VIEW ARTICLE

Disease Control and Pest Management

Correlative in Vitro and in Vivo Behavior of Mutant Strains of Phytophthora palmivora Expressing Different Resistances to Phosphorous Acid and Fosetyl-Na. T. E. Dolan, Department of Plant Pathology, University of California, Riverside 92521-0122, Current address: Department of Biological Sciences, Butler University, Indianapolis, IN 46260; M. D. Coffey, Department of Plant Pathology, University of California, Riverside 92521-0122. Phytopathology 78:974-978. Accepted for publication 19 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-974.

Mutant strains of an isolate (PO376) of Phytophthora palmivora were selected in the laboratory from populations of germinating cysts treated with N-methyl-N’-nitro-N-nitrosoguanidine. Five mutants were selected for their low, moderate, or high resistance in vitro to phosphorous acid (H₃PO₃) buffered to pH 6.2. The EC₅₀ values for inhibition of radial growth of mycelium by H₃PO₃ on 0.5% cornmeal agar for these mutants ranged from 36.6 to 130.3 µg/ml compared to 6.6 µg/ml for the parental isolate. The effect of H₃PO₃ on inhibition of zoosporangium production also was investigated. The parental isolate had an EC₅₀ of 0.1 µg/ml of H₃PO₃, whereas values for the mutant strains ranged from 4.5 to 14.8 µg/ml. Similarly with fosetyl-Na, values for zoosporangium inhibition ranged from 10.6 to 45.6 µg/ml for the mutants, compared to only 0.3 µg/ml for the parental isolate. Using tomato seedlings as a susceptible host for in vivo tests, EC₅₀ values for inhibition of infection ranged from 38 µg/ml for the parental type and 47 to 437 µg/ml for the five mutants. For these mutants, the order of their in vivo sensitivity to both H₃PO₃ and fosetyl-Na paralleled that observed in vitro for inhibition of their mycelial growth by H₃PO₃. The results of these experiments support the hypothesis that H₃PO₃ and fosetyl-Na have a direct mode of action against P. palmivora.