Fitness measurement reveals contrasting costs in homologous recombinant mutants of Botrytis cinerea resistant to succinate dehydrogenase inhibitors.

Botrytis cinerea is a pathogenic ascomycete fungus that causes gray mold on many crops. Chemical control remains the principal method for curbing this disease. However, fungicide efficacy may be compromised by the selection of resistant strains. Assessments of the fitness of resistant strains is important, to evaluate the risk of their establishment in populations. Strains resistant to boscalid, the first succinate dehydrogenase inhibitor (SDHI) registered for the treatment of gray mold on grapevine in France, have recently been detected in the field. Most of these strains harbor mutations of the sdhB gene, encoding subunit B of SDH. In this study, we used sdhB recombinant mutants to investigate the impact of mutations conferring SDHI resistance on the fitness of B. cinerea. We have shown that sdhB mutations (except for the sdhB(H272Y) mutation) affect SDH activity and respiration rate. Our results suggest that different sdhB mutations have different effects on fitness. In particular, mutants displaying an inhibition of SDH activity do not suffer the same effects on fitness. We discuss the results in the context of mutant frequencies in field populations and the possible occurrence of compensatory mechanisms that modulate fitness losses.