Texas A&M University-Kingsville Citrus Center, Weslaco, TX 78596, USA

Background and objectives Citrus is a major crop in the Lower Rio Grande Valley (LRGV) of Texas. Over 95% of the 33,000 acres of grapefruit and oranges are grown on sour orange rootstock, which is sensitive to citrus tristeza virus (CTV). Texas is facing an imminent threat from the brown citrus aphid (BrCA), already established in Florida and in Belize. BrCA is the most efficient vector of CTV. The Texas citrus industry, in partnership with universities and regulatory agencies established a mandatory virus-free budwood program [1,2] to manage the threat of CTV /BrCA complex.

Materials and methods Since 1995, a total of 8,101 citrus samples were collected from non-commercial plantings and variety blocks in LRGV and in east Texas, about 500 miles from the major citrus area. The samples were tested for CTV by enzyme linked immunosorbent assay (ELISA). Mexican lime (Citrus aurantifolia Swingle) plants were inoculated with CTV positive isolates and were maintained in the greenhouse for preliminary biological indexing. This was followed by a standard host range study of six isolates to understand the biological reactions in indicator plants. A program of mass production of virus-free citrus planting material was initiated in cooperation with the citrus industry using shoot tip grafting and indexing techniques. Other research and development programs are also in place, as well as enforcement of appropriate laws and regulations to reduce the effect of the virus-vector complex.

Results and conclusions Of a total of 8,101 citrus samples tested since FY 1995, the CTV infection rate in the LRGV was: 0.62%, 0.39%, and 1.29% in 1995, 96 and 97, respectively. The CTV infection rate in samples from east Texas was: 28.05%, 19.35%, and 28.05% in corresponding years. Thirty three CTV isolates have been maintained in a greenhouse and a partial host range study has been initiated. More characterization studies based on aphid transmission, serology, strain specific nucleic acid probes, and molecular identities at the two coat protein genes and two protease genes of the CTV genome are being proposed. Initial indexing results, though unconfirmed, show evidence of a severe CTV component in one or two of the isolates collected. Over 250 virus-free trees are now available to serve as bud source for commercial nurseries.

1. Skaria, M, Amador JM. 1998. Phytopathology (In Press).
2. Skaria, MJ, Baker C, Kahlke N, Solis-Gracia CN, Roistacher CN, da Gracia JV. 1996. Proc. 8th Int Soc.Citri Culture, 366-368.