Correlative in Vitro and in Vivo Behavior of Mutant Strains ofPhytophthora palmivoraExpressing Different Resistances to Phosphorous Acid and Fosetyl-Na

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Abstract
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 .mu.g/ml compared to 6.6 .mu.g/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 .mu.g/ml of H3PO3, whereas values for the mutant strains ranged from 4.5 to 14.8 .mu.g/ml. Similarly with fosetyl-Na, values for zoosporangium inhibition ranged from 10.6 to 45.6 .mu.g/ml for the mutants, compared to only 0.3 .mu.g/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 .mu.g/ml for the parental type and 47 to 437 .mu.g/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.

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