Abstract

Weekly or fortnightly records of the incidences of three phytoplasma diseases of papaya (dieback, yellow crinkle and mosaic) were taken over 4 years at three sites in central and southern Queensland. Outbreaks of dieback generally occurred between October and December and yellow crinkle between November and March. Disease outbreaks appeared to be associated with weather conditions which caused the surrounding vegetation to dry. No consistent incidence pattern was evident for mosaic disease. Over three sites, the incidence of dieback was 68-85%, yellow crinkle was 2-27% and mosaic was 5-8%. When the time from infection to the expression of disease symptoms was considered, the plants would have been infected between September and November for both dieback and yellow crinkle diseases. The diseases were absent when plants were grown under insect proof netting, implying that the diseases were transmitted by aerial vectors, probably insects. Disease symptoms and PCR testing indicated that the phytoplasmas were present in plants outside the netted area but not inside. Seven different species of planthoppers and 13 species of leafhoppers were collected from papaya. Although some were present from September to November, none gave a positive PCR test for any of the three phytoplasmas. Regular sampling indicated that the insects do not stay for long or breed on the papaya plants, suggesting that papaya is not a favoured host. On this basis, we present the hypothesis that dieback and yellow crinkle are transmitted by leafhoppers or planthoppers. The insects are transported into the district by weather troughs/fronts that involve north-to-south air movement in spring (September-November). Dieback and yellow crinkle outbreaks generally occur in years with dry conditions in late winter and early spring, as the insects are attracted to green papaya plantations surrounded by unattractive dry, brown vegetation.