First Report of Carbendazim-Resistant Field Isolates of Ustilaginoidea virens on Rice in East China

Abstract

Rice false smut caused by the filamentous fungus Ustilaginoidea virens is a devastating grain disease in rice. Carbendazim is usually used to control rice fungal disease. In this study, sensitivities of 264 isolates of U. virens collected from rice fields to carbendazim were determined. Most isolates were completely inhibited at 10.0 µg/ml of carbendazim except for 14 resistant isolates (5.3%). The average effective concentration for one half inhibition of mycelial growth (EC50) value for the sensitive isolates was 0.61 ± 0.03 (SEM) μg/ml, whereas the EC50 value for the resistant isolates ranged from 57.6 to >500 µg/ml. The average mycelial growth rate of five resistant isolates was not different from sensitive isolates, but the conidia production of resistant isolates decreased. Cross-resistance assays showed that there was no cross-resistance between carbendazim and DMI fungicides or QoI fungicides. Sequencing analysis of the partial β-tubulin gene from all 14 carbendazim-resistant isolates revealed that there was no nucleotide change at codon 198 or 200. The molecular mechanism of carbendazim-resistant field isolates of U. virens needs to be further studied. As far as we know, this is the first report of carbendazim resistance in U. virens in East China.