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 widespre ad in Europe, was shown to be caused by the host-specific, 'near-diplo id' Verticillium longisporum comb. nov. Thirty-one cruciferous isolate s (plus one from sugarbeet) of V. longisporum, mainly from oilseed rap e from Germany, Sweden, Japan, France and Poland, were characterized a nd compared with nine typical isolates of V., dahliae and five of V. a lbo-atrum. isolates of V. longisporum were distinguished from those of V. dahliae by three morphological characters, i.e. elongate microscle rotia, long conidia (7.1-8.8 mu m) and mainly three phialides per node on conidiophores, whereas those of V., dahliae had +/- spherical micr osclerotia, short conidia (3.5-5.5 mu m), and 4-5 phialides per node. Isolates of V., longisporum lacked extracellular polyphenol oxidase ac tivity (p.p.o.); they showed mean conidial nuclear diam. (DAPI fluores cence) 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 th ree oligonucleotide primers, isolates of V. longisporum were clearly d istinguishable from those of V. dahliae and V. albo-atrum by their RAP D band profiles. Greenhouse pathogenicity tests, employing five winter oilseed rape cvs, confirmed the pathogenicity of Ir. longisporum, whe reas V. dahliae was non-pathogenic. On the basis of all the above char acters, this host-specific pathotype, V. longisporum, should now be co nsidered 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 dipl oid' state and the origin of four recombinants detected.