Phytoplasma diseases of papaya (Carica papaya L.) in Australia: Phytoplasma classification, pathology and transmission

In Australia, phytoplasmas have consistently been associated with the papaya (Carica papaya L.) diseases known as papaya dieback (PpDB), yellow crinkle (PpYC) and mosaic (PpM). PpDB is the most economically important of these diseases, followed by PpYC. The investigations presented in this thesis have therefore focused primarily on PpDB. Analysis of the DNA sequences of the 168 rRNA gene and the 168-23S rRNA intergenic spacer region (SR) of the PpDB, PpYC and PpM phytoplasmas showed that the PpYC and PpM phytoplasma DNA sequences were identical to each other, but were distinctly different to that of the PpDB phytoplasma. A phylogenetic tree based on 16S rRNA sequences revealed that PpDB is most closely related to the Australian grapevine yellows (AGY) phytoplasma and the Phormium yellow leaf (PYL) phytoplasma from New Zealand, forming a distinct group within subclade xii. PpYC and PpM phytoplasmas are most closely related to the tomato big bud (TBB) phytoplasma from Australia, within subclade iii. It was proposed that the PpDB phytoplasma be included in the taxon "Candidatus Phytoplasma australiense", and that the ppye and PpM phytoplasmas be assigned to a new taxon, "Candidatus P. australiense". Histological studies and mapping of phytoplasma distribution using PCR revealed that it is likely that phytoplasma cells are present in very low titre and that, while the plant appears to limit proliferation of the PpDB phytoplasma, this defence response is associated with a rapid decline of the papaya plant. Immature leaf material was sampled weekly for eight months from 60 plants in a commercial papaya plantation, to estimate the minimum time between inoculation and symptom expression of PpDB, PpYC and PpM. The PpDB phytoplasma was detected by PCR one week prior to, or the same week as, external symptoms were first observed, while phytoplasma DNA was detected between three and eleven weeks prior to expression of PpM symptoms. Examination of lateral shoot regrowth on papaya plants that had recovered from PpDB or were cut back (ratooned) when they initially exhibited PpDB, PpYC or PpM symptoms, revealed that the PpDB phytoplasma did not persist in plants after the initial expression of symptoms. In contrast, the PpYC and PpM phytoplasmas usually persisted in the lower parts of the plant, and then infected the new lateral shoots as they developed. Dodder (Cuscuta australis R. Brown) was used as a phloem bridge between papaya plants affected by PpDB, PpYC and PpM, and periwinkle (Catharanthus roseus G. Don) plants. "Candidatus P. australasiense", but not the PpDB phytoplasma, was transmitted to periwinkle. The inability to transmit the PpDB phytoplasma corresponds with the view that in papaya, this phytoplasma is likely to be present at low titre, is a highly virulent pathogen, and disrupts phloem function before external disease symptoms are observed. Based on the results of this study it is recommended that ratooning of PpDB-affected plants and removal of PpYC- and PpM-affected plants are the best strategies currently available for the management of these diseases. Suggestions for future research and disease control strategies are discussed.