Use of Two- and Three-Way Mixtures to Prevent Buildup of Resistance to Phenylamide Fungicides inPhytophthoraandPlasmopara

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Abstract
Oxadixyl, mancozeb, and cymoxanil were tested, singly and as two-or three-way mixtures, for their potential to control phenylamide-sensitive and -resistant strains of Phytophthora infestans on tomato or potato and Plasmopara viticola on grape under growth chamber and plastic house conditions. Fungicide mixtures provided better disease control of phenylamide-resistant strains than either one of the components alone. Two- and three-way mixtures were compared in terms of selection pressure against mixed populations of sporangium suspensions of both pathogens over a period of eight generations of sporangia. Applications were made three to seven times at 7-14-day intervals. For inoculation, a sporangium suspension with an initial proportion of resistant strains of 0.01, 0.1, 1, 10. 20, or 50% was used. When initial amounts of resistance in the mixed population were 1,0.1, and 0.01%, the two-way mixture oxadixyl/mancozeb did not select for more than about 15% resistance after two, three, and four applications, respectively, each application made on a new sporangium generation. If initial percentage of resistance was 10% or higher, 100% resistant strains were present after four to five sporangium generations. In contrast, mixed populations containing as high as 50% resistance remained fairly stable, when treated with the three-way mixture oxadixyl/mancozeb/cymoxanil; the proportion of the resistant subpopulation even decreased in tendency during a period of eight sporangium generations, each treated with a new fungicide application.