If agricultural authorities around the world do not get their measures under control quickly enough, your morning orange juice could disappear from supermarket shelves forever.

That’s how critical the situation has become in citrus cultivation. Over the past few decades, a disease called Huanglongbing (HLB), also known as “citrus greening” disease, has devastated orchards in Asia, the Americas and several African countries. Although global production has increased in recent years thanks to strong market demand and new plantings, the unstoppable spread of the disease has producers fearing the worst.

The economic consequences are dramatic in some countries. In Brazil, production has fallen by more than 20 percent, in Guadeloupe by 60 percent and in Florida by more than 90 percent.

In Florida, a well-known orange producer, thousands of jobs in the citrus sector have been lost. Drastic quarantine and management measures have been introduced everywhere. The price of orange juice has doubled in a year and producers are now struggling to get hold of fruit.

The disease is caused by a bacterium of the genus Candidatus Liberibacterwhich attacks the sap-carrying vessels, the phloem. The trees react to the bacterium by producing a sugar, which is known as Calluseswhich accumulates in the vessels and constricts them, as in a thrombosis. The photosynthesis products SEE The substances synthesized in the leaves are then no longer distributed throughout the tree. After a long symptom-free phase, internal damage appears in the form of yellow spots on the leaves. The fruits become deformed, discolored unusually and become more bitter, so that they can no longer be sold. Twigs and then branches die. These symptoms begin on one branch and then spread to the entire tree. After a few years, clogged vessels inevitably lead to the death of the trees.

A bacterium transmitted by insects

The bacterium is transmitted by two insects from the psyllid family. These small, winged insects, measuring between 2 and 5 millimeters in size, feed like aphids by inserting their long, thin tubes, called rostrum, into plants to suck the sap. When they feed on very young citrus shoots infected with HLB disease, they can pick up the bacterium, which then grows inside them and is transmitted to other trees, just as mosquitoes transmit the dengue or chikungunya virus. These two species of psyllid are quite specific to citrus fruits and related species such as the jasmine orange (Murraya paniculata), often used as an ornamental plant.

Plant fleas are extremely effective vectors of the bacteria. A single bite can be enough to transmit the disease. The Asian plant fleas (Diaphorina citri) of tropical and subtropical origin has been observed in Asia, South America, the Caribbean, Florida and California. The African psyllid (Trioza erythreae) is better adapted to the cool conditions of certain African plateaus.

Overall, billions of dollars have been invested over the past two decades to combat the disease worldwide, but no lasting solution has been found.

In the Brazilian state of São Paulo, where the disease has been present since 2004, the rate of tree infection has been limited to about 20% over the last two decades by uprooting infected trees and using pesticides (up to 46 treatments per year) to control psyllids. However, in recent months, the rate of infection in medium-sized farms has increased to almost 38%, indicating the development of pesticide resistance in psyllids and a deterioration in the control of the spread of the disease. This situation is leading investors to plant in areas where psyllids are absent or present at a low level, such as the state of Minas Gerais.

Very active research

In the Mediterranean, the control of psyllids is a top priority, particularly through biological control. Portugal and Spain are currently successfully releasing mass parasitoids, small insects that lay their eggs in psyllids and whose larvae then develop at their expense and eat the animals from the inside out. This policy is also to be followed in Cyprus.

Scientists also want to combat the bacteria through biomonitoring throughout the Mediterranean region. For this purpose, samples are taken regularly. If the results are positive, infected trees should be cut down immediately in order to eliminate the sources of contamination.

Finally, it is crucial to inform not only the citrus growing sector but also the general public about the risks associated with the transport of plants, since the import and use of scions from infected areas to non-infected areas is the main cause of new disease outbreaks. The EU has already recognised the problem by classifying HLB bacteria and their psyllid vectors as priority quarantine organisms and requiring Member States to establish appropriate surveillance and contingency plans. In France, the authorities responsible for surveillance and the research community have set up a dedicated unit within the National Platform for Epidemiological Surveillance in Plant Protection, as well as several action-based research projects.

All cultivated citrus fruits are susceptible to HLB and the study of resistant varieties and rootstocks is now an important area of ​​research with a view to developing sustainable production systems. Recent research suggests that certain genotypes of cultivated citrus fruits from the Aurantioideae family, such as the caviar lemon, may be more resistant. For this reason, the French Centre for Agricultural Research for International Development (CIRAD) and its partners are studying this type of variety to understand the biological characteristics that determine the interaction between the bacterium and citrus plants.