Pcr Detection Of The Rickettsia Associated With Papaya Bunchy Top Disease

Papaya bunchy top (PBT) disease is a major limiting factor in the production of papaya (Carica papaya L.) in the American tropics. We recently discovered a small, rodshaped, gram-negative bacterium within the latex-producing cells (laticifers) of PBT-affected, but not healthy, papaya plants. Phylogenetic analysis based on the sequences of four genes indicated that bacterium is.a member of the ee-subdivision of the Proteobacteria and of the genus Rickettsia. Primers for detection of the PBT bacterium by the polymerase chain reaction were designed to amplify a 705 bp fragment of the putative gene for the flavoprotien subunit of succinate dehydrogenase. Using these primers, a single PCR product of the expected size was obtained with DNA extracts from all 12 PBT-affected papaya plants including seven from Puerto Rico and five from Costa Rica; whereas, no amplification of the DNA fragment was detected for any of the 13 DNA extracts from individual healthy plants including six from Florida -and seven from Puerto Rico. Further evidence of the widespread occurrence of the association between PBT and the PBT bacterium was obtained when DNA extracts from papaya samples from Barbados, Dominica, Grenada, Jamaica, St. Lucia, and St. Vincent tested positive by the PCR assay. The rickettsia was also detected by PCR in the leafhopper vector, Empoasca papayae, collected in several different papaya plantings affected by PBT in Puerto Rico. The PCR assay appears to provide the first specific means of detection for PBT.