Control of Papaya ringspot virus in the Philippines by development of PRSV resistant hybrids

• University of the Philippines at Los Banos, Philippines

Papaya ringspot virus occurs in both tropical and sub-tropical regions. It has devastated the papaya industry in Sri Lanka, Florida, and Luzon in the Philippines, and drastically reduced production in Brazil, Hawaii, Taiwan, India, and tropical regions of Africa and Central America. It has recently spread to Australia and Malaysia.

Papaya (Carica papaya) is an important crop in the Philippines. It is grown on large plantations, and also by small landholders, who inter-crop the fruit with pineapples, bananas, coconuts and coffee. PRSV first occurred in the Philippines in 1982. The spread of the virus is such that only the island of Mindanao remains free of it. However, PRSV spread to Mindanao is likely. It has numerous alternative hosts, is spread by many insect vectors and can be transmitted in seeds.

Selection of genotypes of C. papaya that are most tolerant have been identified as Cariflora from Florida and Tainung from Taiwan.

The aim of this project is to transfer known resistance to Papaya ringspot virus (PRSV) from other Carica species into C. papaya by interspecies crosses and then separate out hybrid embryos by in vitro embryo rescue techniques. The primary aims are wide hybridisation of Carica species, and in vitro propagation of selected in vitro lines. Researchers will also attempt other methods of hybridisation for resistance, such as use of gene-transfer techniques.

The collaborators will obtain seed of Carica from known collections and grow it in Queensland and in the Philippines. They will make controlled crosses between C. papaya and other species, and these other species will also be crossed with one another to test the possibility of obtaining bridge species - to be used if incompatibility problems occur.

Fruit will be harvested at various periods after pollination, and embryos rescued in vitro. Zygotic embryos will be identified by isozyme analyses, and resultant plantlets will be inoculated with PRSV and screened for tolerance/resistance in controlled glasshouse experiments. ELISA tests will be used to assess quantitative resistance. Other tissue culture techniques will also be investigated.

Protocols developed will be used for a large-scale field evaluation of the system to ensure 'trueness to type'. This part of the project will involve training two Filipino scientists in the techniques.