Caffeic acid esters as in vitro inhibitors of plant pathogenic bacteria and fungi

Abstract

Seven caffeic acid derivatives were tested for their activity against two plant-pathogenic fungi and seven plantpathogenic bacteria. A determination of minimum inhibitory concentration (MIC) in solid media was used for the fungi, and for the bacteria agar diffusion, as well as determintion of the MIC in liquid media, were used. A distinct concentration related activity was seen for most of the compounds. The molar activity of the caffeoyl esters against the bacteria was similar to that of tartaric acid, higher than that of phenol, but much lower than that of streptomycin. For most host plants, their ‘own’ compounds were less active against their pathogens than foreign ones. Of the tested compounds, ferulic acid, caffeic acid, esculine, esculetin and orobanchoside were the most active against the two fungi, and similarly, ferulic acid and esculetin were the most active against the bacteria.