Baseline Sensitivity, Control Efficacy and Resistance Risk Assessment for Fluazinam in Diaporthe citri

Citrus melanose, caused by Diaporthe citri, is a severe fungal disease that threatens global citrus production and reduces fruit quality and marketability. Fluazinam, a broad-spectrum phenylpyridinamine fungicide, exhibits excellent antifungal effects against several ubiquitous plant pathogens. However, data on its antifungal activity toward D. citri and the potential for pathogen resistance remain scarce. In this study, the sensitivity of 69 D. citri isolates to fluazinam using mycelial growth inhibition assays. The EC50 values ranged from 0.0690 to 0.8199 μg/mL, with a mean value of 0.2102 ± 0.1321 μg/mL. Pot assays and field trials showed that fluazinam provided better control of melanose than mancozeb. Four inheritable fluazinam-resistant mutants were derived from two sensitive strains. No cross-resistance was observed between fluazinam and pyraclostrobin, carbendazim, prochloraz or fludioxonil. Three resistant mutants showed a significantly decreased compound fitness index (CFI), which included biological traits such as mycelial growth, conidiation, conidial germination and virulence, relative to their parental strains, whereas LY1R-1 displayed a comparable CFI. In addition, ATP production in the resistant strains was lower than that in the parental strains. Genetic analysis showed that the expression of genes associated with efflux and detoxification was up-regulated in all fluazinam-resistant mutants compared to their parental strains. Based on these data, we speculated that D. citri had a low-to-moderate resistance risk to fluazinam. The up-regulation of efflux and detoxification genes may be the main reason for fluazinam resistance in D. citri. This study provide valuable guidance for managing citrus melanose in the field.