A study was conducted to investigate phenotypic and genotypic changes within Pythium aphanidermatum populations during the period 2006 to 2011. In total, 92 isolates of P. aphanidermatum (59 in 2006 and 33 in 2011) were obtained from different planting sites (soil) of cucumber from a single greenhouse. Generated sequences of the internal transcribed spacer (ITS) ribosomal DNA showed that all, except one isolate, share an identical sequence of the ITS region. Most (89%) P. aphanidermatum isolates were found to be aggressive on cucumber seedlings, with no significant differences in the aggressiveness level between populations obtained from different planting rows or different years. Sensitivity to metalaxyl among populations of P. aphanidermatum increased significantly from concentration resulting in 50% growth inhibition levels of 0.070 to 1.823 (average 0.824 μg ml–1) in 2006 to 0.002 to 0.564 (average 0.160 μg ml–1) in 2011. Amplified fragment length polymorphism analysis of the 92 isolates produced 92 different genotypes and 985 polymorphic loci. P. aphanidermatum populations from 2006 and 2011 were found to have low levels of genetic diversity (H = 0.1425), which implies introduction of the isolates into the greenhouse via common sources. Results from analysis of molecular variance (FST = 0.0307 in 2006 and 0.0222 in 2011) provided evidence for frequent exchange of Pythium inoculum between different planting locations within the same year. However, the analysis showed moderate levels (FST = 0.1731) of genetic differentiation among populations from the 2 years. This was supported by unweighted pair group method with arithmetic means analysis, which showed clustering of many of the 2006 isolates in separate clusters. The change in the metalaxyl sensitivity of the populations from 2006 to 2011 accompanied by the genetic differences among these two populations may suggest that many of the isolates from 2006 were lost and were replaced by new and highly sensitive P. aphanidermatum isolates by 2011.

Al-Sadi, A. . 2013. Population genetic analysis reveals diversity in Lasiodiplodia species infecting date palm. Citrus, and mango in Oman and the UAE. Plant Dis 97:1363-1369.Lasiodiplodia is a common pathogen causing dieback, gummosis, or root necrosis on the three most important fruit crops in Oman and the United Arab Emirates (UAE): date palm (Phoenix dactylifera). Citrus spp., and mango (Mangifera indica). A study was conducted to examine diversity in 64 Lasiodiplodia isolates infecting date palm (24), Citrus (11), and mango (29) in Oman and the UAE. Identification based on sequences of the internal transcribed spacer (ITS) rDNA and EEla gene showed that date palm isolates belonged to L. hormozganensis (75% of isolates) and L. theobrotnae (25%); Citrus isolates belonged to L. hormozganensis (45%), L. theobromae (45%), and L. iraniensis (10%); and mango isolates belonged to L. theobromae (59%), L. iraniensis (34%), and L. hormozganensis (7%). Amplified fragment length polymorphism (AELP) fingerprinting of the 64 isolates using four primer pair combinations produced 64 genotypes and 972 polymorphic alíeles. Cluster analysis separated the isolates into four clusters representing the three species. A higher level of genetic diversity was observed in L. iraniensis (0.3105) compared to L. hortnozganensis (0.2503) and L. theobromae (0.2331) in Oman. Analysis of molecular variance (AMOVA) indicated the existence of low levels of genetic differentiation among date palm populations of L. hormozganensis obtained from Oman and the UAE (EST = 0.025) and among populations of L. hormozganensis (0.0485) and L. theobromae (0.0703) from date palm. Citrus, and mango. These findings imply a high rate of movement of L. hormozganensis and L. theobromae isolates among date palm. Citrus, and mango and between the two countries. Eindings from the pathogenicity test supported the AMOVA analysis and suggested a lack of host specialization in L. hormozganensis, L. iraniensis, and L. theobromae on date palm, acid lime, and mango. Although this is the first record of L. hormozganensis and L. iraniensis in Oman, the relatively moderate level of genetic diversity in the two species compared to L. theobromae suggests that the two species have been in Oman for a long time but misidentified by morphology and ITS rDNA sequences as L. theobrotnae. This study is also the first record of date palm and acid lime as natural hosts for L. hormozganensis and the first record of L. hormozganensis in the UAE. The diversity in Lasiodipiodia species affecting date palm. Citrus, and mango in Oman and the UAE should be taken into consideration when planning future management programs for diseases caused by these pathogens.

Witches’ broom disease (WBD), caused by ‘Candidatus Phytoplasma aurantifolia’, is a serious disease of acid lime (Citrus aurantifolia) in Oman and the UAE. However, little is known about the distribution of phytoplasma and the expression of WBD symptoms in different geographical locations. A survey was carried out in 18 districts in Oman and the UAE covering 143 orchards and 5823 acid lime trees. ‘Candidatus Phytoplasma aurantifolia’ was detected in acid lime in all the 18 surveyed districts. However, the development of typical symptoms of WBD was only observed in 12 districts. Districts in which the phytoplasma was present but symptoms were not expressed were located either in desert areas or in areas characterized by semitropical conditions. Phylogenetic analysis of 16 phytoplasma isolates from trees developing WBD symptoms and six phytoplasma isolates from trees with no WBD symptoms showed that all isolates share an identical 16S rRNA sequence, belonging to subgroup II‐B. Quantitative PCR analysis showed that the concentration of phytoplasma is 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. The lack of expression of WBD symptoms under certain environmental conditions may suggest that symptom development and phytoplasma are affected by certain unfavourable environmental conditions. These findings could provide a basis for managing WBD through encouraging lime cultivation under climatic conditions less conducive to WBD symptom expression.

Witches’ broom disease of lime (WBDL), caused by ‘Candidatus Phytoplasma aurantifolia’, is a very serious disease of acid limes. The disease destroyed more than one million lime trees in the Middle East. WBDL results in the production of small, clustered leaves in some branches of lime trees. Branches develop symptoms with time and become unproductive, until the whole tree collapses within 4–8 years of first symptom appearance. This study was conducted to investigate differences in minerals between symptomatic and asymptomatic leaves of infected lime trees. The study included one set of leaves from uninfected trees and two sets of infected leaves: symptomatic leaves and asymptomatic leaves obtained from randomly selected acid lime trees. Nested polymerase chain reaction detected phytoplasma in the symptomatic and asymptomatic leaves from the six infected trees, but not from the uninfected trees. Phylogenetic analysis showed that all phytoplasmas belong to the 16S rRNA group II-B. Mineral analysis revealed that the level of Na significantly increased by four times in the symptomatic 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 development of witches’ broom disease symptoms was found to be associated with changes in some minerals. The study discusses factors and consequences of changes in the mineral content of acid limes infected by phytoplasma.

“Candidatus Phytoplasma aurantifolia” is associated with witches’ broom disease of lime in Oman and the UAE. A previous study showed that an infection by phytoplasma may not necessarily result in the physical appearance of witches’ broom symptoms in some locations in Oman and the UAE. This study investigated whether phytoplasma strains belonging to “Ca. P. aurantifolia” (based on the 16S rRNA gene analysis) in locations where disease symptoms are expressed are different from phytoplasma in locations where disease symptoms are not expressed. About 21 phytoplasma strains (15 from areas and trees with disease symptoms and six from areas and trees without disease symptoms) were included in the analysis. The study utilized sequences of the imp and SAP11 genes to characterize the 21 strains. Phylogenetic analysis of both genes showed that the 21 strains are similar to each other and to reference strains in GenBank. The study shows that there is a low level of diversity among all phytoplasma strains. In addition, it shows that phytoplasma in places where witches’ broom symptoms are not expressed are similar to phytoplasma in places where disease symptoms are expressed. This may suggest that disease expression is not linked to the presence of different phytoplasma strains, but may be due to other factors such as weather conditions.