Bioassay results confirmed the role of low molecular weight phytotoxin in the patho-genesis of Verticillium albo-atrum. The metabolites separated from 21-day-old culture fil-trate by adsorption on the resin Amberlite XAD-4, and further chromatographed on Bio-Gel P2 polyacrylamide gel, induced chlorosis and necrosis on the leaflets of tomato and potato cultivars, similar to those caused by the fungus on diseased plants. Leaflets from tolerant cultivars were much less sensitive to the toxin (s)

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... an those from the susceptible ones. In the presence of toxin(s) plant tissues and individual cells showed ion-leakage and cell death to an extent relating to the plants reaction to the fungus. The relative specificity observed during pathogenicity tests between potato and tomato and their related isolates was shown to be related to the action of toxin (s).