Papaya clean seed program

Papaya sticky disease is found across the major papaya growing regions in Australia. Papaya plants infected with the disease have fruit that exudes watery latex which dries on the skin, producing unmarketable fruit. In Australia, the disease is caused by a virus (PMeV2-Aus) that is seed transmitted. The objective of this project was to develop PMeV2-Aus-free parental lines to enable the production of virus-free hybrid seed for growers and develop new knowledge in relation to the virus involved in the disease. The Australian virus has only recently been discovered, and very little is known about its physical characteristics and epidemiology. A specific, sensitive, and rapid test was developed and validated to diagnose PMeV2-Aus in samples of leaf tissue. A qPCR test was also developed for determining the sex of the plants, which was made into a multiplexed single assay with the virus test. Embryo rescue was used to generate virus-free parent lines, with growth in an insect proof shade house. On average over the 506 plants propagated from the ten different parent lines, 96% of embryos tested negative to PMeV2-Aus. Between six and ten plants of the ten parental lines were entered into tissue culture maintenance to safeguard the industry’s investment into developing PMeV2-Aus-free parental lines. A field trial with virus-free plants saw rapid re-infection with the virus, demonstrating that careful management of the parental lines is required to produce virus-free seed. The benefits of growing clean seed will not be fully realised without proper management plans until localised virus pressures drop. Characterisation of PMeV2-Aus, has found significant difference to the situation reported overseas. PMeV2Aus is a double-stranded RNA virus, not a single stranded RNA virus as is reported overseas. The second virus reported overseas as contributing to sticky disease was not found in Australian plants and only PMeV2-Aus was found linked to disease in Australia. There does not seem to be a recognisable coat protein in the virus genome sequence, though virus particles are easily found. This is very unusual for plant viruses and the method by which the virus obtains the proteins to form a capsid is currently still unknown. Many questions remain about the epidemiology of PMeV2-Aus. This can now be explored with a supply of healthy virus-free plant material. Important questions that can impact management decisions include what is spreading virus, how soon after infection can the virus be detected, and how soon after infection can it transmit the virus.