Inhibition of tomato ringspot virus by flavonoids

When applied in a mixed inoculum with tomato ringspot nepovirus (TomRSV), flavonoids and related compounds inhibited infectivity in Chenopodium quinoa. Compounds that showed strong anti-viral activity were: quercetin, quercetin 3-methyl ether, quercetin 7-methyl ether, quercetin 3,7,3′4′-tetramethyl ether, galangin 3-methyl ether, morin, robinin, quercetin 3,7,4′-trimethyl ether, quercetin 7,4′-dimethyl ether, 7,4′-di- O-benzol-quercetin 7-hydroxy-3,4′-dimethyl flavone, 6,3′-dihydroxy-4′-methyl aurone and fisetin 4′-methyl ether. Quercetin applied at a concentration of 5 μg ml −1 caused 70% inhibition of local lesion development. When quercetin was applied to leaves prior to inoculation, there was only slight induced resistance to infection. Quercetin at 5 μg ml −1 did not affect virus multiplication in protoplasts prepared from cucumber cotyledons and transfected with viral-RNA. In meristematic tip cultures, quercetin reduced virus titre by up to 89% over a period of 36 weeks whereas ribavirin caused a 25% reduction over the same period. It is proposed that flavonoids interfere with an early event in the virus life cycle resulting in decreased infectivity and titre in tissue culture.