| Summary: | <p>The Root-lesion nematode, <em>Pratylenchus loosi </em>Loof inhabits all tea growing regions of Sri Lanka depicting a variety of symptoms and damage severities to affected tea. Though much has been researched on biology and control measures, isolate characterization has not been attempted. Hence, male and female morphometric variations and molecular characterization of <em>P. loosi </em>populations isolated from different agro-ecological regions in Sri Lanka viz. PL1 (Cecilton, Balangoda), PL2 (Delmar, Halgranoya), PL3 (Hapugastenna, Ratnapura) PL4 (Mahadowa, Passara), PL5 (Nawalapitiya) and PL6 (Richiland, Deniyaya) were studied. Female morphometrics of <em>P. loosi </em>showed intraspecific variation and clustered in four groups in Principal Component Analysis where PL1 and PL5 were closely-related while PL3 and PL6 clustered separately with the exception of PL2 and PL4. Sequence analysis of the D2/D3 expansion segments of the 28S rDNA gene of the <em>P.</em> <em>loosi </em>populations revealed that PL3 and PL6 were closelyrelated while PL1, PL4 and PL5 were relatively distant. Sequences of the ITS region of rDNA placed PL3 and PL6 in a single clade. The isolates PL1, PL2, PL4 and PL5 were relatively distantly-related and PL2 and PL4 were relatively distant from the other populations. Molecular characterization further validated the relatedness of PL1 and PL5, PL3 and PL6 and, PL2 and PL4 obtained from the morphometric data. The divergence in <em>P. loosi </em>populations shown in this study supports evidence of intra specific isolates resulting different symptomological expressions and thus implies need of specific management strategies in managing nematode infestations in tea plantations in Sri Lanka.</p>
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