Conidial surface antigens from 10 progressive and 10 fluctuating isolates of Verticillium albo-atrum were analysed by indirect fluorescence-staining using antisera raised against pooled spores from five isolates of each type. Cross-absorption of antisera with heterologous conidia indicated that the spore surface from eight progressive isolates contained an antigen which was absent from the surface of eight fluctuating isolates. Analysis of single-spore cultures prepared from these isolates by immunofluorescence substantiated the presence of two serological groups of V. albo-atrum and indicated that the original cultures were antigenically mixed. The immunofluorescence data could be reconciled with the previously reported immunoelectrophoresis data which indicated the existence of at least three serotypes. There is no correlation, however, between the serotypes and the virulence to hop of V. albo-atrum.
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