title: Exploring mechanisms of resistance to dimethachlone in Sclerotinia sclerotiorum.
abstract: BACKGROUND
  The dicarboximide fungicide dimethachlone has been widely used in China for more than 12 years to control the Sclerotinia stem rot caused by Sclerotinia sclerotiorum disease. First signs of resistance in the field are reported at low frequency. In this study, four resistant isolate/mutants were used to explore still unknown mechanisms leading to dimethachlone resistance.
  
  
  RESULTS
  The resistant isolate/mutants had significantly higher EC50 values compared with the sensitive control isolates. Cross-resistance was confirmed between dimethachlone and procymidone, iprodione and fludioxonil. The resistant isolate/mutants revealed a decreased mycelial growth rate, were less pathogenic on leaves of oilseed rape, were more sensitive to osmotic pressure and oxidative stress and released more electrolytes compared with the sensitive isolates. Only in one lab mutant did we find a point mutation (V238A) in the SsOs1 gene of the high-osmolarity glycerol (HOG) signalling pathway. The expression of this gene was lost in the field resistant isolate HN456-1-JBJ and decreased in mycelium that was subjected to either high osmotic pressure or dimethachlone; however, another key gene in the HOG pathway, SsHog1, could be induced in the resistant isolate and mutants with NaCl treatment.
  
  
  CONCLUSIONS
  This study demonstrates that resistance to dicarboximide fungicide dimethachlone in S. sclerotiorum is emerging in China. Several fitness parameters, including mycelial growth rate, sclerotia formed in vitro, aggressiveness on leaves and osmotic and H2 O2 sensitivity, indicate that the resistant strains may not effectively compete with sensitive isolates in the field in the absence of selection pressure. Lost expression or the V238A point mutation in the SsOs1 gene may confer resistance to dicarboximide fungicide dimethachlone in S. sclerotiorum, but this study illustrates that other, yet unknown mechanisms also exist.
authors.affiliation: College of Plant Science and Technology and the Key Lab of Crop Disease Monitoring and Safety Control in Hubei Province, Huazhong Agricultural University, Wuhan, China.
authors.forename: Md Jahangir
authors.initials: MJ
authors.lastname: Firoz
authors.affiliation: College of Plant Science and Technology and the Key Lab of Crop Disease Monitoring and Safety Control in Hubei Province, Huazhong Agricultural University, Wuhan, China.
authors.forename: Xiang
authors.initials: X
authors.lastname: Xiao
authors.affiliation: College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China.
authors.forename: Fu-Xing
authors.initials: FX
authors.lastname: Zhu
authors.affiliation: College of Plant Science and Technology and the Key Lab of Crop Disease Monitoring and Safety Control in Hubei Province, Huazhong Agricultural University, Wuhan, China.
authors.forename: Yan-Ping
authors.initials: YP
authors.lastname: Fu
authors.affiliation: College of Plant Science and Technology and the Key Lab of Crop Disease Monitoring and Safety Control in Hubei Province, Huazhong Agricultural University, Wuhan, China.
authors.forename: Dao-Hong
authors.initials: DH
authors.lastname: Jiang
authors.affiliation: School of Agricultural, Forest and Environmental Sciences, Clemson University, Clemson, SC, USA.
authors.forename: Guido
authors.initials: G
authors.lastname: Schnabel
authors.affiliation: College of Plant Science and Technology and the Key Lab of Crop Disease Monitoring and Safety Control in Hubei Province, Huazhong Agricultural University, Wuhan, China.
authors.forename: Chao-Xi
authors.initials: CX
authors.lastname: Luo
mesh_terms: D001203:Ascomycota / Q000166:cytology / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D002722:Chlorobenzenes / Q000494:pharmacology*; D025141:Drug Resistance, Fungal / Q000235:genetics; D015966:Gene Expression Regulation, Fungal / Q000187:drug effects; D005800:Genes, Fungal / Q000235:genetics; D009997:Osmotic Pressure; D018384:Oxidative Stress / Q000187:drug effects; D017354:Point Mutation; D015398:Signal Transduction / Q000187:drug effects; D013388:Succinimides / Q000494:pharmacology*
chemical_list: D002722:Chlorobenzenes; D013388:Succinimides; C488805:dimethachlon
keywords: Sclerotinia sclerotiorum; dimethachlone; fungicide resistance; osmotic stress; resistance mechanism