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.