Cross-resistance in <i>Alternaria brassicicola</i> from naturally infested broccoli seeds against two succinate dehydrogenase inhibitor fungicides

<i>Alternaria brassicicola</i> is a fungal seed-borne pathogen that can be disseminated via commercial seeds across transplant houses and commercial broccoli fields. Our study provides the first evidence that commercial broccoli seeds can harbor pathogenic <i>A. brassicicola</i> isolates with cross-resistance to two succinate dehydrogenase inhibitor (SDHI) fungicides. We observed that 93% of the <i>A. brassicicola</i> isolates from naturally infested commercial broccoli seeds contained a point mutation that conferred resistance to two SDHI fungicides (boscalid and penthiopyrad). Furthermore, we developed a PCR-based allele-specific assay for the rapid detection and monitoring of fungicide resistance. Our study highlights the importance of seed health testing and potential dissemination of fungicide-resistant isolates locally and globally, thus impacting disease management strategies.