Q1077h And V1109l Mutations in the CesA3 of Phytophthora Nicotianae Confers Resistance to Dimethomorph

Black shank, caused by Phytophthora nicotianae, is a devastating disease of tobacco in the world. Dimethomorph has been widely used to control this disease, but the potential for fungicide resistance is unknown. The baseline sensitivities of 71 P. nicotianae isolates, tested by mycelial growth inhibition, showed a unimodal distribution with a mean EC 50 value of 0.3502 (±0.0332) µg/ml. Eight dimethomorph-resistant mutants were obtained by repeated exposure to dimethomorph in vitro. The mutants with a high resistance level had a resistance factor greater than 220, while the mutants with a low resistance level had a resistance factor of less than five. The resistant mutants had almost equal fitness as their wild-type parents in mycelial growth, temperature sensitivity, and pathogenicity. No cross-resistance between dimethomorph and fluazinam, metalaxyl, or fluopicolide was observed. The results suggest that the risk of P. nicotianae developing resistance to dimethomorph is moderate. A comparison of four full-length cellulose synthase genes between wild-type and dimethomorph-resistant isolates revealed two types of point mutations in the high-resistance mutants. The amino acids changed from glutamine to histidine at position 1077 and from valine to leucine at position 1109, conferring a high resistance to dimethomorph. Based on the point mutations, allele-specific polymerase chain reaction methods were developed to detect dimethomorph resistance in P. nicotianae populations.