Epidemiology of phytoplasma diseases in papaya in northern Australia

Using molecular tools, the spread of phytoplasma diseases in a papaya plantation was investigated for 3 years to identify phytoplasma strains affecting papaya. insect vectors and alternative plant hosts. Five phytoplasma strains (SPLL-V4, TBB, CaWB, StLL and WaLLvar) were associated with papaya yellow crinkle disease and one phytoplasma strain (PDB) was associated with papaya dieback disease. The most prevalent strains were TBB and SPLL-V4 which occurred in 94% of infected papaya. There was a significant correlation between phyllody and TBB. and virescence and SPLL-V4. although other phytoplasma types could also be associated with either phyllody or virescence. No mixed infections were detected in diseased papaya. Disease progress curves for TBB and SPLL-V4 showed a sigmoid response reaching a maximum disease incidence of 16% after 24 months. The rate of disease spread was best described by a logistic model which showed that TBB spread at a slightly higher rate than SPLL-V4. Ten phytoplasma strains were detected in 14 alternative plant species, however. TBB and SPLL-V4 were present in only a few individual plants of some of these species, so these alternative hosts would probably not have provided a significant infection source to papaya. Very few phytoplasmas were detected in leafhoppers collected over 3 years with TBB and SPLL-V4 only detected in Orosius spp.