Rafael Lemos Alves

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.

Wheat blast, caused by Pyricularia oryzae Triticum lineage (PoTl), is one of the most destructive and significant fungal diseases affecting wheat crops. The stability of the G143A mutation in the cytB gene, which confers resistance to Quinone outside inhibitor fungicides (QoIs) in PoTl isolates, has not been extensively studied. This study was conducted to evaluate the stability of fungicide resistance, fitness, and competitive ability of the QoI-resistant (R) PoTl isolates group over nine and five consecutive transfer cycles in vitro and in vivo, respectively, without fungicide exposure. No changes in azoxystrobin sensitivity were observed in either the QoI-resistant or sensitive (S) PoTl isolate groups after the successive transfer cycles in vitro and in vivo. The mycelial growth of the QoI-R PoTl isolate group remained stable, while the conidial germination capacity increased over time. For the QoI-resistant isolates, leaf and head disease, conidial production, and the latent period on wheat leaves did not change between the first and fifth infection cycles. In each transfer cycle, the highest levels of leaf and head disease, as well as the largest quantities of conidia collected from wheat leaves, were observed in isolate mixtures. Also, the G143A mutation responsible for QoI resistance remained stable after five transfer cycles of the QoI-resistant (0S:100R) isolate on wheat leaves. Our findings indicate that the G143A mutation remains stable, and there are adaptive benefits in QoI-R PoTl isolates. We discuss the ecological implications of the wheat blast population’s adaptation and PoTl QoIs resistance stability in wheat-cropping areas in Brazil.