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.
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
were distinguished from those of V. dahliae by three
morphological characters, i.e. elongate microsclerotia, long conidia
and mainly three phialides per node on conidiophores, whereas those of
V. dahliae had ±spherical microsclerotia, short conidia
(3·5–5·5 μm), and 4–5 phialides per node.
of V. longisporum lacked extracellular polyphenol oxidase activity
showed mean conidial nuclear diam. (DAPI fluorescence) of ca
1·85 μm, and ‘near-diploid’ standardized arbitrary
(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 μm 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
by their RAPD band profiles. Greenhouse pathogenicity tests, employing
winter oilseed rape cvs, confirmed the pathogenicity
of V. 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
the origin of four recombinants detected.