Production of phytotoxic spore germination liquids by Alternaria brassicae and A. brassicicola and their effect on species of the family Brassicaceae

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Abstract
Summary: Experiments assessed the susceptibility of Brassica spp. and non‐Brassica spp. in the family Brassicaceae to infection by Alternaria brassicae and A. brassicicola, and determined the sensitivity of the host species to spore germination liquids (SGLs) produced by the pathogens on B. napus leaves. There was a wide range of sensitivity to the pathogens. Brassica spp. were generally more susceptible, and some non‐Brassica spp. (Barbarea vulgaris and Capsella bursa‐pastoris) were immune to A. brassicicola. Measurable damage was caused by SGLs but with significant variation between host species. Non‐hosts and weak hosts also showed necrosis. It was concluded that, in the case of both pathogens, the toxic factors in these SGLs were host‐selective. Selectivity in toxin production was also demonstrated in relation to the host surface or growing medium in which spores germinated. A substantial amount of toxin was produced on all Brassicaceae tested but not on unrelated species (Triticum aestivum, Pisum sativum and Lycopersicon esculentum). Neither pathogen produced measurable amounts of toxin when cultured in Czapek (Dox) broth.