Diverse isolates of the soilborne wilt fungi Verticillium dahliae and V. albo-atrum were studied to understand the nature and origins of those infecting cruciferous hosts. All isolates from cruciferous crops produced microsclerotia, and the majority produced long conidia with a high nuclear DNA content; these isolates were divided into two groups by amplified fragment length polymorphism (AFLP) analysis. One group could be subdivided by other criteria such as rRNA sequences and mitochondrial DNA restriction fragment length polymorphism (RFLP) analysis. Two crucifer isolates were short spored and had a low nuclear DNA content. The results are consistent with the crucifer isolates being interspecific hybrids. The long-spored isolates are best regarded as amphihaploids (or allodiploids) with the AFLP groups probably each representing separate interspecific hybridization events. The short-spored crucifer isolates appear to be derived from interspecific hybrids and are here called 'secondary haploids'. Molecular evidence suggests that one parent in the crosses was similar to V. dahliae. The other parent of the amphihaploids seems to have been more similar to V. albo-atrum than to V. dahliae, but was distinct from all isolates of either species so far studied. The implications for the taxonomy of crucifer isolates are discussed and the use of the name V. longisporum, proposed elsewhere for just some of these isolates, is discouraged.

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The relationships of two host‐adapted pathotypes of Verticillium dahliae have been examined at the molecular level using restriction fragment length polymorphisms. Isolates obtained from and adapted to Mentha×piperita (peppermint), which were presumed to be haploid, formed a distinct subspecific group (referred to as M) related to the previously described non‐host‐adapted subspecific group A of V. dahliae. The limited molecular variation found among the four group M isolates was not related to geographic origin. Isolates from several cruciferous hosts (and one from Beta vulgaris (sugar beet)), which are thought to be natural, stable diploids, formed another distinct group (referred to as D) that was markedly different from all previously described subspecific groupings in both V. dahliae and V. alboatrum. This group of isolates might better be regarded as a separate species. Again, only limited variation was found within the D group. Polymorphisms revealed by two probes distinguished two isolates derived from Brassica rapa (Chinese cabbage) from the six other isolates (four from Brassica napus (oilseed rape) and one each from Raphanus raphanistrum (wild radish) and Beta vulgaris).

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