Fluopyram and acibenzolar-S-methyl seed treatments reduce blackleg infection originating from cotyledons but not from later leaves in canola

Blackleg, caused by Leptosphaeria maculans, remains a major threat to canola production in Canada despite the use of resistant cultivars and crop rotation. Recently registered succinate dehydrogenase inhibitor (SDHI) fungicide seed treatments, including fluopyram (FLUO) and pydiflumetofen (PYD), may provide cost-effective protection, but their duration of efficacy is uncertain. This study evaluated FLUO, PYD and the resistance inducer acibenzolar-S-methyl (ASM) under greenhouse and/or field conditions, with emphasis on cotyledon and lower true-leaf protection. In greenhouse assays, FLUO at the recommended rate (75 g a.i. per 100 kg seed) suppressed cotyledon symptoms but did not consistently reduce blackleg, particularly in field trials, likely due to short soil persistence. In contrast, FLUO at 150 g a.i. per 100 kg seed consistently restricted cotyledon lesion development and reduced stem disease in multiple field trials. PYD at 40 g a.i. per 100 kg seed provided protection equivalent to high-rate FLUO and represents the first documented evidence of its efficacy against blackleg in the field. ASM reduced disease severity at the sixth-leaf stage in greenhouse trials but was effective only against cotyledon inoculation, suggesting a transient response under field conditions. Across trials, combining SDHI with the seed-treatment package Helix® Vibrance® improved cotyledon protection and enhanced blackleg control compared with Helix Vibrance alone. Overall, these findings indicate that FLUO at 75 g a.i. per 100 kg seed is insufficient for reliable efficacy, especially under field conditions, whereas FLUO at 150 g or PYD at 40 g provide consistent protection of cotyledons and subsequent stem disease reduction. Given the risk of fungicide resistance, integration with genetic resistance and careful stewardship will be necessary to sustain SDHI efficacy.