Citrus tristeza virus (CTV) isolates collected from the Lower Rio Grande Valley in south Texas and east Texas were characterized using citrus indicators and molecular methods. The citrus indicators were Mexican lime (Citrus aurantifolia), sour orange (C. aurantium), sweet orange (C. sinensis) grafted to sour orange, Duncan grapefruit (C. × paradisi), and Madam Vinous sweet orange, with some CTV isolates additionally indexed using the Texas commercial grapefruit cvs. Rio Red and Star Ruby, and Marrs and N-33 sweet orange. Severity ratings used 11 biotype groups or cumulative mean relative indices. Molecular characterization was carried out using poly- and monoclonal antibodies, seven strain-specific probes and single-stranded conformational polymorphism, and all were based on the CTV major coat protein or gene. All Texas CTV isolates produced vein clearing symptoms on inoculated Mexican lime plants. Over half of the CTV isolates tested were placed in biotype groups IX and X (causing decline of sweet orange on sour orange, seedling yellows on sour orange and grapefruit seedlings, and stem pitting of grapefruit or sweet orange), and one isolate was in biotype I (mild).
The high incidence of psorosis disease prompted the Texas citrus industry to initiate a voluntary virus-free program in 1948, but by the 1960s it was no longer operating. A similar program in the 1980s was short-lived, but the discovery of Citrus tristeza virus in nursery trees in 1992, the continued dominance of sour orange rootstock, and the presence of viroids and viruses which affect alternative rootstocks were catalysts for a mandatory virus-free budwood program. Selected trees of the commercially important varieties of grapefruit and sweet orange were indexed for several viruses and viroids, subjected to shoot tip grafting and re-indexed. These plants provided the start of a foundation block of trees to which other varieties imported from the California Citrus Clonal Protection Program have been added. There are now over 80 varieties in the foundation block and nearly 280,000 buds have been supplied to nurseries since 1998. Since there is no current expansion or replanting of commercial citrus in the Rio Grande Valley of south Texas, a growing proportion of budwood is for the homeowner market, especially in east Texas.
Citrus exocortis viroid and other viroids were detected in four of the five major citrus cultivars tested from different South Texas locations. Inoculated Etrog citron indicator plants showed symptoms resembling reactions produced by CEVd, CVdII, CVdIII or CVdIV. The infection level was low or nil in two cultivars. The infection rates of the cultivars tested were 100% in Henderson grapefruit, 80% in Marrs orange, 75% in Navel orange, 27% in Rio Red grapefruit, and 0% in nucellar Ruby Red grapefruit.
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