Field isolates of Alternaria alternata from peach were previously characterized for their sensitivity to succinate dehydrogenase (SDH) inhibitor fungicides and the underlying molecular basis of resistance was determined. In the present study we report that isolates resistant to the SDHI fungicide, boscalid, regardless of genotype, were also resistant to pyraclostrobin and thiophanate-methyl. Resistance to pyraclostrobin was due to the G143A mutation in the cytb gene and resistance to thiophanate-methyl was due to a mutation of 167Y in the β-tubulin gene. Representatives of the two most commonly-isolated, SDHI resistance genotypes H277Y in sdh subunit B and H134R in sdh subunit C, as well as genotype D123E in sdh subunit D were selected for fitness evaluations. Genotypes H277Y and H134R suffered no fitness penalties based on mycelial growth on PDA, spore production in vitro, osmotic sensitivity, oxidative sensitivity, germination ability, or the ability to cause disease on peach fruit. Hypersensitivity to oxidative stress and weak sporulation was observed only in genotype D123E. No competitive advantage was detected for sensitive isolates over the course of five consecutive transfers on peach fruit when spores were mixed with genotypes H277Y or H134R. Results suggest that in the absence of fungicide pressure, A. alternata isolates resistant to MBC, QoI, and SDHI fungicides carrying the H277Y mutation in SDHB and the H134R mutation in SDHC may effectively compete with the boscalid-sensitive populations
