Huanglongbing remains a major constraint for sweet orange production in tropical and subtropical regions, with long-term consequences for orchard productivity and viability. The disease is associated with the bacterium 'Candidatus Liberibacter asiaticus' and is transmitted by the Asian citrus psyllid. Once infected, trees show leaf, root, and fruit symptoms that lead to yield loss, fruit drop, and eventual decline.
HLB was detected in São Paulo, Brazil, in 2004 and in Florida, the United States, in 2005. At that time, the São Paulo and Triângulo Mineiro citrus belt produced about 378 million boxes of sweet oranges, while Florida produced roughly 150 million boxes. By the 2023/24 season, production in São Paulo had declined to about 307 million boxes, while Florida's output had fallen to around 18 million boxes. Environmental stresses have contributed to these declines, but HLB remains a central factor. In Florida, the number of fruit-bearing trees has dropped sharply, while Brazil has continued planting new orchards alongside preventive strategies.
HLB affects all commercial citrus species, including mandarins, lemons, limes, and grapefruits. This broad host range complicates the development of resistant cultivars. Some Australian and New Guinea citrus relatives have shown limited bacterial multiplication, and breeding programs are exploring their use in hybrid development. However, producing commercially viable resistant varieties is a long-term process.
Therapeutic approaches for infected trees have shown limited effectiveness. The bacterium colonizes phloem tissues and spreads quickly from shoots to roots, making uniform treatment difficult. Trunk injections of antibiotics, pruning, and heat treatments have produced inconsistent and temporary results. As a result, HLB management relies primarily on preventive measures.
Control strategies focus on reducing bacterial acquisition and transmission by the psyllid. These include removing symptomatic trees and applying insecticides to disrupt the psyllid life cycle. Secondary infections within orchards can be reduced through coordinated tree removal and frequent vector control. Primary infections remain more difficult to manage due to long-distance psyllid dispersal, delayed symptom expression, and external inoculum sources.
Psyllids preferentially feed and lay eggs on young shoots, making early vegetative stages the most vulnerable period for infection. Uneven sprouting patterns, influenced by climate and tree health, extend the window of susceptibility in some regions. Systemic insecticides are most effective on young trees, while older orchards rely mainly on foliar applications with shorter residual effects.
Research has shown that most psyllid movement occurs within the first 200 metres from orchard borders. This has led to recommendations such as intensified monitoring and control at orchard edges, higher planting density in border areas, and coordinated area-wide management.
Given the limitations of orchard-level control, regional strategies that reduce vector pressure and inoculum sources beyond individual farms are increasingly emphasized. Coordinated action among commercial growers and non-commercial citrus owners is viewed as necessary to slow disease spread and delay epidemic development in new production areas.
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