If global agricultural authorities do not act soon, your morning orange juice may disappear from supermarket shelves. The citrus industry is in crisis due to Huanglongbing (HLB), also known as “citrus greening” disease, which has been devastating orchards in Asia, the Americas, and several African countries. Despite increased global citrus production in recent years due to high demand and new plantings, the relentless spread of HLB is causing widespread concern among growers.
The disease has had catastrophic economic impacts in several countries. Brazil, one of the world’s largest orange producers, has seen a 20% drop in production, while Florida’s production has plummeted by more than 90%, leading to the loss of thousands of jobs in the citrus sector. In Guadeloupe, production has fallen by 60%. The price of orange juice has doubled within a year, as manufacturers struggle to obtain sufficient fruit supplies.
HLB is caused by a bacterium of the genus Candidatus Liberibacter, which infects the phloem, the sap-conducting vessels of citrus trees. The trees produce a sugar called callose in response, which clogs the vessels, preventing the distribution of photosynthesis products. After a long asymptomatic period, the disease manifests as yellow mottling on the leaves, deformed and bitter fruit, and eventually, the death of the tree.
The bacterium is transmitted by two psyllid insects: the Asian psyllid (Diaphorina citri) and the African psyllid (Trioza erytreae). These small, winged insects feed on the sap of young citrus shoots, picking up and spreading the bacterium much like mosquitoes transmit viruses. The Asian psyllid, which is the more effective vector, has spread to Asia, South America, the Caribbean, Florida, and California. The African psyllid, better adapted to cooler climates, has been found in Spain and Portugal since 2014 and 2015, respectively. In 2021, the Asian psyllid was identified in Israel, and in the summer of 2023, it was spotted in Cyprus for the first time.
The disease’s impact has been particularly severe in Florida, where orange production has dropped by more than 60% over the past 20 years, resulting in losses exceeding $3 billion and nearly 50% of related jobs. Despite extensive research and billions of dollars invested globally, a lasting solution to HLB has yet to be found.
In Brazil’s São Paulo state, the disease has been present since 2004. Efforts to control it through the removal of infected trees and the extensive use of pesticides up to 46 treatments per year have kept the infection rate around 20% for two decades. However, recent months have seen infection rates rise to nearly 38% on medium-sized farms, suggesting that psyllids are developing resistance to pesticides.
In the Mediterranean basin, where the disease has not yet spread, the focus is on limiting psyllid populations through biological control. Portugal and Spain are successfully using mass releases of psyllid parasitoids small insects that lay their eggs in psyllids, which are then consumed from within by the parasitoid larvae. This approach is also planned for Cyprus.
Scientists are also monitoring the region through regular sampling, with any positive results leading to the immediate removal of infected trees to prevent contamination. Public awareness campaigns are crucial to prevent the spread of the disease through the movement of plants, particularly through the use of grafts from infected to non-infected areas.
The European Union has recognized the severity of the situation by categorizing HLB bacteria and their psyllid vectors as priority quarantine organisms. Member States are required to develop appropriate surveillance and contingency plans. In France, dedicated surveillance units and research projects are in place to combat the disease.
All cultivated citrus varieties are vulnerable to HLB, making research into resistant varieties and rootstocks a key area of focus. The French Agricultural Research Centre for International Development (CIRAD) is studying certain genotypes related to cultivated citrus, such as the caviar lemon, which may be more resistant. Through genetic and genomic studies, researchers aim to identify the genes associated with resistance mechanisms, ultimately leading to the development of HLB-resistant citrus varieties. This could reduce the need for heavy pesticide use and ensure the future of citrus production worldwide.
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