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Spore suspensions of Alternaria brassicae, the causal agent of gray leaf spot in Brassica plants, were incubated on the leaves of cabbage (B. oleracea) and spore germination fluid (SGF) was collected after 48 h. A high molecular weight (HMW) fraction (>10 kDa) was separated from the SGF by ultrafiltration. In a detached leaf assay, the HMW fraction induced visible symptoms only on host leaves and the toxicity was lost by treatment with proteinase K or heat at 60 degrees C for 15 min, indicating the presence of host-specific protein toxin(s). A protein toxin in the HMW fraction was purified by several chromatography steps. The toxin induced water-soaked symptoms followed by chlorosis at concentrations of 0.5 to 1 microg/ml on host leaves, but not on nonhost leaves even at 50 microg/ml. The toxin also had infection-inducing activity when added to spore suspension of a nonpathogenic isolate of A. alternata, causing symptoms similar to the infection of A. brassicae only on host leaves. These results indicate that a new host-specific protein toxin named ABR-toxin is released from germinating spores of A. brassicae on host leaves. ABR-toxin migrated as a protein of 27.5 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric point of ABR-toxin was estimated to be approximately 7.0 and 21 N-terminal amino acid residues were sequenced.

The protective effect of autoclaved water extract from spent mushroom substrate (AWESMS) and autoclaved spent mushroom substrate (ASMS) of the edible mushrooms Lyophyllum decastes (hatakeshimeji) and Pleurotus eryngii (eringi) against fungal and bacterial diseases was investigated on cucumber plants. When the plants were treated with AWESMS by spraying the first true leaves and inoculated with the target pathogen 7 days later, AWESMS of hatakeshimeji significantly reduced powdery mildew by Podosphaera xanthii, angular leaf spot by Pseudomonas syringae pv. lachrymans, but not Corynespora leaf spot by Corynespora cassiicola and scab by Cladosporium cucumerinum. When the plants were grown in a mixture (1 : 2, v/v) of ASMS of hatakeshimeji and soil, a significant disease reduction was observed in powdery mildew, scab and angular leaf spot. The protective effect was also observed against anthracnose on plants treated with AWESMS or on plants grown in a mixture of ASMS of eringi (1 : 3, v/v). Our results indicated that AWESMS and ASMS, independently of the mushroom type, provide a protective effect against fungal and bacterial diseases. Therefore, SMS should be considered an easily available source of active compounds to protect plants from fungal and bacteria infections, helping alleviate the waste disposal problem in the mushroom industry and creating an environmentally friendly method to reduce plant pathogens.

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