Sensitivity of Leptosphaeria maculans to Q O I fungicides in Australia

Abstract BACKGROUND Blackleg disease (caused by Leptosphaeria maculans ) is one of the most important diseases of canola, and in Australia and other countries around the world there is a reliance on the use of fungicides for control. Quinone outside inhibitor (Q O I) fungicides are classed as high risk of evolving resistance, highlighting the importance of establishing baseline sensitivity levels to ensure effective monitoring and early detection of resistance. RESULTS Spore germination of L. maculans isolates was more sensitive to azoxystrobin than mycelial growth, and inhibition of spore germination is the more reliable method of measuring azoxystrobin sensitivity. Sequencing of isolates collected from azoxystrobin treated plants and deep amplicon sequencing of field populations revealed no mutations in the coding region of the gene encoding cytochrome b ( cyt b ). Whilst high levels of disease were detected on azoxystrobin fungicide treatments in the population screen, no resistant isolates were confirmed. CONCLUSIONS No resistance to azoxystrobin was detected in this study suggesting that Australian blackleg populations are currently sensitive to the Q O I fungicides. In vitro and in planta assays are now established for continued monitoring of populations. Lastly, an additive effect was observed when combining Q O I and demethylation inhibitor (DMI) fungicides suggesting Q O I fungicides should be released only in combination with other actives to increase fungicide efficacy. © 2025 Society of Chemical Industry.