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.

中文翻译：

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伊朗法尔斯省与柑橘进行性下降相关的生物和非生物因素

在伊朗南部柑桔种植区，柑桔的减少已成为一个毁灭性的问题。受影响的树木显示出叶片逐渐发黄，冠层变薄，枯萎和完全倒塌。尽管进行了许多尝试，但下降的病因仍然不确定。在本研究中，我们评估了各种生物和非生物因素与伊朗法尔斯省衰退的潜在关联。生物剂包括受访暂定Liberibacter胭脂，螺原体螨，植原体，半穿刺线虫和根感染真菌和卵菌。研究的非生物因素包括土壤和水的盐度，盛行温度的变化以及其他环境条件。在我们的调查中，钙。单独或与其他因素结合使用的亚洲刺李具有最高的关联频率（92％）与下降关联，其次是柠檬链球菌（75％）。钙。在所有未衰落的树木中均未检出积雪草。腐霉属或疫霉属物种以及茄形镰刀菌（Fusarium solani）种复合物的真菌也从许多下降的树木的根中分离出来。仅在少数情况下发现了植原体，柑桔线虫的种群通常低于经济损失的阈值。在大多数情况下，土壤/水盐度都适合柑橘种植。建议下降是由Ca引起的。L. asiaticus感染，已知会削弱根系，使其容易受到机会性土壤真菌和卵菌的感染。柠檬葡萄球菌，夏季温度，低空气湿度和树木的霸道似乎是造成这种疾病强度的其他潜在因素。