The effectiveness of Quinone outside Inhibitor (QoI) fungicides against grape downy mildew in European vineyards has significantly decreased in the last decade. One nucleotide polymorphism, G143A in the cytochrome b gene of Plasmopara viticola, is involved in resistance to QoIs. Previous genetic examination on the mitochondrial genomes showed four major haplotypes (IR, IS, IIR, IIS) coexisting in European vineyards. A resistant allele (G143A) was present in IR and IIR haplotypes. The purpose of the present study was to estimate the diversity of the different mitochondrial haplotypes and their distribution in QoI-resistant populations before evaluating the potential cost of the resistant mutation G143A in P. viticola population. From 2000 to 2004, the frequencies of resistant isolates ranged from 0% to 23.25% with an average of 4.64 % among the populations examined. To evaluate the fitness of sensitive and resistant isolates, a comparison of different biological parameters including latent period, spore production and infection frequency was performed, enabling a fitness index (FI) to be determined. Resistant isolates exhibited greater infection frequency than sensitive isolates, whereas no significant difference was found in sporulation ability and latent period between sensitive and resistant isolates. To further investigate competitiveness among isolates, an assay including two resistant isolates in different proportion with a sensitive isolate was conducted on eight asexual growing cycles in the absence of a QoI fungicide. The competitiveness of resistant isolates varied according to their fitness parameters, suggesting that there is no noticeable cost of QoI resistance in controlled conditions in Plasmopara viticola.Read less <