title: Molecular characterization of boscalid- and penthiopyrad-resistant isolates of Didymella bryoniae and assessment of their sensitivity to fluopyram.
abstract: BACKGROUND
  Didymella bryoniae has a history of developing resistance to single-site fungicides. A recent example is with the succinate-dehydrogenase-inhibiting fungicide (SDHI) boscalid. In laboratory assays, out of 103 isolates of this fungus, 82 and seven were found to be very highly resistant (B(VHR) ) and highly resistant (B(HR) ) to boscalid respectively. Cross-resistance studies with the new SDHI penthiopyrad showed that the B(VHR) isolates were only highly resistant to penthiopyrad (B(VHR) -P(HR) ), while the B(HR) isolates appeared sensitive to penthiopyrad (B(HR) -P(S) ). In this study, the molecular mechanism of resistance in these two phenotypes (B(VHR) -P(HR) and B(HR) -P(S) ) was elucidated, and their sensitivity to the new SDHI fluopyram was assessed.
  
  
  RESULTS
  A 456 bp cDNA amplified fragment of the succinate dehydrogenase iron sulfur gene (DbSDHB) was initially cloned and sequenced from two sensitive (B(S) -P(S) ), two B(VHR) -P(HR) and one B(HR) -P(S) isolate of D. bryoniae. Comparative analysis of the DbSDHB protein revealed that a highly conserved histidine residue involved in the binding of SDHIs and present in wild-type isolates was replaced by tyrosine (H277Y) or arginine (H277R) in the B(VHR) -P(HR) and B(HR) -P(S) variants respectively. Further examination of the role and extent of these alterations showed that the H/Y and H/R substitutions were present in the remaining B(VHR) -P(HR) and B(HR) -P(S) variants respectively. Analysis of the sensitivity to fluopyram of representative isolates showed that both SDHB mutants were sensitive to this fungicide as the wild-type isolates.
  
  
  CONCLUSIONS
  The genotype-specific cross-resistance relationships between the SDHIs boscalid and penthiopyrad and the lack of cross-resistance between these fungicides and fluopyram should be taken into account when selecting SDHIs for gummy stem blight management.
authors.affiliation: Department of Plant Pathology, University of Georgia, Tifton, GA 31793, USA. havenot@uga.edu
authors.forename: Hervé F
authors.initials: HF
authors.lastname: Avenot
authors.forename: Anna
authors.initials: A
authors.lastname: Thomas
authors.forename: Ronald D
authors.initials: RD
authors.lastname: Gitaitis
authors.forename: David B
authors.initials: DB
authors.lastname: Langston
authors.forename: Katherine L
authors.initials: KL
authors.lastname: Stevenson
mesh_terms: D000595:Amino Acid Sequence; D001203:Ascomycota / Q000187:drug effects* / Q000235:genetics* / Q000302:isolation & purification / Q000502:physiology; D001549:Benzamides / Q000494:pharmacology*; D001713:Biphenyl Compounds / Q000494:pharmacology; D025141:Drug Resistance, Fungal; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D005659:Fungicides, Industrial / Q000494:pharmacology*; D008969:Molecular Sequence Data; D009536:Niacinamide / Q000031:analogs & derivatives / Q000494:pharmacology; D010935:Plant Diseases / Q000382:microbiology*; D011720:Pyrazoles / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology*; D016415:Sequence Alignment; D013385:Succinate Dehydrogenase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013876:Thiophenes / Q000494:pharmacology
chemical_list: D001549:Benzamides; D001713:Biphenyl Compounds; D005656:Fungal Proteins; D005659:Fungicides, Industrial; D011720:Pyrazoles; D011725:Pyridines; D013876:Thiophenes; D009536:Niacinamide; C550088:2-chloro-N-(4-chlorobiphenyl-2-yl)nicotinamide; D013385:Succinate Dehydrogenase; C572868:N-(2-(3-chloro-5-(trifluoromethyl)-2-pyridyl)ethyl)-alpha,alpha,alpha-trifluoro-o-toluamide; C539106:penthiopyrad