Autofluorescence of necrotic phloem cells and laticifers, and a reduced latex flow: new symptoms for papaw dieback disease in Australia

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.