some species of helicotylenchus steiner, 1945 found in rapeseed fields of north khorasan province

abstract in order to identify the plant parasitic nematodes of rapeseed fields in north khorasan provice, during years 2007-2008, 50 soil and root samples were collected. nematodes were extracted by centrifugal flotation technique and transferred to glycerin according to the modified de grisse method (1969). the permanent slides were prepared from the extracted nematodes. the nematodes were identified by light microscopy based on morphological and morphmetrical characters. in this study, eight species of helicotylenchus were identified as following: helicotylenchus digonicus, h. vulgaris, h. exallus, h. pseudorobustus, h. californicus, h. canadensis, h. minzi and h. crassatus. three species h. californicus, h. canadensis and h. crassatus were reported for the first time from iran. h. californicus is recognizable from related species by the hemispherical lip region, functional spermatheca in female and shape terminus projection of tail,from h. canadensis by the truncate lip region with 4-5 annulations, long stylet and tail terminus hemispherical and from h. crassatus by the truncate lip region and tail shape.