Survey Methods for Assessment of Citrus Tristeza Virus Incidence When Toxoptera citricida Is the Predominant Vector
Tóm tắt
Citrus tristeza virus (CTV) incidence may be assessed by sampling groups of citrus trees, recording the groups as CTV positive (one or more infected trees) or CTV negative (no infected trees), and then calculating disease incidence at the scale of the individual tree by means of a formula involving incidence at the group scale and the number of trees per group. This procedure works well when the CTV status of a tree can be regarded as independent of the CTV status of other trees in the same group. This is the case when the main vector species is Aphis gossypii and the groups comprise four adjacent trees, because the spatial pattern of CTV incidence at the within-group scale can be regarded as random. However, when the main vector species is Toxoptera citricida, this simple procedure is not appropriate, because the spatial pattern of CTV incidence at the within-group scale cannot be regarded as random. Using field data and computer simulation, an alternative procedure for assessment of CTV incidence when the main vector species is T. citricida was devised and tested. In the alternative procedure, the sampling scheme is operationally identical to that used when the main vector species is A. gossypii, but the calculation of CTV incidence at the scale of the individual tree is based on incidence at the group scale and an effective sample size. The analysis of CTV-incidence data collected from a number of citrus blocks in reasonable geographical and temporal proximity and the use of CTV-detection methods more sensitive than the enzyme-linked immunosorbent assay used here are also discussed.
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Tài liệu tham khảo
Bennett S., 1989, GLIM Newsl., 17, 12
Central California Tristeza Eradication Agency. 1998. Field Procedures Manual. CCTEA, Tulare, CA.
Cochran, W. G. 1977. Sampling Techniques, 3rd ed. John Wiley & Sons, Inc., New York.
Colquhoun, D. 1971. Lectures on Biostatistics. Clarendon Press, Oxford.
Garnsey, S. M., and Cambra, M. 1991. Enzyme-linked immunosorbant assay (ELISA) for citrus pathogens. Pages 193-216 in: Graft Transmissible Diseases of Citrus. C. N. Roistacher, ed. FAO (Food Agric. Organ. U.N.), Rome.
Gottwald, T. R., Garnsey, S. M., Cambra, M., Moreno, P., Irey, M., and Borbón, J. 1996. Differential effects of Toxoptera citricida vs. Aphis gossypii on temporal increase and spatial patterns of spread of citrus tristeza. Pages 120-129 in: Proc. Conf. Int. Organ. Citrus Virol., 13th. J. V. De Graça, P. Moreno, and R. K. Yokomi, eds. IOCV (Int. Organ. Citrus Virol.), Riverside, CA.
Healy, M. J. R. 1988. GLIM: An Introduction. Oxford Science Publications, Oxford.
Kish, L. 1965. Survey Sampling. John Wiley & Sons, Inc., New York.
Mathews, D. M., Riley, K., and Dodds, J. A. 1995. Comparison of ELISA and PCR for the sensitive detection of citrus tristeza virus (CTV) in pooled leaf samples from sweet orange groves with a low incidence of infection. Pages 12-16 in: Proc. Conf. Int. Organ. Citrus Virol., 13th. J. V. De Graça, P. Moreno, and R. K. Yokomi, eds. IOCV (Int. Organ. Citrus Virol.), Riverside, CA.