HUANGLONGBING: A DESTRUCTIVE NEWLYEMERGING CENTURYOLD DISEASE OF CITRUS
A detailed account is given of the history, aetiology, biology, epidemiology, detection, geographical distribution, and control of huanglongbing (HLB), a destructive disease of citrus that represents a major threat to the world citrus industry, and is slowly invading new citrusgrowing areas. HLB, whose name in Chinese means “yellow dragon disease”, was first reported from southern China in 1919 and is now known to occur in next to 40 different Asian, African, Oceanian, South and North American countries. The agent is a phloem-restricted, non cultured, Gram-negative bacterium causing crippling diseases denoted “greening “ in South Africa, “mottle leaf” in the Philippines, “dieback” in India, “vein phloem degeneration” in Indonesia. The HLB bacterium belongs to the genus Candidatus Liberibacter, three species of which are currently known, Candidatus Liberibacter asiaticus, occurring in Asian countries and, to a lesser extent, in Brazil and the USA (Florida), Candidatus Liberibacter africanus with its subspecies “capensis”, recorded from African countries, and Candidatus Liberibacter americanus present in Brazil. The suggestion is that each liberibacter species has evolved in the continent after which it is named. HLB symptoms are virtually the same wherever the disease occurs. Infected trees show a blotchy mottle condition of the leaves that results in the development of yellow shoots, the early and very characteristic symptom of the disease. Trees are stunted, declining and bear a few, small-sized, and deformed (lop-sided) fruits, that are poorly coloured (greening) and with coloration starting at the peduncular end (colour inversion). HLB can be transmitted by grafting from citrus to citrus and by dodder to periwinkle. The psyllids Trioza erytreae and Diaphorina citri are natural vectors. Two different types of HLB are known: the heat-sensitive African form transmitted by T. erytreae, which develops at temperatures of 22-25°C, and the heat-tolerant Asian form, transmitted by D. citri, which stands temperatures well above 30°C. Although the HLB pathogen can be identified by electron microscopy, other laboratory methods are used for routine detection. ELISA with monoclonal antibodies is not recommended. Better systems are dot blot hybridization with a DNA probe, and various PCR formats (one-step, nested, multiplex) using species-specific primers based on 16S rRNA or rplKAJL-rpoBC operon sequences. Because no curative methods of HLB are available, control is preventive and largely based on inoculum elimination by removal of infected trees and chemical treatments against vectors. Strict quarantine measures must be implemented to impair further international spread of HLB agents and their vectors.