Point Mutation in Cytochrome b Gene Conferring Resistance to Strobilurin Fungicides in Erysiphe graminis f. sp. tritici Field Isolates

A field isolate of Erysiphe graminis f. sp. tritici that was more than 200-fold less sensitive to strobilurin fungicides (respiration inhibitors at the Qo center of the cytochrome bc1 enzyme complex) was compared to a sensitive isolate using molecular methods. With degenerate primers, 60% of the cytochrome b gene of E. graminis was cloned and sequenced, including all known positions of mutations affecting sensitivity. Sequence comparison between resistant and sensitive isolates revealed a single point mutation leading to a change at amino acid (aa) position 143 from glycine to alanine. This change is known to occur also in the naturally tolerant basidiomycete Mycena galapoda. It is assumed that this aa change is responsible for resistance to strobilurin fungicides in E. graminis. In resistant isolates, DNA from all mitochondria seem to contain the mutation. Because of mitochondrial inheritance, resistance to strobilurins will be donated after sexual recombination only if it is present in the maternal crossing partner. The described mode of resistance and the frequency, migration, and fitness of resistant isolates in field populations have to be considered when strategies concerning the method of delaying resistance buildup under field conditions are defined.