IV Morphology and reproduction: Strobilurins and sex in Mycosphaerella graminicola

Over the past three seasons, resistance to strobilurin fungicides has rapidly evolved in the foliar wheat pathogen Mycosphaerella graminicola. This resistance is conveyed by a point mutation in the cytochrome b gene in the mitochondrial genome (p. 108, IVp-15) Over the past three seasons, resistance to strobilurin fungicides has rapidly evolved in the foliar wheat pathogen Mycosphaerella graminicola. This resistance is conveyed by a point mutation in the cytochrome b gene in the mitochondrial genome. In order to study the inheritance of strobilurin resistance, we crossed resistant and sensitive isolates of M. graminicola on wheat seedlings that were pre-treated with azoxystrobin (Amistar). Three such crosses were made at each of the following concentrations based on the full recommended dose: 0% (control), 3%, 6%, 12.5%, 25%, 50%, 100%, and 200%. Ratios of resistant to sensitive progeny were determined by comparing the number of germinating vs. non-germinating ascospores after discharge onto water agar versus water agar amended with 1 ppm azoxystrobin in a total of over 17,000 ascospores counted. These ratios were confirmed by a diagnostic strobilurin resistance PCR screen on more than 1,000 isolates from a random collection of over 3,000 ascospores discharged onto unamended water agar. To our surprise, all crosses were successful under all preventive applications of azoxystrobin. Hence, in planta, sensitive isolates can very effectively and consistently overcome the disruption of mitochondrial respiration and even participate in sexual reproduction under the aforementioned fungicidal pressure range. We observed a strong maternal/ paternal preference in control crosses on untreated plants as indicated by segregation ratios in progeny of approximately 1:0 or 0:1 for strobilurin resistance : sensitivity. When there was no maternal preference for the strobilurin resistant isolate, a pre-treatment of just 6% of the full recommended dose of azoxystrobin was sufficient to shift the progeny to nearly all resistant. We hypothesize that this is an example of stress-induced male behavior. These findings can help explain the unexpected rapid development and distribution of strobilurin resistance in the Western European M. graminicola population.