Sensitivity of Species of Colletotrichum to Fluxapyroxad and Efficacy of the Fungicide to Control Infection on Detached Apples

ABSTRACT

Colletotrichum chrysophilum , C. melonis and C. nymphaeae are causal agents of apple bitter rot (BR), Glomerella leaf spot (GLS) and Colletotrichum fruit spot (CFS). In Brazil, site‐specific fungicides belonging to the demethylation inhibitor (DMI; FRAC 3), quinone outside inhibitor (QoI; FRAC 11), and methyl benzimidazole carbamate (MBC; FRAC 1) groups are widely used, often in combination with protectant fungicides (FRAC M3). Since the 2015/2016 season, the succinate dehydrogenase inhibitor (SDHI) fluxapyroxad (FRAC 7) has been registered for BR management. This study aimed to characterize the sensitivity of Colletotrichum spp. to fluxapyroxad and to evaluate its efficacy in managing fruit infection in ex vivo essays using detached apples. A total of 71 isolates collected from Brazilian apple orchards between 2010 and 2017 were assessed using mycelial growth inhibition assays. Baseline sensitivity was determined using isolates collected prior to the introduction of fluxapyroxad (2010–2015; n  = 25). EC ₅₀ values were estimated and compared among the three species. Isolates with reduced sensitivity (EC ₅₀  > 100 μg mL ⁻¹ ) were detected in both baseline and post‐exposure populations, with an increased frequency in C. melonis (from 25% to 31.6%) and C. nymphaeae (from 20% to 42.9%). For C. chrysophilum , isolates with EC ₅₀  > 100 μg mL ⁻¹ were observed only in the baseline population (16.7%). Ex vivo assays conducted on wounded ‘Gala’ apples showed that fluxapyroxad was ineffective in controlling infection.