INCREASE AND PATTERNS OF SPREAD OF CITRUS TRISTEZA VIRUS-INFECTIONS IN COSTA-RICA AND THE DOMINICAN-REPUBLIC IN THE PRESENCE OF THE BROWN CITRUS APHID, TOXOPTERA-CITRICIDA

Citrus tristeza virus (CTV) was monitored for 4 years by monoclonal an tibody probes via enzyme-linked immunosorbent assay in four citrus orc hards in northern Costa Rica and four orchards in the Dominican Republ ic following the introduction of the brown citrus aphid, Toxoptera cit ricida. The Gompertz nonlinear model was selected as the most appropri ate in most cases to describe temporal increase of CTV. Ordinary runs analysis for association of CTV-positive trees failed to show a spatia l relationship of virus status among immediately adjacent trees within or across rows. The beta-binomial index of dispersion for various qua drat sizes suggested aggregations of CTV-positive trees for all plots within the quadrat sizes tested. Spatial autocorrelation analysis of p roximity patterns suggested that aggregation often existed among quadr ats of various sizes up to four lag distances; however, significant la g positions discontinuous from the main proximity pattern were rare. S ome asymmetry was also detected for some spatial autocorrelation proxi mity patterns. These results were interpreted to mean that, although C TV-positive trees did not often influence immediately adjacent trees, virus transmission was common within a local area of influence that ex tended two to eight trees in all directions. Where asymmetry was indic ated, this area of influence was somewhat elliptical. The spatial and temporal analyses gave some insight into possible underlying processes of CTV spread in the presence of T. citricida and suggested CTV sprea d was predominantly to trees within a local area. Patterns of longer-d istance spread were not detected within the confines of the plot sizes tested. Longer-distance spread probably exists, but may well be of a complexity beyond the detection ability of the spatial analysis method s employed, or perhaps is on a scale larger than the dimensions of the plots studied.