Pathogenicity and colonization studies on wild–type and auxotrophic isolates of Verticillium albo–atrum from hop

Three fluctuating (M18, M33 and M50) and three progressive (PVl, PV2 and PV3) hop wilt isolates of Verticillium albo–atrum Reinke and Berthold were tested against the hop ( Humulus lupulus L.) cultivars, Fuggle, Wye Challenger and Wye Target. Significant differences in pathogenicity were observed between progressive and fluctuating isolates and although wilt symptoms varied in intensity from season to season, the relative order of pathogenicity and host resistance usually remained the same. Using a detached leaf procedure, differences were observed in the frequency of successful re–isolations of Vertieillium from hop plants infected with wild–type isolates. This procedure is discussed as a possible additional method for distinguishing between progressive and fluctuating hop wilt isolates. Auxotrophic and morphological mutants derived from these six wild–type isolates were tested under glasshouse conditions against hop and the non–selective alternative host Antirrhinum majus L. (cv. Maximum). Although auxotrophic mutants generally showed a reduction in pathogenicity to, and ability to colonize, both hop and Antirrhinum , certain auxotrophs retained near–parental levels of colonization and were therefore suitable for studies on possible parasexual recombination in these hosts.