Laboratory studies were conducted to evaluate the risk of developing field resistance to
zoxamide, a new Oomycete fungicide which acts on microtubules.
Zoxamide,
metalaxyl and
dimethomorph were compared with respect to the ease with which fungicide-resistant mutants could be isolated and their level of resistance. Attempts to generate mutants of Phytophthora capsici and P infestans with resistance to
zoxamide by mycelial adaptation on fungicide-amended medium were unsuccessful. Similarly, changes in sensitivity to
zoxamide were small (resistance factors < or = 2.2) in mutants of P capsici isolated by chemical mutagenesis of zoospore
cysts. In parallel experiments with
metalaxyl, highly resistant mutants were obtained using both adaptation (P capsici or P infestans) and chemical mutagenesis (P capsici). For
dimethomorph, chemical mutagenesis (P capsici) yielded moderately resistant mutants (maximum resistance factor = 20.9), and adaptation (P capsici or P infestans) did not induce resistance. It is proposed that failure to isolate mutants resistant to
zoxamide results from the diploid nature of Oomycete fungi and the likelihood that target-site mutations would produce a recessive phenotype. Our studies suggest that the risk of a highly resistant pathogen population developing rapidly in the field is much lower for
zoxamide than for
metalaxyl. However, as with any site-specific fungicide, appropriate precautions against resistance development should be taken.