Citrus australasica

Taxonomy

This species was first described by Australian botanist Ferdinand von Mueller, and was published in the first volume of his massive work Fragmenta phytographiæ Australiæ in 1858. American botanist Walter Tennyson Swingle, in a 1915 review of the genus Citrus, placed all four Australian species in a new genus Microcitrus, based on morphological features which he said gave "[...] these plants a very different aspect from the commonly cultivated species of Citrus." However, in a paper published in 1998, British botanist David Mabberley discussed the mix of morphological features present throughout the subtribe Citrineae (genera Clymenia, Eremocitrus, Fortunella, Microcitrus and Poncirus) and the ease with which species can be crossed, and subsequently restored the finger lime to Citrus.

Distribution and habitat

The finger lime is native to the southeastern corner of Queensland and the northeastern corner of New South Wales, from the area around Brisbane to the Clarence River. It grows in coastal rainforest and extends into neighbouring open forest, often on soils derived from basalt. It is favoured by disturbance.

Conservation

As of December 2024, this species has been assessed to be of least concern by the International Union for Conservation of Nature (IUCN) and by the Queensland Government under its Nature Conservation Act.

Pests and diseases

The finger lime is susceptible to a range of insect pests, including scale insects, katydids, larvae of various moths and butterflies, some bugs, grasshoppers and others. However, it is not a host to the Queensland fruit fly Bactrocera tryoni, saving Australian growers the extra burden of treatment before export.

Research conducted since the 1970s indicated that a wild selection of C. australasica was highly resistant to Phytophthora citrophthora root disease, which has resulted in a cross-breeding program with finger lime to develop disease-resistant citrus rootstock. In 2020, researchers began working with C. australasica to develop solutions for Citrus greening disease.

Cultivation and uses

History

First Peoples living along the east coast of Australia used finger limes for hydration and to treat skin infections and cuts. The fruits contain the nutrients vitamin C, folate and potassium, as well as the antioxidants vitamin E and polyphenols, and research has shown the efficiency of vitamin C as a treatment for bacterial infections.

Early settlers consumed the fruit and retained the trees when clearing for agriculture. Colonial botanists suggested that they should be cultivated, due to the lack of citrus alternatives.

Rising demand

The finger lime has been popularised as a gourmet bushfood. The globular juice vesicles (also known as pearls) have been called "lime caviar", and can be used as a garnish or added to various recipes. The fresh vesicles have the effect of a burst of effervescent tangy flavour as they are chewed. The juice is acidic and similar to that of a lime. Marmalade and pickles are also made from finger lime. Finger lime peel can be dried and used as a spice.

Commercial use of finger lime fruit started in the mid-1990s with boutique marmalades made from wild harvested fruit. By 2000 the finger lime was being sold in restaurants, and exported fresh.

The finger lime is grown on a commercial basis in Australia in response to high demand for the fruit. There is an increasing range of genetic selections which are budded onto citrus rootstock. With the sudden high market demand for the fruit the primary source of genetic material for propagation has been selections from wild stock.

The CSIRO has also developed several Citrus hybrids by crossing the finger lime with standard Citrus species, creating cultivars which produce finger limes in many different colours ranging from light pink to deep blue-green. Finger lime is thought to have the widest range of colour variation within any Citrus species.