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Whatsapp Workers prepare a citrus fruit display during the annual La fete du citron (Lemon Festival) in Menton, France.

Despite the advice of dieticians, many of us still start the day with a glass of orange juice. Yet orange juice, and in fact the entire citrus growing industry, is in serious peril. Citrus greening disease wiped out the citrus industry in Asia and is damaging it in the US, and Australia could be next warns Professor David Mabberley.

Orange blossom and cologne, Curaçao and Cointreau, Earl Grey tea and marmalade, orangeries and pomanders, Nell Gwynn and sun tan oil: there’s certainly more to citrus than orange juice. Orange juice, however, is the most familiar citrus product and citrus growing is the most important fruit industry in warm countries, contributing significantly to the economies of Australia, the United States, Brazil, South Africa, Israel and southern Europe.

No citrus was known to Europeans until Alexander the Great reached Persia and India and saw Citrus medica, the citron. In writing up the botanical findings of that expedition, the botanist and doctor Theophrastus explained that the citron was not eaten but placed amongst clothes to deter moths.

The simple fact is that orange juice—and indeed the whole citrus industry worldwide—is in peril.

He also discussed its use as a mouthwash, but the most famous medicinal use of citrus in the west has to be the alleviation of scurvy, a fatal disease caused by lack of vitamin C. From 1795, British sailors were issued with daily draughts of lemon juice, usually added to their rum ration, and the improved health of the British Navy even gave Nelson a strategic advantage over Napoleon’s fleet at the Battle of Trafalgar.

Vitamin C is still one of the most popular dietary supplements on the market, though its use in treatment of colds has been vastly overrated, largely because of the prominence of Linus Pauling’s 1970 book Vitamin C and the Common Cold.

Today orange juice is the most important citrus commodity and its production is a huge industry. So typical is it of the western breakfast that even though many dieticians say that the sugar rich morning OJ is a recipe for fat deposition, the popular perception is that it is wholesome. Indeed, citrus fruits in general are seen as clean and natural, as witnessed by the number of cleaning products that are lemon-scented.

The popularity of orange juice dramatically increased with the development of the commercial juice industry in America in the late 1920s. In 1944, a way was found to concentrate fruit juice in a vacuum and freeze it without compromising flavour or vitamin content, and frozen juice was on the market the very next year. Commercial aseptic packaging now allows juice to be sold without refrigerated storage and the global trade is worth more than $2.3 billion.

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The biggest markets for orange juice are the United States, Canada, Europe and Japan. Although some of the orange crop in Australia is used for juice, most is grown for the fresh fruit trade, with over 28,000 hectares maintained by about 1,900 growers. Citrus is the largest fresh fruit export business in Australia and is worth more than $200 million a year.

It’s a little known fact that Australia has more wild species of citrus than any other country. There are six, including bush tucker finger limes, and none of them are found anywhere else.

There are some 25 wild citrus species growing naturally around the world, but the bulk of the global industry is based on hybrid plants.

We now know that oranges are hybrids derived from crosses between two wild species: the sweet, thin-skinned mandarin and the sour, thick-skinned pomelo. It would seem that thousands of years ago, the Chinese brought the two species together and bees did the rest.

Lemons are crosses between this pomelo-mandarin hybrid complex and the citron. Culinary limes with many seeds appear to be crosses between the citron and yet another species. The seedless or Tahiti lime in your gin and tonic has probably got lemon in its ancestry too, which means it has four different species in its makeup: pomelo, mandarin, citron and kaffir lime.

The clonal nature of these crops is important because it means much of the industry is reliant on a very narrow genetic base. A narrow genetic base lays open crops—and the industry and economy based on them—to serious danger. The Irish Potato Famine is a classic example.

Today, there’s a very real threat to the citrus industry: citrus greening disease. It’s one of the most severe plant diseases in the world. There is no known cure, and the disease has already wiped out millions of trees in the US.

No citrus, hybrid or wild—not even our own finger lime—is resistant to this bacterial disease. It’s spread by tiny insects, psyllids, which feed on citrus phloem. Grafting with infected scions is also a major source of infection in Asia. Plants can look healthy for months or years before the disease shows, during which time they can infect other plants.

The fact that there is no resistance at all in any species of citrus suggests the disease has jumped from another kind of plant.

This severe form of the disease in Asia has prevented the establishment of a viable modern citrus industry there. Between 1960 and 1970, three million trees were destroyed in Indonesia alone and by 1983 commercial groves in most parts of Java and Sumatra were abandoned.

By 1980 up to 95 per cent of the trees in the northern and eastern provinces of Thailand were affected. It was first recorded in Papua New Guinea in 2002 and in 2004 it was found in Brazil and is now rampant in South and Central America.

The Asian citrus psyllid was first found in the United States in 1998; in 2005 a pomelo near Miami was found to be infected with ‘asiaticus’. The psyllid and disease soon reached the other southern states. Today it is found throughout the citrus-growing parts of Florida, where the disease is now a serious threat to the $9 billion industry.

In Australia, a country with more wild species of citrus than any other as well as a major commercial industry, we should be alarmed. What can be done?

Pesticides don’t really help. In Asia their use has led to trees surviving for up to 15 years in well-managed orchards, but even with 35-52 sprays a year, often with up to four active ingredients, pesticides have failed to prevent spread of the disease.

Worse, intensive insecticide application schedules lead to the selection of insecticide-resistant psyllids. Some studies, however, indicate that white oils used to control other pests and diseases may be as effective as conventional insecticides without the risk of evolution of resistance.

Biological control of the Asian citrus psyllid is also possible, but introduced parasitoids have been unable to prevent the spread of the disease. In desperation, all kinds of other ideas have been put forward, including the application of various nutrients, the use of antibiotics and water-saturated hot air treatments. None have proven truly effective. Nowhere in the world is there a successful management regime.

In the United States the situation is dire, but only recently has the industry come out and admitted it has a major problem. Florida produces 63 per cent of citrus fruits grown in the country and the industry supports some 76,000 jobs, but yields are falling year on year.

Since 2005 about 200,000 acres have been lost, most significantly to diseases, principally citrus greening disease. The local economy is being severely affected. The 2014 Farm Bill allotted the first $25 million of $125 million for research over the next five years. In the meantime, more and more citrus has to be imported from other countries, especially Brazil.

The simple fact is that orange juice—and indeed the whole citrus industry worldwide—is in peril.

The disease is closing in on Australia. Quarantine is crucial to prevent the entry of the bacteria and its insect hosts and to ensure the continued wellbeing of our crops and growers, as well as our native citrus biodiversity.

Ockham’s Razor is a soap box for all things scientific, with short talks about research, industry and policy from people with something thoughtful to say about science. This is an edited transcript of Professor David Mabberley's comments on Ockham's Razor.

