Resistance of Alternaria spp. Causing Strawberry Black Spot to Boscalid in China

Strawberry black spot, caused by <i>Alternaria</i> spp., is an emerging disease that threatens both leaves and fruits during strawberry growth and postharvest storage. This study investigated the boscalid sensitivity of 49 <i>Alternaria</i> isolates collected from symptomatic strawberry leaves. Boscalid has been widely used to control diseases in strawberry in China for several years. The EC₅₀ values for the tested isolates ranged from 0.0884 to 266.3289 µg/mL, indicating that most isolates exhibited varying levels of resistance to boscalid based on resistance ratio values. A substitution of SDHC-H134R was detected from most high-resistance isolates. Fitness cost assessment revealed that highly resistant isolates had a reduced conidial germination rate; however, their mycelial growth and conidia production were increased. No significant virulence deficiency was observed, suggesting low fitness cost in resistant isolates. Furthermore, the highly resistant isolates exhibited positive cross-resistance to fluopyram and fluxapyroxad. Molecular docking analysis revealed that the SDHC-H134R mutation reduced the binding affinity between boscalid and mitochondrial complex II. These findings suggest that resistance management strategies, such as fungicide rotation or combinations of fungicides with different action modes, should be implemented to control strawberry diseases, minimizing the development of fungicide resistance and improving overall disease control efficacy.