الملخص الإنجليزي
Witches' broom disease of lime (WBDL), caused by 'Candidatus Phytoplasma aurantifolia', is a very serious disease of acid limes. This study investigated the distribution of phytoplasma and the expression of WBD symptoms in different geographical locations in Oman and UAE. Also, it investigated whether phytoplasma in locations where disease symptoms are expressed, are different from phytoplasma
in locations where disease symptoms are not expressed. Beside that the mineral contents in symptomatic and asymptomatic lime leaves were studied to see if there is any relationship between WBD development and mineral imbalance in limes. The survey showed that WBD symptoms were apparent in most locations (subtropical), except in desert (dry) and in the monsoon areas (semitropical). The symptoms were characterized by the mass production of branches and small light green to yellow leaves. However, 'Ca. Phytoplasma aurantifolia' was detected in acid lime in all the surveyed districts, including symptomless trees in desert and semitropical areas. Quantitative PCR analysis showed that the concentration of phytoplasma was significantly higher (8800-801 000 copies) in leaves developing WBD symptoms compared to 2–268 copies in symptomless leaves from the same trees and 8–874 copies in acid lime trees from areas where disease symptoms were not expressed.
Phylogenetic analysis of 15 phytoplasma isolates from treesdeveloping WBD symptoms and six phytoplasma isolates from trees with no WBD symptoms showed that all isolates shared an identical 16S rRNA sequence, belonging to subgroup II-B. Phylogenetic analysis of both genes imp and SAPII showed that the 21 strains were similar to each other and to reference strains in GenBank. The study showed that there was a low level of diversity among all phytoplasma strains. In addition, it showed that phytoplasma in places where witches' broom symptoms were not expressed were similar to phytoplasma in places where disease symptoms were expressed The study also compared the level of minerals in symptomatic acid lime leaves in comparison to asymptomatic leaves from the same trees and also to healthy trees. Mineral analysis revealed that the level of Na was significantly higher by four times in the symptomatic acid lime leaves compared to the non-symptomatic leaves and to the uninfected leaves. In addition, symptom development resulted in a significant increase in the levels of P and K by 1.6 and 1.5 times, respectively, and a significant decrease in the levels of Ca and B by 1.2 and 1.8 times, respectively. There was no significant effect of WBDL on the levels of N, Cu, Zn, and Fe. The study showed that 'Ca. Phytoplasma aurantifolia' can result in WBD symptoms in most regions in Oman, but symptom expression may be affected under certain environmental conditions. Thus, future studies should investigate the potential of moving lime production to some areas which are less conducive for the disease. In addition, WBD development in lime may be associated with mineral imbalance. Managing minerals in limes should be investigated as a potential way for managing the disease
