Breeding papaya for resistance to ringspot virus disease in Malaysia

Papaya (Carica papaya) is one of the most important export fruits in Malaysia, grossing RM59 million in export revenue in 2000. However, the two cultivars; commonly grown in the country, 'Sekaki' and 'Eksotika', are very susceptible to the papaya ringspot virus (PRSV). First detected in Johor in 1991, this devastating disease had effectively stopped papaya growing in the south and forced cultivation to shift to the central and northern regions. The Malaysian Agricultural Research and Development Institute (MARDI) initiated a breeding programme for developing papayas with resistance to PRSV in 1991. The programme was divided into four phases, 1. Hybridization of the susceptible 'Eksotika' with tolerant cultivars, viz. 'Cariflora' and 'Tainung 5', followed by field evaluation of the F-1 hybrids. 2. Using the Single Seed Descent Method to develop F-5 inbred lines from the F-1, population by repeated selfpollination at each generation. 3. Screening and selection of F5 seedlings for PRSV disease resistance by manual inoculation. 4. Evaluation of field tolerance and performance of selected lines. The progress made at each phase of the programme was highlighted, culminating in the selection and field-testing of eleven lines over 2 'hotspot' diseased locations. Four lines, L248, L41, L90 and L13, were found to be most tolerant. L248 was the most tolerant in plant disease symptoms and was the highest yielding. L90 with near symptom-free fruits was the most tolerant insofar as fruit symptoms were concerned. Many of the tolerant selections, however, have low TSS of 9-10% and their fruit sizes were too large. L13 was found to have TSS and fruit weight closest to 'Eksotika' and had disease tolerance only just below that of the three most tolerant lines. L13 and possibly L248 may have the best potential for interim release while L41 and L90 would make useful parents for development of F-1 hybrids in future breeding programmes.