The project HORT/2012/113 - Integrated disease management strategies for the productive, profitable and sustainable production of high quality papaya fruit in the southern Philippines and Australia had the goal to contribute to economic growth in the Philippines through increased income and improved livelihoods of smallholder farmers including tropical fruit growers in the southern Philippines. This project delivered this goal by building research capacity, developing skills and undertaking comprehensive studies on the epidemiology of bacterial crown rot (BCR) and phytoplasma diseases to help generate sustainable management approaches.
The specific objectives of this project were:
1. To characterize the Erwinia sp. associated with BCR and its survival (Philippines).
2. To develop and evaluate sustainable management practices for BCR (Philippines). 3. To develop and evaluate sustainable management practices for dieback of papaya (Australia and Philippines)
4. To disseminate a package of technology (IDM strategies) for papaya (Philippines and Australia).
Bacterial Crown Rot
Molecular techniques were used and standardized to recover bacterial crown rot (Philippines) and papaya phytoplasma from diseased plants, planting material, weeds and insects. These techniques helped provide a better understanding of papaya disease epidemiology, insect vector relationships and effective disease management strategies.Erwinia mallotivora has been confirmed as the causal organism of BCR in the Philippines and has shown a 99.2% similarity to the BT - MARDI strain that causes BCR in Malaysia. The results of targeted gene sequencing of 16s rDNA, gyrB, rpoB, and recA have also shown no genetic variation among the many Erwinia mallotivora isolates recovered from BCR affected papaya in the Philippines.
Six isolates of Erwinia mallotivora have been sequenced at PCARI-SGCL using whole genome sequencing and these will be deposited at the NCBI database. Access to these sequences will allow for rapid and precise identification, which will help determine the origin and spread of the BCR pathogen. BCR tolerant lines tested under glasshouse conditions have shown significant differences in fruit weight, length and width, TSS, flesh colour, peel thickness, treatable acidity, firmness and percent edible portion. The field screening of a few selected tolerant lines showed a high incidence of BCR recently under wet weather conditions and indicated that the F2 population is still segregating and requires further work to stabilise the material. Foliar applications of Phosphro and BacillusQST 719 were found to be more effective at managing BCR than copper fungicides. Regular sprays with these products could be integrated into the disease management plan.
Real-time and nested PCR was consistent in the identification of phytoplasma from papaya plants, weeds and insect samples. The phytoplasma groups were identified using 16 sRNA sequencing and phylogenetic analysis. Ca .P. aurantifolia, which belongs to the 16 Sr II group of phytoplasmas was identified as the causal organism of a previously unidentified phytoplasma disease in Mindanao, the Philippines.
An extensive survey of papaya growing in commercial plantations using molecular techniques showed that three phytoplasma species were associated with papaya dieback, yellow crinkle disease and mosaic disease in Australia. These were Candidatus Phytoplasma australiense (dieback; PpDB), Candiadatus Phytoplasma australasia (yellow crinkle and mosaic disease ;PpYC and PpM) and Candidatus Phytoplasma aurantifolia(Australian Lucerne Yellows , ALuY). In Australia ‘Ca P. aurantifolia, and ‘Ca. P. australasia has been found associated with yellow crinkle and mosaic disease in northern Queensland. Another phytoplasma disease Australian Lucerne Yellows (ALuY) which is closely related to yellow crinkle and mosaic was recovered from one papaya plant with dieback symptoms. Six weeds/plant species; Praxelis, Sand Spurge, Wire weed, Shrubby Stylo, Sun-hemp and one unidentified weed which were collected in late summer in Australia had little leaf and leaf proliferation symptoms and were positive for yellow crinkle and mosaic. Two weeds; Praxelis and Shrubby Stylo were also positive for Australian lucerne yellows (ALuY) phytoplasma. These weeds were found in and around a phytoplasma-infected papaya plantation and were a potential food source for insect vectors and reservoir of phytoplasma disease.
The leafhopper Orosius orientalis (Cicadellidae) and a lace bug (Tingidae) collected from (QLD) tested positive for the 16 Sr II group of phytoplasma ‘Ca. P. aurantifolia and ‘Ca. P.australasia. This is the first report of phytoplasma detection from a lace bug and needs further study to determine if it is a vector of phytoplasma diseases. This finding also indicates the need to explore other groups of insects not only the known phloem feeding vectors of phytoplasma leafhoppers/plant hoppers and psyllids.
Papaya sticky disease caused by Meleira virus (P Mev) has been recorded for the first time in Australia. Papaya researchers have gained a better understanding of phytoplasma and Meleira virus diagnostic techniques through collaborations with virologists at the Department of Agriculture and Fisheries, Eco sciences Precinct in Brisbane. Attendance at National Biosecurity Training Meetings and a workshop on in field molecular diagnostic techniques (LAMP) improved our technical capacity to detect plant pests and diseases and in so doing helped reducing risk and provided security for crop production.
This project improved facilities and equipment in both the countries and provided opportunity for three UPLB students to complete their graduation and master’s research studies in Plant Breeding, Plant Pathology and Entomology. The project team members were also given support to attend and present their research at national, international conferences and symposia. The workshops, training sessions, field visits and growers meetings with researchers, student’s, project collaborators, DMPI growers and the Australian papaya industry conducted throughout the project also helped strengthen partnerships between researchers and farmers.
Future ACIAR funding for papaya breeding in the Philippines will allow them to develop Papaya ringspot virus (PRSV) tolerant lines with improved flavour by backcrossing to the desired cultivars. They could also be hybridised with the BCR tolerant lines to develop other hybrids with resistance to both PRSV and BCR then backcrossed to another cultivar to infuse a delayed ripening trait. In addition, there is also the opportunity to explore further the use of the metabolites of endophytic bacteria which have been shown to enhance resistance to diseases such as BCR and Phytophthora root rot.