A New Mutation Genotype of K218T in Myosin-5 Confers Resistance to Phenamacril in Rice Bakanae Disease in the Field.

In 2017 and 2018, a total of 294 isolates were collected from bakanae-diseased rice plants in Jinhua, Shaoxing, and Jiaxing in Zhejiang Province, China. Phenamacril sensitivity of these isolates was determined by the 50% effective concentration value or minimum inhibitory concentration methods. Our results indicated that the phenamacril resistance frequency of increased from 18% in 2017 to 47% in 2018, and rice plants infected with -resistant isolates could not be protected effectively with 50 mg/liter of phenamacril. Phenamacril-resistant isolates obtained from rice fields showed stable resistance, because their fitness levels (i.e., mycelial growth, sporulation, and pathogenicity) were similar to the phenamacril-sensitive isolates. In addition to the point mutation at codon 219 in the - gene that conferred resistance to phenamacril, our results also showed another point mutation at codon 218 (AAG→ACG) in -5 that also conferred resistance to phenamacril. In this study, we found rapid development and persistence of diversified genotypes of phenamacril resistance, highlighting the importance of proper use of phenamacril in rice fields. Our results may also help researchers develop new fungicides or new control strategies using combinations of different fungicides in the control of phenamacril-resistant isolates.