A Comprehensive Study of Witches' Broom Disease of Lime in Southern Iran

Witches' broom disease of lime (WBDL) caused by Candidatus Phytoplasma aurantifolia is the most devastating disease of Mexican lime in Iranian southern provinces. In order to study the mode of spread of Ca. Phytoplasma aurantifolia in infected Mexican lime, 10 healthy lime seedlings were graft inoculated with WBDL phytoplasma under greenhouse condition. Samplings were carried out monthly from the various parts of the inoculated seedlings including roots, crown bark, apical leaves, leaves above and below the grafting area and checked by PCR for detection of phytoplasma. Two months after inoculation, the phytoplasma was detected in root of 7 out of 10 seedlings. Phytoplasma was detected by nested PCR, in roots of all 10 seedlings, in crown bark of 5 seedlings and in young apical leaves of only one of the seedlings three months after inoculation. Six months after graft inoculation, WBDL phytoplasmal DNA was detected by nested PCR in crown barks and apical leaves of all seedlings. Six months after graft inoculation, the first typical symptom of the disease was observed on only one of the seedlings while all seedlings showed the symptom of WBDL after 9 months. Based on these results, Ca. Phytoplasma aurantifolia spread sequentially from inoculated point to main stem , root , young leaves and then to other parts of the lime seedling. This model of phytoplasma movement and distribution was verified by other expereiments : taking samples from different parts of inoculated lime seedlings and grafting on healthy lime seedlings as well as PCR detection of phytoplasma in naturally infected lime trees. Pruning experiments were conducted to determine if removal of symptomatic branches or the entire canopy would eliminate WBDL phytoplasma infection. Disease symptoms reappeared on pruned trees and ineffectiveness of pruning to control WBDL was proved. In an experiment, Valencia sweet orange trees on Mexican lime rootstock were used to study passage and possible propagation of witches΄ broom disease of lime (WBDL) phytoplasma in different organs. Results of this experiment showed nonpropagative passage of witches΄ broom disease of lime phytoplasma through sweet orange phloem.