First and sixth authors: United Graduate School of Agricultural Sciences, Tottori University, Tottori 680-8553, Japan; second and third authors: Servizio Fitosanitario-Regione Emilia-Romagna, Via Corticella 133, Bologna 40129, Italy; fourth author: Laboratory of Applied Microbiology, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan; fifth and seventh authors: Laboratory of Plant Pathology, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan
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Accepted for publication 8 July 1999.
Culture filtrates of a pathogenic isolate (IT37) of Stemphylium vesicarium, causing brown spot of European pear, induced veinal necrosis only on pear leaves susceptible to the pathogen. Two host-specific toxins, SV-toxins I and II, were purified from culture filtrates of IT37 by successively using Amberlite XAD-2 resin adsorption, cellulose thin-layer chromatography, and high-performance liquid chromatography under three different sets of conditions. Susceptible cultivars showed veinal necrosis at a SV-toxin I concentration of 0.01 to 0.1 μg/ml, whereas resistant cultivars were insensitive to the toxin at 1,000 μg/ml. SV-toxins I and II caused a dose-dependent increase in electrolyte loss from susceptible leaf tissues. No increase in electrolyte loss was detected in leaf tissues from resistant cultivars. The results of physiological studies indicated that SV-toxins appear to have an early effect on plasma membranes of susceptible leaves. Spores of a nonpathogenic isolate induced necrotic lesions on susceptible leaves in the presence of a small amount of toxin. SV-toxins were detected in intercellular fluids obtained from diseased leaves after inoculation with the pathogen. The results indicate that SV-toxins I and II produced by S. vesicarium can be characterized as host-specific toxins.
© 1999 The American Phytopathological Society