Journal
PHYTOPATHOLOGY
Volume 113, Issue 1, Pages 80-89Publisher
AMER PHYTOPATHOLOGICAL SOC
DOI: 10.1094/PHYTO-05-22-0166-R
Keywords
antimicrobial or fungicide resistance; disease control and pest management; oomycetes
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Since its reemergence in 2004, Pseudoperonospora cubensis (CDM) has exhibited changes in fungicide sensitivity, with frequent applications needed to combat the disease. Fungicides such as carboxylic acid amides (CAA) and quinone outside inhibitors (QoI) are used, but resistance has developed due to single nucleotide polymorphism (SNP) mutations. This study found that CAA resistance is more prevalent in clade 2 isolates, while QoI resistance is widespread across both clades. The occurrence of fungicide resistance mutations in specific clades highlights the importance of understanding population dynamics for effective fungicide programs.
Since its reemergence in 2004, Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew (CDM), has experienced significant changes in fungicide sensitivity. Presently, frequent fungicide applications are required to control the disease in cucumber due to the loss of host resistance. Carboxylic acid amides (CAA) and quinone outside inhibitors (QoI) are two fungicide groups used to control foliar diseases in cucurbits, including CDM. Resistance to these fungicides is associated with single nucleotide polymorphism (SNP) mutations. In this study, we used population analyses to determine the occurrence of fungicide resistance mutations to CAA and QoI fungicides in host-adapted clade 1 and clade 2 P. cubensis isolates. Our results revealed that CAA-resistant genotypes occurred more prominently in clade 2 isolates, with more sensitive genotypes observed in clade 1 isolates, while QoI resistance was widespread across isolates from both clades. We also determined that wild cucurbits can serve as reservoirs for P. cubensis isolates containing fungicide resistance alleles. Finally, we report that the G1105W substitution associated with CAA resistance was more prominent within clade 2 P. cubensis isolates while the G1105V resistance substitution and sensitivity genotypes were more prominent in clade 1 isolates. Our findings of clade-specific occurrence of fungicide resistance mutations highlight the importance of understanding the population dynamics of P. cubensis clades by crop and region to design effective fungicide programs and establish accurate baseline sensitivity to active ingredients in P. cubensis populations.
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