Abstract
Spore suspensions ofAlternaria brassicicolapathogenic toBrassicaspp. were incubated on the surface ofBrassicaleaves. The spore germination fluid was collected and examined for biological activity. In a detached leaf assay, the fluid induced water-soaking symptoms followed by brown necrotic lesions onBrassicaleaves susceptible to the pathogen but did not cause visible symptoms on non-host leaves. When spores of the non-pathogenic speciesAlternaria alternatawere combined with the fluid and inoculated on the leaves, the spores could invadeBrassicaleaves, just as the pathogen did. These results indicate thatA. brassicicolaproduces a host-specific toxin(s) and releases it during spore germination on the leaves. Toxin activity of the fluid was retained by an ultrafiltration membrane with a 5 kDa cut off. The toxin was purified by ion exchange chromatography and gel filtration HPLC. The molecular weight of the toxin was estimated to be 35 kDa by SDS-polyacrylamide gel electrophoresis. The activity of the toxin was heat labile and was also lost after treatment with proteinase K. Thus, the toxin named AB-toxin is probably a protein, the first described among the many host-specific toxins produced byAlternariaspecies.
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