Genetics governing the differential response to boscalid of Microsphaeropsis tanaceti and Stagonosporopsis tanaceti

Microsphaeropsis tanaceti, the cause of tan spot disease of pyrethrum (Tanacetum cineariifolium), is an emerging pathogen in Australian crops. Through its emergence, it has displaced Stagonosporopsis tanaceti, the cause of ray blight disease, as the dominant pathogen during the spring growing period. Screening of the fungicide response of these pathogen populations indicated that M. tanaceti has developed levels of resistance to the fungicide boscalid, while S. tanaceti has maintained susceptibility. This is despite both species being exposed to boscalid over an equal time period. Characterisation of the succinate dehydrogenase (Sdh) gene complex was conducted to examine the basis for the response profiles of the two species. Point mutations at codon 277 of the ShdB gene were evident in the majority of boscalid-resistant M. tanaceti individuals. These mutations change the amino acid from histidine (susceptible) to arginine or tyrosine (resistant) in the resultant protein. Additionally, a smaller proportion of boscalid-resistant individuals had mutations resulting in amino acid translation changes at one of three codons in the ShdC, or at one codon in the ShdD gene. Mutations in the Shd gene complex corresponding with fungicide resistance were absent in the S. tanaceti individuals sequenced.