Molecular Analyses of Citrus tristeza virus Subisolates Separated by Aphid Transmission

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Abstract
Citrus tristeza virus (CTV) exists in field isolates as a complex of virus isolates. This complex may contain both mild and severe CTV. Using single and multiple aphid transmissions, subiso-lates of the various field isolates were separated. Some CTV isolates that tested negative with the monoclonal antibody MCA13 consisted of MCA13-positive subisolates. Using primers to specific and variable regions of the CTV genome, molecular profiles of the isolates and subisolates were generated and compared. The profiles of the subisolates sometimes were very different from the parent field isolates from which they were transmitted. Citrus tristeza virus (CTV) is the causal agent of one of the most destructive diseases of citrus. Numerous biological strains of CTV exist and cause different symptoms such as stem pitting, decline and death of sweet orange and grapefruit on sour orange rootstock, seedling yellows, vein clearing, and vein corking. These various symptoms also can occur on different citrus hosts. CTV exists in field trees as a mixture of different biological virus types. This mixture or complex may therefore contain both mild and severe CTV. Using the brown citrus aphid as a tool, we have found that different parts (subisolates) of this mixture can be separated. We studied what is transmitted from three different CTV isolates using aphid transmissions and tested these components with serological and molecular analysis. We showed the transmission of severe CTV from mild virus sources. This has implications for regulatory agencies. Understanding what constitutes various CTV field isolates is important in devising methods to ...