Stemphylium leaf blight (SLB) caused by Stemphylium vesicarium is the most common foliar disease of onion in Ontario, Canada. Management of SLB relies on repeated fungicide applications and most fungicides are in the succinate dehydrogenase inhibitor (SDHI) mode of action group. The strobilurin fungicide, azoxystrobin was applied regularly until very high resistance was confirmed in 2021. Both groups of fungicides have poor efficacy in the field. Isolates of S. vesicarium from New York State, classified as highly resistant to SDHI fungicides, frequently had one of three mutations, C-G79R, C-H134R and C-C135R, in the gene encoding succinate dehydrogenase subunit C (sdhC). Also, resistance to the strobilurin fungicides in New York State was associated with the G143A gene in cytochrome b. In the current study, Kompetitive Allele Specific PCR (KASP) assays were designed to detect the single nucleotide polymorphisms (SNPs) of C-G79R, C-H134R and C-C135R in sdhC and the G143A gene. The assays were tested on 70 isolates of S. vesicarium collected in Ontario from 2012-2023 that were previously classified as either sensitive or resistant to these fungicides. Resistant isolates were sequenced to detect additional mutations. Mutation C-H134R was identified in 29%, C-G79R in 7% and C-C135R in 3% of isolates. An additional mutation, C-H134N, was detected by sequencing in 23% of isolates. The G143A mutation was found in all 67 isolates collected after 2016. The KASP assays were fast and efficient. They are effective for monitoring fungicide resistance in S. vesicarium and could be developed for other fungi.
- Vegetables