Evaluation of East Asian citrus germplasm as scions and rootstock

• Southern Fruit Research Institute, Vietnam
• Research Institute of Fruit and Vegetables, Vietnam
• Chinese Academy of Agricultural Sciences, China
• CSIRO Plant Industry, Australia

The health and productivity of a citrus industry depends greatly on the availability of suitable rootstocks and of good, disease-free cultivars. This is because rootstocks have a major effect on the 'tree' (technically called the scion) that is grafted on to them. Tree size, tolerance to severe conditions, fruit size, total yield and the quality of the fruit juice are all largely determined by the rootstock. Ideally rootstocks must be adapted to local conditions and be able to survive any pathogens that have built up in pre-existing orchards. Salinity, alkalinity and root rot problems are prime concerns, and stocks vary in their tolerance of these.
In Southeast Asia and Australia there is a need for improved disease-tolerant rootstocks that are better adapted to local soil and climatic conditions. An increase in the variety and performance of scions is also necessary to adapt to changing consumer preferences and to expand marketing opportunities. Any new cultivars must be virus-free and able to tolerate various climatic zones.
To achieve this, new citrus types must be found in the wild. The main regions of citrus diversity are the subtropical and tropical parts of Asia and the Malay Archipelago. But human pressure is affecting natural citrus species occurring in mountain areas of China. This project addressed the problem by starting to collect, evaluate and exchange citrus germplasm. It followed on from a previous project with China on citrus rootstock improvement. Its scope was broadened to include evaluation of scions as well as rootstocks and collaboration with research institutions in Vietnam.

The specific objectives of the project were: to collect and exchange citrus rootstocks, clones, public access hybrids and species native to each country; to cooperate in the evaluation of citrus rootstock material including germplasm characterisation, assessment of tolerance to pathogenic and environmental stresses, and assessment of horticultural potential; to collect and exchange citrus scion germplasm of economic significance to each country; to assess, improve and maintain the health status of citrus scion germplasm; to investigate the feasibility for citrus seed introduction and the reduction of the juvenile period to provide fruiting wood (in comparison to the time taken for budwood introductions to be available for trial); to establish source trees of important new citrus scion cultivars in each country.

The scientists collected rootstocks native to China and Vietnam, looking for good horticultural performance and high tolerance to fungal diseases. In exchange Australia made available a range of hybrids. Rootstocks were moved in the form of seed, to minimise disease transfer. Scions were also collected as seed, shoot tip grafts or budwood. Where necessary they underwent treatment to eliminate disease problems. Full virus testing also took place, and scions free of virus were placed in an insect-proof repository.
Molecular 'fingerprinting' of material, using isozyme analysis, was undertaken at CSIRO in order to characterise hybrids and cultivars more precisely. Material was then screened for horticultural attributes and salt tolerance, also tolerance to the fungus Phytophthora (which causes several economically important citrus diseases) and the citrus tristeza virus (CTV). Finally, field evaluations of lemon, mandarin, grapefruit and orange cultivars took place in New South Wales.

The project successfully fostered the exchange of citrus rootstock and scion germplasm between Australia, China and Vietnam for the benefit of citrus improvement programs in each country and promoted the further collection and conservation of native germplasm in China and Vietnam. The subsequent screening of accessions for horticultural performance, salt tolerance and disease tolerance identified important sources of variation in key attributes relevant to current rootstock improvement programs.
Training programs have been highly effective in broadening the scope of citrus improvement programs in both China and Vietnam and in promoting the wider use of rootstocks to improve orchard productivity. A Chinese scientist received a John Allwright Fellowship to undertake a PhD program in Australia under the guidance of project scientists. The research focused on the mechanism of cross-protection against severe strains of CTV. Successfully dealing with CTV is a critical aspect of citrus improvement programs in Australia and Asia.
Virus-free budwood of 20 public-access varieties was supplied to Vietnam from Australia along with seed of commercially important citrus rootstocks. Seeds of 13 citrus species and hybrids were supplied to China from germplasm collections in Australia together with budwood of public-access navel orange clones. In all, a total of 47 new accessions have been introduced into China. Forty-four new scion and rootstock accessions were introduced as seed into Australia from Vietnam and 35 accessions from China, including seeds from crosses made in China to investigate salt tolerance in Poncirus trifoliata. Along with the rootstock material introduced during the first project this represents a unique germplasm resource which will be of considerable long-term benefit to citrus improvement programs in Australia and internationally throughout the region.
Screening experiments conducted in Australia for Phytophthora, CTV, salt tolerance and early indications of graft incompatibility demonstrated wide variability among accessions. New salt accumulating and excluding germplasm was identified. Zaoyang zhi, a P. trifoliata accession from China has shown no indication of yellow-ring incompatibility (a major problem in commercial lemons) with Eureka lemon in early screening trials, and has great promise as an alternative replant rootstock for Eureka lemons.
Nine replicated preliminary field screening trials have been successfully established in Australia to assess horticultural performance and disease and salt tolerance of rootstock accessions under field conditions. Trials of Valencia orange, Navelina, Lane Late navel, Imperial mandarin, Eureka lemon and Fino lemon scion cultivars are revealing differences among accessions in effects on initial yield and yield efficiency relative to tree size. The field screening trials are being conducted as part of the National Citrus Rootstock Screening Program with funding support from Horticulture Australia.
The marketing period for citrus in China has traditionally been quite short but the introduction of new citrus varieties has served to extend the supply period and boost the industry. Among the introduced cultivars Lane Late navel has been extensively propagated and 100,000 trees planted in south China. The ACIAR project has also been the stimulus for the establishment in China of a 'National Centre for Citrus Virus Exclusion' at the Citrus Research Institute (CRI).
Collecting trips conducted in seven provinces of southern China resulted in 124 accessions (old local varieties and sub-wild types) being added to the National Citrus Germplasm Repository. The CRI has been designated the primary citrus germplasm repository in China. Isozyme analysis and molecular techniques have been used to sort and confirm the identity of germplasm material, and the direction given for the replanting of repository blocks and field evaluation trials.
Substantial progress has been made in assessment of rootstock and varietal evaluation at both the Research Institute for Fruit and Vegetables (RIFAV) in and the Southern Fruit Research Institute (SOFRI) in Vietnam. In both north and south Vietnam local and introduced cultivars have been pathogen indexed and mother trees maintained in an insect-proof environment to provide high-health status propagation material. This has been taken one step further in southern Vietnam with the establishment of a nursery certification scheme.
This project was included in an adoption study of projects completed in 2001-2002, which is available as an ACIAR publication.