Morphological and molecular characterization of Verticillium longisporum comb. nov., pathogenic to oilseed rape

Verticillium wilt in winter-sown oilseed rape (Brassica napus L. ssp, oleifera), which has not-yet been reported in the U.K. but is widespread in Europe, was shown to be caused by the host-specific, 'near-diploid' Verticillium longisporum comb. nov. Thirty-one cruciferous isolates (plus one from sugarbeet) of V. longisporum, mainly from oilseed rape from Germany, Sweden, Japan, France and Poland, were characterized and compared with nine typical isolates of V., dahliae and five of V. albo-atrum. isolates of V. longisporum were distinguished from those of V. dahliae by three morphological characters, i.e. elongate microsclerotia, long conidia (7.1-8.8 mu m) and mainly three phialides per node on conidiophores, whereas those of V., dahliae had +/- spherical microsclerotia, short conidia (3.5-5.5 mu m), and 4-5 phialides per node. Isolates of V., longisporum lacked extracellular polyphenol oxidase activity (p.p.o.); they showed mean conidial nuclear diam. (DAPI fluorescence) of ca 1.85 mu m, and 'near-diploid' standardized arbitrary DNA values (Feulgen DNA microdensitometry) of 0.89-1.17 (mean, 1.02). For isolates of V. dahliae, extracellular p.p.o., activity was detectable, and the corresponding figures for conidial nuclear diam. were ca 1.16 mu m and DNA values of 0.45-0.65 (mean, 0.57), respectively. Using three oligonucleotide primers, isolates of V. longisporum were clearly distinguishable from those of V. dahliae and V. albo-atrum by their RAPD band profiles. Greenhouse pathogenicity tests, employing five winter oilseed rape cvs, confirmed the pathogenicity of Ir. longisporum, whereas V. dahliae was non-pathogenic. On the basis of all the above characters, this host-specific pathotype, V. longisporum, should now be considered as a distinct species. Evidence is presented to suggest that it may have evolved by parasexual hybridization between a strain of V. albo-atrum and a strain of V. dahliae, thus explaining its 'near diploid' state and the origin of four recombinants detected.