Quelles conséquences pratiques au vignoble ?
Summary
Fenhexamid is a narrow spectrum fungicide from the hydroxyanilide family, introduced in France in 2000 to control grey mould in vineyards. It inhibits the 3-cetoreductase enzyme involved in sterol biosynthesis at the C4-demethylation steps. Nevertheless, several resistant phenotypes have been detected in vineyards: two phenotypes, HydR3- and HydR3+ acquired specific resistance to this fungicide and two MDR phenotypes are weakly resistant to fenhexamid and other modes of action.
Remarkably, HydR1 strains represent the Botrytis cinerea cryptic species named Botrytis pseudocinerea naturally resistant to fenhexamid and preexisting fenhexamid introduction. For this newly described species, the resistance mechanism is only partially determined by target alteration, but mainly related to fenhexamid metabolisation due to a cytochrome P450 monooxygenase, such mechanism is rare in phytopathogenic fungi. The moderate frequency of B. pseudocinerea in populations does not lead to efficacy losses in the field. Considering phenotypes HydR3- and HydR3+ respectively moderatly and highly resistant to fenhexamid, mutations were found in the gene encoding fenhexamid target.
The development of isogenic mutants of B. cinerea by gene replacement demonstrated the role of these mutations in resistance as well as their fitness penalty on several parameters, especially for the HydR3+ strains, the most frequent in the vineyard. This biological cost may contribute to explain the moderate evolution of the HydR3+ isolates since 2003, time of first detection. Resistance management implies to monitor the evolution of HydR3+ strains in the various vineyards, which could be achieved with a new PCR technique allowing detecting several resistant alleles simultaneously.
Key words: target mutation, metabolisation, grey mould, Botrytis cinerea, Botrytis pseudocinerea, natural resistance, acquired resistance, fitness, fungicide, fenhexamid, sterol biosynthesis inhibitor, PCR.
