Papers by Mekki Ben Khalifa
Phytopathologia Mediterranea, 2018
Sunflower broomrape ( Orobanche cumana Wallr.) was detected in 2010 for the first time in sunflow... more Sunflower broomrape ( Orobanche cumana Wallr.) was detected in 2010 for the first time in sunflower fields of Tunisia, in the Beja region. No information is available on the race composition and genetic diversity of the broomrape populations parasitizing sunflower in that area. Plant tissue and seeds were collected from nine populations in Beja Sud (ORD, ORE), Amdoun (ORF, ORG), and Beja Nord (ORH to ORL) areas of the Beja region. Virulence studies on populations ORD, ORH and ORK revealed that the ORD population was essentially race E, whereas race G individuals were present in ORH and ORK poulations, with greater frequency in the ORK population. Cluster analysis of inter-population relatedness indicated that Tunisian populations were more related to Eastern European than to Spanish populations, with the Beja Nord populations clustering separately from the rest of Tunisian populations. Analysis at the intra-population level of the ORD, ORG, ORH and ORK populations revealed the exist...
Canadian Journal of Plant Pathology, 2011
‘Candidatus Phytoplasma mali’ was detected, for the first time, in plum (Prunus domestica L.) tre... more ‘Candidatus Phytoplasma mali’ was detected, for the first time, in plum (Prunus domestica L.) trees, with symptoms of plum decline in Tunisia. Infected trees showed leaf browning, leaf and fruit wilting followed by tree death in a few weeks. Phytoplasma was detected by polymerase chain reaction (PCR) using universal phytoplasma primer pairs P1/P7. A band with expected size was observed in samples collected from symptomatic, but not symptomless, plum trees. PCR using group specific primers yielded a band of 1050 bp with 16SrX group specific primers fO1/rO1. Universal PCR products (1.8 kb ) were used for restriction fragment length polymorphism analysis (RFLP) after digestion with endonuclease Ssp I. RFLP patterns obtained were similar to those previously reported for the Apple Proliferation phytoplasma (AP, 16SrX-A), the causal agent of apple proliferation disease. Blast analysis revealed that universal PCR sequences obtained from the infected Tunisian plums were identical and had the highest identity (99%) with the sequence of Ca. Phytoplasma mali. Phylogenetic analysis based on 16S rDNA gene sequences showed that the plum decline phytoplasma strain from Tunisia clustered with ‘Candidatus Phytoplasma mali’ strains. This is the first report on detection of 16S rDNA of ‘Ca. Phytoplasma mali’ in plum decline in Tunisia.
JOURNAL OF PLANT PATHOLOGY
SUMMARY Samples from apparently healthy European pears (Pyrus communis) from Tunisian orchards an... more SUMMARY Samples from apparently healthy European pears (Pyrus communis) from Tunisian orchards and from trees showing symptoms recalling those of the "slow de- cline" form of pear decline, i.e. reduced growth, pro- duction of fewer and smaller leaves that turn reddish and drop early in autumn, were analyzed by polymerase chain reaction (PCR) using the universal phytoplasma primers P1/P7. A
Plant Pathology, 2009
ABSTRACT The genetic structure of Potato virus Y (PVY) populations was investigated in naturally-... more ABSTRACT The genetic structure of Potato virus Y (PVY) populations was investigated in naturally-infected pepper (Capsicum annuum) fields, collected at eight different localities in northern Tunisia, where 23% of the sampled plants were homozygous for the pvr21 recessive resistance allele, while the other plants carried the dominant susceptibility allele pvr2+. Restriction fragment length polymorphism analysis at three PVY genome segments revealed a high level of viral diversity, with a majority of cases showing co-infection of individual plants by several PVY haplotypes and a strong genetic differentiation of viral populations collected in the different localities. Geographic distances affected the differentiation of PVY populations and isolation by distance among these populations was significant. However, the occurrence of the pvr21 resistance allele did not contribute to the structure of viral populations, suggesting that the virulence properties of the virus did not significantly affect its fitness. Consequently, greater deployment of the pvr21 gene would probably not be a suitable strategy to control PVY, and other resistance genes should be preferred.
Plant Pathology, 2012
The pathogenic properties of Potato virus Y (PVY; genus Potyvirus, family Potyviridae) isolates c... more The pathogenic properties of Potato virus Y (PVY; genus Potyvirus, family Potyviridae) isolates collected in naturally infected pepper (Capsicum annuum) fields in Tunisia were evaluated against recessive resistance alleles at the pvr2 locus of pepper. Two pathotypes were observed. Pathotype (0,1,3) isolates were able to infect plants carrying the susceptibility allele pvr2 + , together with pvr2 1 ⁄ pvr2 1 and pvr2 3 ⁄ pvr2 3 plants, but not pvr2 2 ⁄ pvr2 2 plants. Pathotype (0) isolates were only able to infect pvr2 + ⁄ pvr2 + plants. On the other hand, sequence data and phylogenetic analyses revealed three major groups of isolates, each characterized by particular amino acid residues in the central part of the VPg, the pathogenicity factor towards pvr2. Correspondence between pathogenic properties and phylogeny suggested a single evolutionary step for pathogenicity towards the pvr2 1 and pvr2 3 resistances, possibly under the selective pressure of pvr2 1. Indeed, 23% of the pepper plants in this area were shown to carry the pvr2 1 resistance, while pvr2 3 was not detected. The data suggested that pathogenicity towards pvr2 1 and pvr2 3 were not costly for PVY to infect susceptible pepper genotypes and supported the matching allele model for pepper-PVY interactions.
Phytoparasitica, 2011
Candidatus Phytoplasma prunorum was detected for the first time in almond (Prunus dulcis Mill.) c... more Candidatus Phytoplasma prunorum was detected for the first time in almond (Prunus dulcis Mill.) cv. ‘Abiod’ in Tunisia. Infected trees showed emergence of new growth during dormancy and leafed out before flowers opened in addition to early defoliation in summer. Phytoplasma was detected by nested polymerase chain reaction (PCR) using universal phytoplasma primer pairs P1/P7 and F2n/R2. A band with
Samples from apparently healthy European pears (Pyrus communis) from Tunisian orchards and from t... more Samples from apparently healthy European pears (Pyrus communis) from Tunisian orchards and from trees showing symptoms recalling those of the " slow decline " form of pear decline, i.e. reduced growth, production of fewer and smaller leaves that turn reddish and drop early in autumn, were analyzed by polymerase chain reaction (PCR) using the universal phytoplasma primers P1/P7. A band with the expected size (1.8 kbp) was obtained from samples collected from symptomatic trees only. PCR products were used for restriction fragment length polymorphism (RFLP) analysis after digestion with the endonucleases AluI, RsaI and SspI. RFLP patterns obtained were consistent with those reported for Candidatus Phytoplasma pyri, thus providing evidence of the occurrence in Tunisia of this hitherto un-recorded pathogen.
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Papers by Mekki Ben Khalifa