Sunflower Coumarin Phytoalexins Inhibit the Growth of the Virulent Pathogen Sclerotinia sclerotiorum

The fungus Sclerotinia sclerotiorum causes serious damage to sunflower crops. The effect of two major sunflower phytoalexins, ayapin and scopoletin, on the growth of S. sclerotiorum has been examined; both inhibited mycelial growth in vitro, but at different rates. The concentration necessary to obtain 50% inhibition of fungal growth (IC50 values) determined for ayapin and scopoletin were 0.16 mm and 1.5 mm, respectively. Other sunflower phenolic metabolites such as caffeic acid, chlorogenic acid, and isoliquiritigenin had no inhibitory effect on S. sclerotiorum growth at concentrations as high as 2 mm. These results, together with previously reported data, suggest that coumarin biosynthesis-accumulation in sunflower may be another target for plant breeding and genetic engineering strategies aimed at obtaining sunflower varieties with enhanced resistance to Sclerotinia rot.