The eggplant little-leaf agent was graft transmitted to tomato causing big-bud symptoms. Transmission from the big-bud tomato to potato by grafting or the leafhopper Orosius argentatus resulted in the development of purple top wilt symptoms. Thin-section electron microscopy revealed mycoplasma-like organisms present in the phloem sieve elements of a big-bud tomato plant and purple top wilt potato plants infected by grafting or leafhoppers. When tubers from graft-infected potato plants were planted, 73% produced spindly shoots and 44% of these later developed purple top wilt symptoms. When scions from either field-infected or experimentally infected potato plants showing purple top wilt symptoms were grafted onto tomato plants, 24% and 62% respectively developed big-bud symptoms. The results provide strong evidence for the mycoplasmal aetiology of some, if not all, potato purple top wilt in Queensland.

Two distinct double-stranded RNA (dsRNA) patterns, each comprising two major bands, were detected in extracts from apparently healthy Carica papaya plants. The slowest migrating band was present in both patterns and had an estimated Mr of 2.8 x 106 . The faster migrating bands had estimated Mrs 1.3 x 106 or 1.2x106. When papaw total and organellar DNA was extracted and hybridized with 32P-end labelled papaw dsRNA, the probe hybridized strongly with total and chloroplast DNA, but only very weakly with mitochondria1 and nuclear DNA.

When petiole and root sections taken from papaw plants affected by dieback disease were examined under a fluorescence microscope, specific autofluorescence was observed in some phloem cells and/or laticifers. Such autofluorescence was never observed in similarly treated sections taken from healthy plants. Light and electron microscopic examination of areas showing specific autofluorescence revealed the presence of necrotic phloem cells, but close examination of these cells failed to show a parasitic agent. Papaw plants affected by dieback disease and showing necrosis of laticifers also displayed a marked reduction in latex flow. The results indicate that autofluorescence of necrotic phloem cells and laticifiers, and a reduced latex flow, are new symptoms of papaw dieback disease in Australia.