The resistance of Vicia Faba L. to infection by botrytis

Abstract

The mechanisms of resistance of Vicia faba L. to infection by Botrytis have been examined. Inoculation of leaves detached from growth room grown plants with the avirulent species B. allii, B. cinerea and B. elliptica caused lesion development to occur at very few sites. At lesion free sites spore germination and germ tube growth were inhibited on the leaf surface. The cause of the inhibition of B. cinerea was shown to be a combination of nutritional competition between epiphytic bacteria and fungal spores and also the release into inoculum droplets of a water soluble inhibitor, probably arising from the interaction between conidia and epicuticular wax. The virulent pathogen, B. fabae was not inhibited on the leaf surface and caused lesion development at all inoculation sites. The mechanism by which this species may overcome inhibitors acting on the leaf surface is discussed. Inhibition of B. cinerea germination was much less frequent on leaves from field grown plants indicating that post-penetration resistance mechanisms may be of more significance to the restriction of avirulent Botrytis spp. in the field. Whereas B. cinerea typically produced limited lesions in field grown leaves, B. fabae produced lesions which ultimately spread throughout the leaf. Phytoalexin production by V. faba has been strongly implicated in the differential pathogenicity of Botrytis towards the broad bean plant. The antifungal activity of five phytoalexins from V. faba, the furanoacetylenes wyerol, wyerone, wyerone acid and wyerone epoxide and the isoflavanoid medicarpin has been examined. All the phytoalexins were more active against germ tubes of B. cinerea than B. fabae but differential activity was more marked with wyerone derivatives than with medicarpin. The inhibitors were ranked in the following order of activity; wyerone epoxide, wyerone acid, wyerone, medicarpin and wyerol. The activity of wyerone acid was influenced by pH. B. fabae was found to be less sensitive to wyerone acid than six other species of Botrytis and studies with four isolates of both B. cinerea and B. fabae showed that tolerance to wyerone acid correlated with virulence. At concentrations slightly above those causing complete inhibition of fungal growth, wyerone, wyerone acid and wyerone epoxide were also shown to be toxic to B. cinerea and B. fabae. Results suggested that the phytoalexins probably acted by binding to a specific receptor site within hyphae, but their precise mode of action was unclear. Studies on the relationships between fungal growth and metabolism of wyerone, wyerone acid and wyerone epoxide showed that the differing abilities of B. cinerea and B. fabae to grow in the presence of the inhibitors was due primarily to differences in sensitivity rather than ability to metabolize the phytoalexins. The time course of the growth and death of infection hyphae and accumulation of phytoalexins in epidermal tissue has been examined. Results suggested that antifungal concentrations of phytoalexins (particularly wyerone acid) accumulated sufficiently early, and in the correct location to account for the observed cessation in growth of B. cinerea. Within lesions caused by equally large numbers of conidia of B. fabae, however, phytoalexins failed to reach fungistatic concentrations. The biochemical mechanisms underlying the pathogenicity of B. fabae to V. faba are discussed.