A New Survey Method for Citrus Tristeza Virus Disease Assessment

Abstract

Various citrus-producing states in the U.S. as well as the Caribbean, Central American, and South American countries have chosen to survey for severe citrus tristeza virus (CTV) strains to identify their location and level of infestation. Several survey methods have been used in the past but most have proven to provide estimates of incidence that are insufficiently accurate and precise for the purposes required. The assessment of CTV incidence by sampling involves laboratory testing by ELISA of plant material collected in the field. Using field data and computer simulation, methods of field data collection were compared. One method is similar to that used until recently by the Central California Tristeza Eradication Agency (CCTEA) program, in which about 4% of the trees in a planting block are sampled and the material from each tree sampled is assayed separately. This method is 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. The number of assays this new method requires is, therefore, equivalent to 6.25% of the tree population. Our comparative study indicated that the latter method results in increased accuracy and precision of estimates of citrus tristeza disease incidence without appreciable increase in the number of laboratory assays required. The new alternative method described here has recently been implemented by the CCTEA. An adaptation of this method is also presented to account for differences in spatial patterns of CTV when the brown citrus aphid is present or absent. In many citrus-growing regions of the world, citrus is planted over very large areas. These plantings can either be contiguous or diffuse throughout the region in patchwork fashion. In either case, individual commercial plantings often consist of very large numbers of individual trees. Citrus tristeza virus (CTV) is often a concern for commercial citrus and examples exist that demonstrate its destructiveness to plantings in various locations around the world. The damage caused by CTV infection is dependent upon virus isolate, cultivar/ rootstock combination, and isolate-cultivar/rootstock interaction. At times new citrus plantings or entire regions may be established with virus-free material. In other locations , the isolates that exist may not be particularly deleterious to the Mention of a trademark, warranty, proprietary product, or vendor does not constitute a guarantee by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.