How the Scheme Operates

Budwood supply

Budwood source trees are located at the Agricultural Research and Advisory Station, Dareton, with a supplementary supply of new varieties located at Monash in South Australia, under the management of the South Australian Citrus Improvement Society (SACIS). Budwood of some varieties is also cut from indexed trees maintained at the Department of Primary Industries Research Station at Griffith, NSW.

Auscitrus supplies two categories of budwood:

Premium - all new varieties released from plant quarantine and most commercially grown varieties are supplied as premium status budwood. source trees are subjected to routine indexing for certain pathogens (eg citrus exocortis viroid and psorosis virus).

Standard - standard varieties are usually of minor commercial importance or backyard varieties such as cumquats, kaffir lime and Meyer lemon. some carry minor pathogens and are supplied as "best available" because no pathogen-free source of the variety exists in Australia.

Rapid multiplication

New varieties released from post-entry quarantine undergo rapid nursery and field multiplication at Dareton. The aim of rapid nursery multiplication is to quickly increase the quantity of high quality budwood available, thus reducing the time to achieve commercial production of new varieties.

Importation of new citrus varieties

Since 1986 more than sixty public citrus varieties have been imported for the Australian citrus industry, most of these by Auscitrus. The major focus has been on importation of new mandarin and navel orange varieties.

In future, most new varieties, of both local and overseas origins, will be patented, and require acquisition fees and payment of tree and/or production royalties. Auscitrus is entering into agreements with 'commercialisers' of these patented varieties to maintain foundation trees, to provide independent horticultural evaluation and to multiply budwood and seed of high health status.

Horticultural evaluation

Auscitrus carries out horticultural evaluation of new varieties in cooperation with NSW Department of Primary Industries as part of the Australian Citrus Improvement Program project, funded by Horticulture Australia. New varieties are rapidly evaluated by reworking mature Valencia orange trees to provide an initial assessment of variety characteristics and provide fruit for test marketing. New varieties are also established in the field on a range of rootstocks to provide information on suitability of common rootstocks for each variety. Field evaluation sites are located in the major citrus growing regions across Australia to provide information relevant to local soil types and climatic conditions.

Repositories of virus-free and pre-immunised trees

The Auscitrus repositories for virus free and pre-immunised clones are located in insect-proof screenhouses at NSW Agriculture's Elizabeth Macarthur Agricultural Institute (EMAI) at Camden, with a field repository at Dareton.

To protect against stem pitting strains of tristeza virus, initially grapefruit, and now all citrus clones, are being pre-immunised with a mild isolate of tristeza virus, a process referred to as mild strain cross protection.

These foundation trees are the insurance policy for the Australian citrus industry and ensure that high health status material will be available in the advent of a disease outbreak.

Pathogen testing

Many viruses and viroids can exist in citrus without expressing obvious symptoms. However these pathogens can be very destructive when infected scions are grafted to sensitive rootstocks or transmitted mechanically on secateurs or budding knives, or by insect vectors, to susceptible scions.

Auscitrus budwood and rootstock seed trees are tested for graft transmissible citrus viruses and viroids using both glasshouse (biological) and laboratory techniques, referred to as indexing. All source trees are indexed on a rotational basis. Suspect trees are indexed as required. Indexing takes place at the Elizabeth Macarthur Agricultural Institute (EMAI) at Camden.

All trees providing Premium budwood are indexed once every three years on Etrog citron indicator plants for Citrus Exocortis Viroid and for milder viroids that may cause dwarfing. Trees showing suspect symptoms on Etrog citron are tested further using a laboratory technique called sequential polyacrylamide gel electrophoresis (sPAGE) to determine which viroid (CVd I-IV, CEV), if any, is present.

Trees are indexed once every ten years on sweet orange seedlings for orange stem pitting strains of Citrus Tristeza Virus and for psorosis virus.

Grapefruit trees in the budwood blocks undergo annual biological indexing to determine the severity of CTV strains present and if necessary, molecular tests are carried out to ensure there has been no ingress of severe strains.

Pathogen elimination

Shoot-tip grafting in conjunction with heat therapy, is used to remove viruses and viroids from infected parent trees. The process involves excising a 0.15mm shoot tip from the infected plant and grafting it onto a 2 week old rough lemon seedling growing in sterile media in a test tube. The resulting plant is indexed to confirm that all viruses and viroids have been eliminated.

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