Are Citrus Going the Way of the Dodo?

World citrus production is under serious threat by a disease, citrus greening, also known as huánglóngbìng or HLB (the Chinese name means yellow dragon disease). It’s caused by one of three closely-related bacteria, Candidatus Liberibacter asiaticus, C. L. africanus and C. L. americanus. If not controlled, citrus greening  may well wipe out citrus production on a planetary scale.

It affects not only orange trees, but all citrus, including lemons, grapefruits and clementines, just to mention the best known. In fact, other plants of the citrus family, the Rutaceae, can also be infected and may even serve as a secondary host for the disease.

Bacteria Need a Vector

Bacteria can’t spread from plant to plant on their own, however: they need a vector. In the case of citrus greening, it’s a small jumping insect called the citrus psyllid. There are two species: the Asian citrus psyllid (Diaphorina citri) is prevalent in Asia, America and Oceania while the African citrus psyllid (Trioza erytreae) is most current in Africa.

A secondary source of infestation comes from grafting a healthy citrus onto an infected one.

The bacteria is introduced into the plant when the psyllid pierces its young leaves and shoots. It then multiples and is carried throughout the tree via its vascular system, right down to its roots. Infested trees begin to produce branches with chlorotic leaves (yellow with green veins), a symptom that could easily be mistaken for a deficiency in iron or some other mineral. The tree’s flowering also becomes sporadic and more flowers than usual abort.

Any fruits produced are small or deformed and don’t ripen equally, often leaving a green section, hence the name citrus greening disease. To top things off, the fruits have a bitter taste. That means that not only are fruits unsaleable because of their appearance, but they can’t even be used to make juice!

History of a Disaster

Detected for the first time in China in 1943, citrus greening disease has since spread all over the world. It was discovered in South Africa in 1947, in Brazil in 2004, in Florida in 2005, in Mexico in 2009 and so on. It is now found all the continents except Antarctica. The first citrus greening infested tree was found in California in 2012. Citrus greening is now prevalent in every part of the world where citrus grow commercially except mainland Japan.

There is currently no treatment for a tree infested with citrus greening other than to destroy it (generally it is pulled out by heavy machinery, as the roots also have to be extracted, then the whole tree is burned).

Florida orange groves, once carpeted with symmetrical green plantings as far as the eye could see, now look like a patchwork mix of dead and dying trees, still green trees, and bare patches. More than 65,640 hectares of citrus production have been infected in Florida alone and orange production has fallen by almost two-thirds, from 242 million boxes in 2007 to an expected 89 million boxes in 2016 (the info count is not yet in). 75,000 Floridian citrus workers are out of a job. The price of oranges has skyrocketed, as has that of lemons, and grapefruits… and the price is expected to continue to climb over the years as more or more citrus trees are bulldozed and burned.

Current Treatments

Since nothing can presently be done once a citrus tree is infected other than destroy it, treatment so far has mostly been limited to prevention and that means spraying insecticides to kill the psyllids that transmit the disease before they reach disease-free trees. As a result, the number of annual insecticide treatments needed to produce citrus fruit has increased from 3 to 8 in Floridian orange groves. Despite these treatments, the disease continues to spread (it takes only one psyllid that escapes treatment to start a new infestation), thus citrus production continues to plummet.

Of great concern are home gardeners with a citrus tree or two in their yard, a common-enough situation in most citrus-growing areas. They are less likely to notice the first symptoms of the disease and also less likely to want to destroy a tree that still seems fairly healthy. Home gardens thus become a reservoir for both psyllids and citrus greening disease.

Ongoing Experiments

Obviously, there is a lot of money is being invested in finding a cure for citrus greening.

A parasitic wasp, Tamarixa radiata, originally from Pakistan, has been found capable of controlling disease-carrying psyllids. In California, where the disease is not yet well established, more than one million of these wasps are released annually in an effort to nip infestation in the bud. Other predatory or parasitic insects are also being tested.

Researchers are also actively looking for any specimens of citrus that are naturally resistant to citrus greening disease. For years, none had been seen anywhere in the world, but the University of Florida’s Citrus Research and Education Center announced in April 2016 that researcher Jude Grosser had found, among the thousands of conventional crosses he made, a single orange tree that appears citrus greening-resistant. Although infected with the bacteria for 5 years now, it has remained green and healthy and continues to produce abundantly. Seedlings grown from this “mother tree” have also been found to be resistant to the disease.

Other scientists are working on ways to effectively inject antibiotics (including penicillin) into affected trees, since citrus greening is a bacterial disease and bacteria are sensitive to antibiotics. One method being tested uses lasers to puncture mini-holes in the leaves through which the product can be injected.

It’s already possible to genetically modify citrus trees by inserting into them a gene from another plant (to date, spinach and arabidopsis genes have been found effective), a treatment that provides strong resistance to the disease. Researchers are however concerned the public will not accept this kind of genetic manipulation, since the fear of GMOs (genetically modified organisms) is widespread, notably in Europe.

Another possibility would be to “switch off” the gene that causes the fatal reaction of citrus trees to the presence of citrus greening bacteria, thus giving a tree that would grow healthily despite the presence of the disease. This is done by genetic engineering and the concern is therefore that the population will consider such a plant a GMO and therefore unacceptable even though no gene transfer had taken place.

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In conclusion, the citrus industry is in serious trouble and consumers are already being affected. You simply will not be able to find citrus fruits as readily in a few years, or if so, only at exorbitant prices, because even if an “effective solution” is found, it will take 20 years or more to implement.

Make sure you enjoy your next glass of orange juice: it may be your last!