Quantification of synergistic interactions of fungicides against Plasmopara and Phytophthora

Abstract

Three fungicides with different modes of action—oxadixyl (phenylamide; systemic), mancozeb (dithiocarbamate; protectant), and cymoxanil (penetrant)—were tested alone and in different combinations against Plasmopara viticola on grapes and Phytophthora infestans on tomatoes. The fungicidal activities against sensitive and phenylamide-resistant strains were compared. The fungicidal activity of the mixture oxadixyl/mancozeb decreased significantly with increasing levels of resistance, whereas the mixture oxadixyl/mancozeb/cymoxanil was almost equally active against all strains. The level of synergistic interaction of fungicide mixtures was found to be dependent on the proportion of the single components in the mixture as well as on the activity of the individual compounds against the fungal strains. Cymoxanil-containing mixtures showed equally high or even higher levels of synergism against resistant compared with sensitive strains, whereas in oxadixyl/mancozeb mixtures synergism was lower in resistant situations. The methods used are suitable for finding those combinations of fungicides which give rise to maximum synergistic interactions: this was achieved with the uses of isobolograms, which were adapted to three-component mixtures leading to three-dimensional figures.