title: Laboratory studies to assess the risk of development of resistance to zoxamide.
abstract: 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.
authors.affiliation: Rohm and Haas Co, Research Laboratories, Spring House, Pennsylvania 19477, USA. dyoung@dow.com
authors.forename: D H
authors.initials: DH
authors.lastname: Young
authors.forename: S L
authors.initials: SL
authors.lastname: Spiewak
authors.forename: R A
authors.initials: RA
authors.lastname: Slawecki
mesh_terms: D000409:Alanine / Q000031:analogs & derivatives* / Q000494:pharmacology; D000577:Amides / Q000737:chemistry / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004352:Drug Resistance, Microbial / Q000235:genetics; D005659:Fungicides, Industrial / Q000737:chemistry / Q000494:pharmacology*; D009025:Morpholines / Q000494:pharmacology; D009154:Mutation; D010838:Phytophthora / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development; D010935:Plant Diseases / Q000382:microbiology
chemical_list: D000577:Amides; D005659:Fungicides, Industrial; D009025:Morpholines; C451427:zoxamide; C028175:metalaxyl; C076154:dimethomorph; D000409:Alanine