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One major dsRNA of molecular weight (MW) 13.3 × 10⁶ and two others (MW 1.9 × 10⁶ and 0.8 × 10⁶) were routinely detected by polyacrylamide gel electrophoresis in extracts from sweet orange {Citrus sinensis) or citron (Citrus medica) infected with each of 66 isolates of citrus tristeza virus (CTV). Several additional dsRNA were also commonly detected, usually as weakly stained bands in reproducible positions in gels, but some were very prominent, e.g., a dsRNA of MW 1.7 × 10⁶ associated with a seedling yellows isolate (sy-1). No dsRNA was detected in equivalent extracts from noninoculated sweet orange and citron. End-labeled [³²P] probes were made from purified full-length viral RNA or polyacrylamide gel-purified full-length dsRNA of a nonseedling yellows (nsy-1) and a seedling yellows (sy-1) isolate of CTV. Each of the four probes was able to hybridize to all major and most minor dsRNAs of both isolates in composite polyacrylamide/agrarose gels, including the 1.7 × 10⁶ dsRNA specific to the seedling yellows isolate, and could readily detect CTV nucleic acid sequences in extracts from bark of infected sweet orange plants spotted onto nitrocellulose membranes. One dsRNA (MW 0.5 × 10⁶) was very prominent in some isolates and much less so, or undetectable, in other isolates and 66 isolates have been screened for the presence of this dsRNA. There was a strong correlation between inability to detect the 0.5 × 10⁶ dsRNA and the designation of an isolate as neither a seedling yellows type nor a stem pitting isolate of grapefruit; these properties were typical for isolates of CTV from southern California.

Two strains of CTV (T505 and SY560) were maintained in sweet orange and reliable, reproducible dsRNA profiles were obtained in several analyses in stained 6.0% polyacrylamide gels. The positions of some dsRNA segments in electrophoresed gels were the same for both strains and others were unique for each strain. When these strains were graft inoculated from sweet orange to sweet orange, no effect on dsRNA profiles was observed. When the strains were grafted from sweet orange to grapefruit, the quantity of dsRNA was less than in sweet orange, particularly for dsRNAs other than the replicative fonn (RF) (MW = 13.3 x lo6), and the number and intensity of stained bands were altered in a reproducible way.A dsRNA (MW = 1.4 x 1W) specific for T-505 in sweet orange was not detected in grapefruit and it did not return to detectable levels in subsequent graft inoculation back to sweet orange, Citrus excelsa, lemon or grapefruit. A dsRNA (MW = 1.7 x lo6) specific for SY-560 in sweet orange was barely detected in grapefruit. On subsequent graft inoculation from grapefruit, this dsRNA returned to readily detectable amounts in sweet orange and C. excelsa, but remained barely detectable or undetectable in subinoculated grapefruit and lemon. These results suggest that these two strains of CTV are probably mixtures, and that the ratios of virus components in such mixtures is affected by the host used to propagate the strain.

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