Survey methods for assessment of citrus tristeza virus incidence

The assessment of citrus tristeza virus incidence by sampling involves laboratory testing by enzyme-linked immunosorbent assay of leaf material collected in the field. Using field data and computer simulation, methods of field data collection were compared. One method was similar to that used by the Central California Tristeza Eradication Agency, in which 4 to 6% of the trees in a planting block are sampled and material from each tree sampled is assayed separately. This method was compared with an alternative method in which about 25% of the trees in a block are sampled, and material from groups of four trees is bulked and assayed together. Our comparative study indicated that the latter method results in increased accuracy and precision of estimates of citrus tristeza virus incidence without increasing unduly the number of laboratory assays required.