The phytopathology of Australian papaya dieback: a proposed role for the phytoplasma

Australian papaya dieback is a severe phytoplasma related disease of papaya in which plant apical death can be observed within 2 weeks of first visibl e external symptom expression, and 3 weeks of first detection of phytoplasm a in host tissue. The pathological mechanisms involved in this disease, and indeed in phytoplasma diseases in general, are unclear. We have attempted to contribute to this understanding by physiological characterization of th e host response to the disease. Photosynthetic rate (CO2 exchange rate) of mature leaves was reduced by approx. 50 % in dieback affected plants relati ve to healthy plants by the time of first visible symptom expression. Photo synthesis effectively ceased in mature leaves of dieback affected plants wi thin 2 days of first visible symptom expression, although mature leaves wer e free of phytoplasma as judged by PCR analysis. Carbohydrate levels increa sed in leaf tissue of diseased plants but decreased in stem and root tissue . A model is presented for the pathogenicity of Australian papaya dieback i n which a pathogen induced or produced xylem mobile metabolite is proposed to be initially responsible for a disruption of phloem transport, and later for a loss of cellular integrity in leaves (X-Y flecking symptom) and in t he stem apex (meristem necrosis). (C) 2001 Academic Press.