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SUMMARYForty isolates of the ergot fungus Claviceps purpurea collected from nineteen gramineous host species were used to inoculate male‐sterile wheat. The isolates segregated into highly and weakly infective groups. The marked pathogenicity, on wheat, of the fungal strains occurring on certain grass species has been correlated with distinctive patterns of alkaloids within the sclerotia.Analysis of the alkaloid content of 241 samples of naturally occurring ergot sclerotia from twenty gramineous host species has confirmed the existence of host restricted strains characterized also by their particular spectra of alkaloids.Similarity of the alkaloid spectra of ergot sclerotia from blackgrass (Alopecurus myosuroides) and wheat, ease of cross‐infection from blackgrass to wheat and an association between blackgrass infestation and the occurrence of ergot sclerotia in surveyed wheat crops have confirmed the hypothesis that the presence of this early flowering weed grass increases the risk of high levels of ergot infection in wheat.

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