Biotic and abiotic factors associated with citrus progressive decline in Fars Province,Iran

Citrus decline has become a devastating problem in citrus-growing regions of southern Iran. The affected trees show progressive yellowing of leaves, thinning of the canopy, dieback and total collapse. Despite a number of attempts, the aetiology of the decline has remained uncertain. In the present study, we evaluated the potential association of various biotic and abiotic factors with the decline in the Fars Province of Iran. Biotic agents surveyed included Candidatus Liberibacter asiaticus, Spiroplasma citri, phytoplasmas, Tylenchulus semipenetrans and root infecting fungi and oomycetes. Abiotic factors studied were soil and water salinity, changes in prevailing temperature and other environmental conditions. In our surveys, Ca. L. asiaticus, either alone or in combination with other factors, had the highest frequency of association (92%) with the decline, followed by S. citri (75%). Ca. L. asiaticus was not detected in any of the non-decline trees examined. Pythium or Phytophthora species and a fungus of theFusarium solani species complex were also isolated from roots of many declining trees. Phytoplasmas were found only in few cases, and populations of citrus nematode were often below the threshold of economic loss. Soil/water salinity were in the suitable range for citrus cultivation in most cases. It is suggested that the decline is initiated by Ca. L. asiaticus infection, which is known to weaken the root system and make it vulnerable to infection by opportunistic soil fungi and oomycetes. S. citri, summer temperatures, low air humidity and overbearing of the trees seem to be other potential factors contributing to the intensity of the disease.