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Did you know? ... Citrus originated in Australia

It has long been assumed that the Citrus genus originated in Asia. But as recent work shows, Citrus actually originated in Australasia. An Australian taxonomist called David Mabberley, currently based at the Royal Botanic Gardens Kew, in England, has shown that Citrus comprises about 25 species, and that the earlier division of the genus into Citrus, Eremocitrus (Australian desert lime), Fortunella (kumquat), Microcitrus (Australian finger limes) and Poncirus (trifoliata rootstock) was incorrect. Molecular genetics – the study of the structure of genes – shows that these former divisions are not supported; that is, all 25 species are descended from a single common ancestor.

Of those 25 species, more than half originated in Australasia – Australia, New Guinea and New Caledonia. The rest originated in India, South-East Asia or China. This distribution shows that the genus Citrus originated in Australasia (possibly Australia itself) and spread from there.

Cultivated citrus

Most of the citrus fruits we eat today are hybrids whose ancestry is tangled. Those that are original species include kaffir lime (C. hystrix), kumquat (C. japonica), citron (C. medica), pomelo (C. maxima), and mandarin, tangerine and satsuma (all C. reticulata). The hybrids include lime (C. ×aurantiifolia), orange and grapefuit (C. ×aurantium), lemon (C. ×limon) and Tahitian lime (C. ×latifolia). (The multiplication symbol here indicates a hybrid.)

Huanglongbing

In most of the rest of the world, a devastating citrus disease is slowly destroying the citrus industry. The disease, officially known as huanglongbing (Chinese for “yellow dragon disease”, a mistranslation of “yellow shoot disease”), is caused by a bacterium, Candidatus Liberibacter. This is the most destructive disease of citrus in the world. We in Australia are very lucky not to have it ... yet.

The bacterium is transmitted by two sucking insects, the Asiatic citrus psyllid, Diaphorina citri, and the African citrus psyllid, Trioza erytreae. Neither insect has made it to Australia yet. Rigid quarantine procedures are essential to keeping them out – along with huanglongbing – if the Australian citrus industry is to survive.

Genetic diversity

The presence of so many species of Citrus and its relatives (including the ornamental plant Murraya paniculata) as endemic (native) species in Australia has mixed blessings for the citrus industry. On the one hand, there is plenty of genetic variation that could be incorporated into citrus for breeding of resistance to the disease – not only in Australia but worldwide.

But on the other hand, there is potentially an enormous amount of susceptible plants down eastern Australia that could allow huanglongbing to spread like wildfire, not only destroying the Australian citrus industry, but also causing the extinction of many native plant species and the animals that depend on them.

What you can do

While the disease and its insect vectors remain outside Australia, citrus growers might be able to develop export markets to parts of the world where the disease is present. But you can support the Australian Quarantine and Inspection Service by not accepting any citrus material from outside Australia.

Further reading

Beattie GAC, Holford P, Mabberley DJ, Haigh AM, Bayer R, Broadbent P. 2008. Origins of Citrus, huanglongbing and the Asiatic citrus psyllid. Sixth Viet Nam Conference on Entomology, Hanoi, 9-10 May 2008, 820-834.