English abstract
Acid lime (Citrus aurantifolia Swingle) is the fourth largest fruit crop in terms of cultivated area and production in Oman. However, over half a million lime trees were lost in Oman over the past 35 years due to witches' broom disease of lime (WBDL) which is caused by Candidatus Phytoplasma aurantifolia. Surveys from different parts of Oman showed that WBDL is present largely in all districts where 95 out of 122 (78%) surveyed farms were found WBDL positive. AFLP analysis of 143 Phytoplasma-free acid lime samples from Oman, 2 from Brazil and one from Pakistan using 4 primer pair combinations produced 980 polymorphic loci (100%) and 146 AFLP genotypes. Despite the long history of acid lime cultivation in Oman, populations of lime from different districts showed low levels of genetic diversity
(0.0888-0.2284). AMOVA analysis indicated the existence of high level of genetic differentiation (Fst = 0.271) among populations of acid lime from Oman and Brazil, which suggests that both populations have evolved independently for a considerably long period of time. On the other hand, AMOVA analysis showed that only 11% of the genetic variation exists among populations from the 18 different districts in
Oman, which suggests a constant level of gene flow (Nm = 2.04) of acid lime across geographical regions in Oman. Findings from this study suggest that the low level of genetic diversity of acid lime in Oman together with the constant rate of gene flow among districts are two main factors which may have contributed to the rapid spread and high susceptibility of acid limes to WBDL in the country. Unless proper management strategies are undertaken, these factors may make acid lime in Oman more vulnerable to future disease outbreaks.
