Occurrence and molecular characterization of strobilurin resistance in cucumber powdery mildew and downy mildew

Between 1998 and 1999, control failure of powdery mildew (Podosphaera fusca ) and downy mildew (Pseudoperonospora cubensis) by the strobilurin fungicid es azoxystrobin and kresoxim-methyl was observed in cucumber-growing areas of Japan. Results from inoculation tests carried out on intact cucumber pla nts and leaf disks clearly showed the distribution of pathogen isolates hig hly resistant to azoxystrobin and kresoxim-methyl. Fragments of the fungici de-targeted mitochondrial cytochrome b gene were polymerase chain reaction amplified from total pathogen DNA and their sequences analyzed to elucidate the molecular mechanism of resistance. A single point mutation (GGT to GCT ) in the cytochrome b gene, resulting in substitution of glycine by alanine at position 143, was found in resistant isolates of downy mildew. This sub stitution in cytochrome b seemed to result in high resistance to strobiluri ns in this pathogen. The same mutation was found in some but not all resist ant isolates of powdery mildew. This study suggests that a mutation at posi tion 143 in the target-encoding gene, resulting in an amino acid substituti on, was probably a major cause of the rapid development of high strobilurin resistance in these two pathogens.