title: Resistance mechanism to carboxylic acid amide fungicides in the cucurbit downy mildew pathogen Pseudoperonospora cubensis.
abstract: BACKGROUND
  Pseudoperonospora cubensis, the causal oomycete agent of cucurbit downy mildew, is responsible for enormous crop losses in many species of Cucurbitaceae, particularly in cucumber and melon. Disease control is mainly achieved by combinations of host resistance and fungicide applications. However, since 2004, resistance to downy mildew in cucumber has been overcome by the pathogen, thus driving farmers to rely only on fungicide spray applications, including carboxylic acid amide (CAA) fungicides. Recently, CAA-resistant isolates of P. cubensis were recovered, but the underlying mechanism of resistance was not revealed. The purpose of the present study was to identify the molecular mechanism controlling resistance to CAAs in P. cubensis.
  
  
  RESULTS
  The four CesA (cellulose synthase) genes responsible for cellulose biosynthesis in P. cubensis were characterised. Resistant strains showed a mutation in the CesA3 gene, at position 1105, leading to an amino acid exchange from glycine to valine or tryptophan. Cross-resistance tests with different CAAs indicated that these mutations lead to resistance against all tested CAAs.
  
  
  CONCLUSIONS
  Point mutations in the CesA3 gene of P. cubensis lead to CAA resistance. Accurate monitoring of these mutations among P. cubensis populations may improve/facilitate adequate recommendation/deployment of fungicides in the field.
authors.affiliation: Institute of Botany, University of Basel, Basel, Switzerland. mathias.blum@gmx.ch
authors.forename: Mathias
authors.initials: M
authors.lastname: Blum
authors.forename: Maya
authors.initials: M
authors.lastname: Waldner
authors.forename: Gilberto
authors.initials: G
authors.lastname: Olaya
authors.forename: Yigal
authors.initials: Y
authors.lastname: Cohen
authors.forename: Ulrich
authors.initials: U
authors.lastname: Gisi
authors.forename: Helge
authors.initials: H
authors.lastname: Sierotzki
mesh_terms: D000577:Amides; D019662:Cucurbitaceae / Q000382:microbiology; D025141:Drug Resistance, Fungal / Q000235:genetics*; D005659:Fungicides, Industrial; D005964:Glucosyltransferases / Q000235:genetics* / Q000378:metabolism; D009868:Oomycetes / Q000201:enzymology / Q000235:genetics*; D010935:Plant Diseases / Q000382:microbiology; D017354:Point Mutation
chemical_list: D000577:Amides; D005659:Fungicides, Industrial; D005964:Glucosyltransferases; C478648:cellulose synthase