Longidorus azarbaijanensis n. sp. is described and illustrated using morphological and molecular data. It was recovered in West Azarbaijan province, northwestern Iran, from the rhizospheric soil of foxtail weed. The new species is characterized by having 5.4 to 6.8 mm long females, offset, anteriorly flat lip region and separated from the rest of the body by a shallow constriction, funnel-shaped amphidial pouches, the guiding ring at 23 to 27 µm from the anterior end, 73 to 81 and 44 to 50 µm long odontostyle and odontophore, respectively, 95.0 to 113.5 µm long pharyngeal bulb, didelphic-amphidelphic reproductive system with long tubular uteri lacking sperm cells, vulva located at 52.0% to 56.4%, conical tail dorsally convex, ventrally flat, with bluntly rounded wide tip, for juvenile developmental stages and absence of males. The general body shape of the new species is similar to that of five known species of the genus namely L. euonymus, L. perangustus, L. persicus, L. protae and L. sturhani. The morphological differences of the new species with the aforementioned species are discussed. For all the aforementioned species (except L. protae, currently lacking molecular data) the differences of the new species was also confirmed with differences in molecular sequences of D2-D3 expansion domains of 28S rDNA and the corresponding phylogenetic analyses. The partial sequence of the internal transcribed spacer 1 (ITS1) of the new species was also used in phylogenetic analyses. In partial 28S tree, the clade including the new species and six other species (L. attenuatus, Longidorus sp. and four above-mentioned species having molecular data for this fragment) was well supported in Bayesian inference. In the ITS1 tree, the new species formed a clade with L. euonymus, L. perangustus and L. persicus, as in 28S tree. This is one of the cases from which the morphologically similar species are separated using molecular sequences.
