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Reverse transcription—polymerase chain reaction (RT-PCR) was compared with enzyme-linked immunosorbent assay (ELISA) and direct tissue blot immunoassay (DTBIA) for detection of non-decline-inducing and decline-inducing isolates of Citrus tristeza virus (CTV) in 21 field sweet orange and grapefruit plants on sour orange rootstock in Fort Pierce, FL. Among these samples, seven, six, and eight were infected with decline-inducing, non-decline-inducing, and both decline-inducing and non-decline-inducing isolates of CTV, respectively. However, there was not a good correlation between field symptoms and detection of the decline-inducing isolate. The results confirmed that RT-PCR is not only able to detect and differentiate decline-inducing and non-decline-inducing isolates of CTV in Florida, but also can detect both isolate types in a single field sweet orange or grapefruit tree. For most samples, results from RT-PCR, ELISA, and DTBIA were the same. However, the 320-bp fragments produced only from decline-inducing isolates were amplified from two sweet orange and two grapefruit samples that did not react with decline-inducing CTV-specific monoclonal antibody MCA13 in ELISA or DTBIA, indicating that RT-PCR has a higher sensitivity than these immunological tests for field sweet orange or grapefruit samples. Thus, RT-PCR is a simple, rapid, and specific procedure for CTV identification applicable to both research and diagnostic needs. [ABSTRACT FROM AUTHOR]