Continuous Azoxystrobin Selection Enhances the Fitness of QoI-Resistant Pyricularia oryzae Triticum Lineage in Wheat

Pyricularia oryzae Triticum lineage (PoTl), the fungal pathogen responsible for wheat blast disease, has demonstrated increased fitness, stable resistance to quinone outside inhibitor (QoI) fungicides, and no associated fitness cost in the absence of fungicide selection pressure. In this study, we assessed whether the fitness-related epidemiological traits and competitive abilities of QoI-resistant (R) PoTl isolates increased or remained unchanged under continuous QoI applications across multiple disease infection cycles in wheat plants, compared to untreated plants. Approximately 48 h before inoculating each group of QoI-R or sensitive (S) PoTl isolates, wheat plants were treated with 10 μg mL−1 azoxystrobin. The EC50, incubation and latent periods, as well as the germination capacity of the QoI-R isolates inoculated on wheat leaves treated with 10 μg mL−1 azoxystrobin, remained unchanged throughout five successive infection cycles. However, continuous azoxystrobin applications throughout successive disease cycles increased head blast severity by 1.33-fold and conidial production on wheat leaves by 36.8-fold in QoI-resistant (QoI-R) PoTl isolates compared with untreated plants. Conidia germination and germ tube development by QoI-R isolates on the abaxial surface of wheat leaves and/or head rachis treated with 10 μg mL−1 azoxystrobin, six hours after inoculation (h.a.i), were observed using a scanning electron microscope (SEM). These findings demonstrate that repeated azoxystrobin exposure enhances fitness-related epidemiological traits of QoI-R PoTl isolates. Consequently, alternative evolutionarily informed disease management strategies, including fungicide rotation, multisite fungicides, and integrated disease management, are urgently needed.