First author: Institute of Ecology and Resource Management, University of Edinburgh, West Mains Road, Edinburgh EH9 3JG, Scotland; and second author: USDA Agricultural Research Service, U.S. Horticultural Research Laboratory, 2120 Camden Road, Orlando, FL 32803
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Accepted for publication 3 March 1999.
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.
brown citrus aphid,
polymerase chain reaction.
© 1999 The American Phytopathological Society