Sensitivity of Ustilaginoidea virens to Albendazole and Fitness of Albendazole-Resistant Mutants

Rice false smut (RFS), caused by Ustilaginoidea virens, severely reduces rice yield and grain quality, threatening food security. Although chemical control is the primary management strategy, prolonged use of fungicides has led to widespread resistance in U. virens. To address this issue, we evaluated the inhibitory potential of albendazole, a promising candidate fungicide, against U. virens. The sensitivity of 103 U. virens strains from five Chinese provinces was assessed using the mycelial growth rate method. The sensitivity baseline exhibited a unimodal distribution, with EC 50 values ranging from 0.4246 to 2.6600 μg/ml (mean: 1.0984 ± 0.4590 μg/ml), indicating strong inhibition of U. virens by albendazole. Despite its efficacy, the resistance risk of U. virens to albendazole remains unclear. To evaluate the potential for resistance development, we generated albendazole-resistant laboratory mutants through fungicide domestication and UV mutagenesis, with resistance frequencies of 3.3 and 6.7%, respectively. Among the nine resistant mutants, eight displayed high-level resistance (resistance factor [RF] > 100), and one showed moderate resistance (RF > 80), with stable inheritance of resistance. Resistant mutants displayed significant fitness costs, including reduced mycelial growth, sporulation, biomass production, inhibitory effect on the radicle of rice seeds, and cold tolerance. Cross-resistance assays revealed no cross-resistance with azoxystrobin or propiconazole but confirmed cross-resistance with carbendazim. Based on resistance frequency and fitness penalties, the resistance risk was evaluated as moderate. These findings provide critical guidance for the rational use of albendazole and highlight its potential as an alternative fungicide in RFS management.