Effect of flavonoids on mycelial growth of Verticillium albo-atrum

Twenty-five flavonoids were examined for their effect on the mycelial growth of a crop pathogen, Verticillium albo-atrum. The minimum inhibitory concentrations (MIC) for the two most active compounds, flavone and flavanone, were 1 and 5 ppm, respectively. Other flavonoids inhibited hyphal growth at concentrations above 5 ppm and some compounds were ineffective at 200 ppm, the highest concentration used. Active compounds did not share a common pattern of substitutions. The unsubstituted flavonoids were stronger growth inhibitors and, in most cases, increasing the number of substitutions (hydroxylation, methoxylation, and glycosylation) resulted in the loss of antifungal activity.