Phytoalexin production and degradation in relation to resistance of clover leaves to Sclerotinia and Botrytis spp

Abstract

The pathogenicity to red clover of three Sclerotinia spp. and three Botrytis spp. was compared with their sensitivity to, and ability to degrade, the clover phytoalexins maackiain and medicarpin. Sclerotinia fruticola formed no lesions at all, whereas S. fructigena and Botrytis allii formed flecks under the inoculation drop. B. cinerea and B. fabae formed limited lesions slightly larger than the inoculation drop, and S. trifoliorum formed spreading lesions. In 24 h, S. trifoliorum and B. cinerea degraded both phytoalexins in vitro to less inhibitory hydroxylated derivatives. B. fabae also degraded the phytoalexins but to different products. The other Sclerotinia and Botrytis spp. did so little or not at all. In lesions caused by S. trifoliorum and B. cinerea in clover leaves, the same phytoalexin degradation products were detected as in vitro. S. trifoliorum degraded at a faster rate than B. cinerea in vitro, and in vivo S. trifoliorum was apparently able to metabolize medicarpin and maackiain at a sufficient rate to reduce their concentrations to sub-inhibitory levels. B. cinerea was less able to do so, so that high phytoalexin concentrations were reached in spite of some degradation by the fungus. Following inoculation with S. fructigena, S. fructicola and B. allii, high phytoalexin concentrations were reached in tissue and in diffusates and no degradation products were detected. These three fungi were more sensitive to the phytoalexins with respect to mycelial growth. The oxygen uptake of S. fructicola was also more strongly inhibited by maackiain than that of S. trifoliorum or B. cinerea.