EPIDEMIOLOGY OF PHYTOPLASMA-ASSOCIATED PAPAYA DISEASES IN QUEENSLAND,AUSTRALIA

Three phytoplasma-related diseases of papaya (Carica papaya), dieback, yellow crinkle, and mosaic, are recognized within Australia. Immature leaf material was sampled every week for 8 months from a cohort of 60 female plants, located within a commercial papaya plantation, to dete rmine the minimum time between infection and symptom expression. Phyto plasma DNA was detected using the polymerase chain reaction (PCR) with primers specific for phytoplasmas in general, and for the stolbur gro up of phytoplasmas. The dieback-associated phytoplasma was detected 1 week prior to (four cases) or the same week (nine cases) as symptom ex pression, while phytoplasma DNA was detected between 3 and 11 weeks pr ior to expression of mosaic symptom (six cases). Lateral shoot regrowt h on the lower stem of plants which had suffered dieback disease faile d to generate stolbur-specific PCR products in 15 cases. A dual infect ion with dieback and yellow crinkle or mosaic was diagnosed in a furth er two cases, using restriction fragment length polymorphism digests, and both cases were interpreted as secondary infections by the dieback -associated phytoplasma. Regrowth in three of seven cases of yellow cr inkle- and three of nine cases of mosaic-affected plants tested positi ve for phytoplasma-specific DNA. Ratooning of dieback-affected plants and removal of yellow crinkle- or mosaic-affected plants is suggested for the management of these diseases.