Field performance of papaya lines selected for tolerance to ringspot virus disease

Y.K. Chan and C.A. Ong

Abstract

Papaya is one of the most important export fruits in Malaysia, grossing RM79 million in export revenue in 2001. However, the two varieties commonly grown in the country i.e., Sekaki and Eksotika are very susceptible to the papaya ringspot virus (PRSV) disease. First detected in the south 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. MARDI initiated a breeding programme for resistance to PRSV in 1991, using Tainung No. 5 and Cariflora as tolerant parents. The F1 hybrids with Eksotika showed very good tolerance. From this stage, the single seed descent method was used to generate the F2 and by subsequent inbreeding to the F5. Seedling screening for PRSV tolerance by artificial inoculation at the F5 resulted in selection of 11 lines. These lines were tested over 2 ‘hot-spot’ diseased locations in Ulu Tiram and Kluang in Johor to evaluate their field tolerance and fruit characteristics.Four lines i.e. L41, L90, L248 and L13 were the most tolerant. L248 was the most tolerant in plant disease symptoms and was the highest yielding with 41.7 kg/tree over 12 months harvest. L90 was the most tolerant insofar as fruit symptoms were concerned. Fruit of L90 rarely showed any ringspot symptoms and had an extremely low disease score of 0.47 compared with 6.08 from Eksotika. A comparison was made between tolerant lines with the susceptible Eksotika and the tolerant benchmark Tainung No. 5 on the onset and expression of disease symptoms. Eksotika and the tolerant lines showed a similar pattern in symptom build-up but tolerant lines consistently have much lower disease severity scores. Tainung No. 5 had an entirely different symptom build-up pattern. It was more tolerant than the selected lines at the early stages but disease severity rose sharply after the 4th month and it was just as susceptible as Eksotika by the 7th month.Many of the fruit of the selections have low TSS of 9–10% with two lines i.e. L25 (11.1%) and L13 (11.0%) exceeding that of Eksotika (10.3%). A ranksum method for simultaneous selection for disease tolerance and fruit attributes (yield and TSS) was used to select the best overall performers. With consideration of these two sets of selection criteria, L13 and L248 were adjudged as having the best potential for interim recommendation.

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