Laboratory studies to assess the risk of development of resistance to zoxamide

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 di-methomorph were compared with respect to the c ase with which fungicide-resistant mutants could be isolated and their leve l 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 zoxa mide were small (resistance factors less than or equal to2.2) in mutants of P capsici isolated by chemical mutagenesis of zoospore cysts. In parallel experiments with metalaxyl, highly resistant mutants were obtained using bo th adaptation (P capsici or P infestans) and chemical mutagenesis (P capsic i). For dimethomorph, chemical mutagenesis (P capsici) yielded moderately r esistant mutants (maximum resistance factor = 20.9), and adaptation (P caps ici 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 resis tant 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. (C) 2001 Society of Chemical Industry.