulfills Koch’s postulates for S. citri and the etiology of this disease. Using an artificial diet incorporated with S. citri, the beet leafhopper, Circulifer tenellus, was shown to acquire and transmit the carrot strain of S. citri to carrots and periwinkle with an efficiency of 15% and 50%, respectively. Purple leaves in carrots and small, chlorotic leaves in periwinkle became evident 10-45 days after plant exposure to insects fed on S. citri plus diet. Confirmation of plant infection was made by spiroplasma re-isolation, and PCR confirmation of bacterial identity. In another experiment using the same artificial feeding system, the leafhopper was shown to acquire and transmit S. citri strains from carrot or citrus to healthy seedlings of both plants regardless of the pathogen host origin. These findings confirm the conclusions of a previous report that carrot is a host of S. citri. Carrots are grown in the western U.S. in areas where the beet leafhopper occur seasonally. Since carrot is not a preferred host of the beet leafhopper, our findings suggest that transmission of S. citri to carrot likely results from transitory feeding during migration of infected leafhoppers.
Technical Abstract: Carrot purple leaf disease (CPLD) was associated with the presence of Spiroplasma citri in 2006 in the state of Washington, USA (Lee et al. 2006). The objectives of this work were to (1) confirm S. citri as the causal agent of CPLD by fulfilling Koch’s postulates, (2) to determine whether carrot strains of S. citri can be transmitted by the beet leafhopper, Circulifer tenellus Baker, and (3) determine whether both carrot and citrus-derived spiroplasma strains are pathogenic to both plant species. Adults of the S. citri leafhopper vector, C. tenellus, were given a 24-h acquisition access period to feeding sachets containing a diet incorporated with spiroplasmas isolated from infected carrots. After a 30-day latent period on healthy sugar beet, insects were transferred to healthy carrot seedlings (5 leafhoppers/ plant). Carrot and periwinkle plants exposed to insects fed on diet alone served as negative controls, while periwinkle plants exposed to insects fed on diet plus S. citri were used as positive controls. Confirmation of plant infection was based on development of expected symptoms in each host, spiroplasma re-isolation from hosts, and PCR confirmation of bacterial identity. Purple leaves in carrots and small, chlorotic leaves in periwinkle became evident 10-45 days after plant exposure to insects fed on S. citri-buffer. No symptoms developed nor was S. citri detected by PCR or culturing in plants exposed to diet only-fed insects. Only symptomatic plants of both species yielded spiroplasma cultures and amplicons of expected size by PCR. S. citri was transmitted to 15% of carrot test plants and 50% of periwinkle test plants exposed to infected leafhoppers. Insects exposed to feeding sachets containing S. citri strains from carrot or citrus acquired and transmitted the pathogen to both host of origin and to the heterologous plant host (carrot or citrus), therefore showing no strain-host specificity. Our findings confirm the conclusions of Lee et al. (2006) that carrot is a host of S. citri. Although carrot is not a preferred host of C. tenellus, our findings suggest that the occurrence of CPLD is likely due to feeding during migration of infected leafhoppers on carrot.