Citrus Huanglongbing: Review, Present status and Future Strategies

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Abstract

Citrus huanglongbing (HLB), formerly known as greening, is a highly destructive disease of citrus, especially on sweet orange and mandarin varieties. A range of primary and secondary leaf symptoms are associated with HLB, making field diagnosis difficult, unless the typical lop-sided greened fruit are present. To date, the causal organism has not been cultured on artificial media and a diagnostic polymerase chain reaction technique has been developed to confirm the presence of the pathogen. The disease is caused by two closely related phloem-limited bacterial species, “Candidatus Liberibacter asiaticus” and “Candidatus L. africanus”. The former, which causes the more severe Asian form, is found in Asia from southern Japan in the east through southeast and south Asia to eastern Iran in the west, as well as Saudi Arabia, Mauritius and Reunion. It is transmitted by the psylla, Diaphorina citri. The milder, yet still serious, African form is less heat tolerant, and is transmitted by another psylla species, Trioza erytreae. This form is found in Yemen, throughout eastern and southern Africa, as well as in Mauritius and Reunion. Of concern is the fact that both psylla species are capable of transmitting both bacterial species under experimental conditions. In Reunion, propagating healthy trees and releasing hyperparasites for vector control achieved almost complete control. Elsewhere, HLB is best controlled through integrated disease management involving the use of healthy nursery material, removal of infected trees or branches, and integrated vector control. The recent arrival of the vectors in citrus producing areas previously regarded as HLB-free highlights the potential threat of one of the most serious diseases of citrus, thus emphasizing the need for effective quarantine services.