Abstract: Diagnosis and detection are important aspect of citrus Huanglongbing research In the past,diagnosis of citrus Huanglongbing was mainly based on its sympomatology,namely,yellow shoot and mottle leaf During 1977-1978,the causative bacterium like organism (BLO) was found in sieve cells of diseased leaf under electron microscope After then,the electron microscope has been used as a major tool for diagnosis of citrus Huanglongbing disease An improvement came about when it was found that the BLO can be transmitted from citrus to periwinkle by dodder and partially purified by differential centrifugation,which made it possible to prepare antisera,both monoclonal and polyclonal antibodies,for detection and diagnosis of Huanglongbing BLO Recently,polymerase chain reaction (PCR) has been developed for detection and quanfitication of Huanglongbing BLO The PCR-based methods are highly sensitive and effective in detecting the presence of the BLO in diseased plant and insect vectors carrying the BLO Furthermore the BLO in the samples can be quantitated by quantitative PCR making it possible to compare the amounts of BLO in different samples.