Citrus rootstock development

• Chinese Academy of Agricultural Sciences, China

Citrus industries depend on the availability of suitable rootstocks. In both China and Australia, salinity and alkalinity problems are concerns in citrus-producing areas. Researchers are seeking improved, disease-tolerant rootstocks that will adapt to local soil and climatic conditions.

Chinese scientists are now establishing arboreta of wild citrus relatives in Hunan and Sichuan Provinces, using isozyme analysis to help identify native species, and are evaluating selections in field trials. Wild citrus collected in south-western China with potential as rootstocks include Poncirus trifoliata variants, natural P. trifoliata hybrids, several Fortunella species, wild mandarins, and selections of Citrus ichangensis and C. junos.

Australian expertise in rootstock evaluation is widely recognised. Research programs have been enhanced by improved identification of local selections, importations from overseas breeding programs, and limited introductions from areas where citrus originated (China, India and Southeast Asia). The only citrus native to Australia are Microcitrus, Eremocitrus and Clymenia.

The aims of the project are to:

collect and exchange citrus rootstock material, including species native to China and Australia, local clonal selections and hybrids arising from breeding programs; and

cooperate in the evaluation of potential rootstock material with respect to isozyme analysis, disease tolerance, salt tolerance and horticultural attributes.

For disease tolerance, attention will be focused on Phytophthora infection and tristeza. Pathogenic strains of the citrus tristeza virus (CTV) and species of the fungus Phytophthora vary in each country.

Australian and Chinese collaborators will assess the range of germplasm available in each country and select suitable trees for seed collection. They will evaluate selected material by:

identifying each species by the usual botanical and horticultural methods, by isozyme analysis and possibly by restriction fragment length polymorphism (RFLP);

screening for tolerance to Phytophthora root and collar rots by collar rot inoculations, by testing of seedlings in aerated nutrient solution containing zoospores, and by evaluating root-rot tolerance in Phytophthora-infested soil in the screenhouse or field under a range of scions;

screening for tristeza tolerance using CTV-infected scions, inoculating potential rootstocks with virulent CTV strains, and assessing immunity or resistance to CTV with ELISA;

screening for salt tolerance using a glasshouse method based on rootstock seedlings growing in peat/sand media irrigated with nutrient solution containing salt, or in aerated nutrient solution with a high salt content; and

screening for scion incompatibility by anatomical assessment of bark ring grafts.

Screening for disease and salinity tolerance and horticultural attributes will be carried out in each country, and will be followed by short-term field evaluations. Promising material (mostly Poncirus variants, mandarins, and natural Poncirus hybrids) may be used in further breeding programs.