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Fitness and Competitive Ability of Alternaria alternata Field Isolates with Resistance to SDHI, QoI, and MBC Fungicides 
Z. FAN (1), G. Schnabel (2). (1) Clemson Unversity, Central, SC, U.S.A.; (2) Clemson Unversity, Clemson, SC, U.S.A.

Alternaria rot, caused by <i>Alternaria alternata</i>, was observed in well managed, commercial peach orchards in South Carolina. In total, 64 isolates were collected in 2013 and 2012 from fruit of problem orchards. Most isolates were resistant to boscalid, pyraclostrobin, and thiophanate-methyl. Resistance to boscalid was due to point mutations in the SDH subunits, resistance to pyraclostrobin was due to the G143A mutation in the <i>cyt</i>b gene, and resistance to thiophanate-methyl was due to 167Y in the β-tubulin gene. The four most commonly found genotypes based on mutations in the SDH subunits (H277Y in <i>sdh</i>B, H134R in <i>sdh</i>C, D123E in <i>sdh</i>D) were selected for fitness evaluations. Genotypes H277Y and H134R didn’t suffer fitness penalties based on mycelia growth on PDA, spore production in vitro, osmotic sensitivity, oxidative sensitivity, germination ability, and ability to cause disease. Hypersensitivity to oxidative stress and weak sporulation capacity was observed in genotype D123E. No competitive advantage was detected for sensitive isolates over the course of five generations when mixed with genotypes H277Y or H134R. The results suggest that in the absence of fungicide pressure, isolates with resistance to the three active ingredients may still effectively compete with the wildtype population.  

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