Chocolate spot of beans (Vicia faba L.)—interactions between phenolase of host and pectic enzymes of the pathogen

SUMMARY Botrytis fabae and B. cinerea, pathogens which cause ‘chocolate spot’ of beans (Vicia faba L.) produced polygalacturonase abundantly in media containing pectic substances or soluble cellulose derivatives; only filtrates from cultures grown on poly‐pectate had pectinesterase activity and even this was relatively low. B. cinerea filtrates from media containing cellulose had appreciable cellulase (Cx) activity, but did not degrade insoluble wood cellulose; B. fabae filtrates were inactive in both respects. Cultures of both fungi on certain types of media readily macerated sections of bean stems and caused them to become brownish black in colour. Filtrates from cultures of both fungi had considerably lower polygalacturonase activities when leaf extracts were present in otherwise suitable media. The latent phenolase present in water extracts of leaves was activated by poly‐pectate, polygalacturonic acid, pectin and carboxymethylcellulose. Activation was most rapid near pH 4.5 and was negligible at pH 5.5 which was the optimum value for sodium dioctyl sulphosuccinate, one of the best activators so far described. Solutions of the polymers could be diluted with no great effect on activations. Lesions are produced more readily on old than on young leaves, and extracts of old leaves discolour more rapidly than do extracts of young leaves when exposed to activated phenolase from bean leaves. This is because young leaves contain some system which retards the development of the coloured products which are normally produced following the oxidation of phenols. Water extracts of bean leaves inactivate the polygalacturonase of both fungi. The polygalacturonase of B. fabae differed from that of B. cinerea in being rapidly inactivated after filtrates had been diluted with water, and by being inactivated quickly by cyanide. B. fabae polygalacturonase was inactivated relatively quickly by products formed after oxidation of dihydroxyphenylalanine, but was less affected when catechol was used; B. cinerea behaved in the opposite way. The mechanisms by which both fungi cause lesions on bean leaves are discussed in the light of the results summarized above.