Destruxin B, the major phytotoxin produced by Allernaria brassicae, was not host‐specific on 30 different plant species, causing necrotic and chlorotic symptoms both on host and non‐host plants. Isolation of sufficient amounts of two minor destruxins, homodestruxin B and destruxin B2, made it possible for the first time to show that they were both phytotoxic to leaves of Brassica napus. There were significant differences between taxonomic plant groups in their sensitivity to destruxin B. Brassica species were most sensitive to the toxin, and sensitivity decreased as relatedness of plant groups became more distant: thus it is suggested that destruxin B is host‐selective in nature. The dilution‐end‐point of destruxin B was 0.2–3.8 μg/ml for the most sensitive host species, and 15 120 μg/ml for the least sensitive. The sensitivity of non‐host species was between 15 and 750 μg/ml. The symptoms caused by destruxin B appear to be light‐dependent. There were significant differences between taxonomic plant groups in their sensitivity to destruxin B. Brassica species were most sensitive to the toxin, and sensitivity decreased as relatedness of plant groups became more distant, thus it is suggested that destruxin B is host‐selective in nature. Destruxin B appears to be a virulence factor, contributing to the aggressiveness of A. brassicae by conditioning the host tissue and thereby determining the susceptibility of the host.

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