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SPECIAL SESSION: The Promise And Limitations of SDHI fungicides - Panel Discussion

Assessing the risk of resistance towards SDHI fungicides in France
Anne-Sophie Walker - INRA Bioger.

Succinate dehydrogenase inhibitor (SDHI) fungicides have been registered since 2006 on a large number of crops in France and resistance has been detected in nine fungal species, including Botrytis cinerea, Ustilago nuda, Helminthosporium teres, Zymoseptoria tritici and Sclerotinia sclerotiorum. In most cases, resistance is associated with a wide variety of mutations in SDHI subunits B, C, or D. Depending on the mutation and the mutation location, contrasting levels of resistance and cross-resistance spectra are observed for SDHIs. Moreover, contrasted resistance costs (e. g. in B. cinerea) were also measured, depending on the mutation.

In Z. tritici, the pathogen responsible for septoria leaf blotch on wheat, resistance is associated with two distinct mechanisms, not yet combined in populations. Increased efflux (leading to multidrug resistance or MDR) generates low levels of resistance to several modes of action, while target site-based resistance is associated with low to high resistance factors. Crosses between isolates carrying the two distinct mechanisms revealed semi-isogenic progeny that we used to characterize resistance. Progeny with both mechanisms of resistance had increased resistance factors and limited fitness penalty with regard to sexual and asexual traits. Population studies showed that resistance based on mutations in SDHI subunits is emerging in French populations and that MDR also evolves rapidly. This ultimately increases the risk of natural recombination by sexual reproduction between isolates carrying the two distinct mechanisms. Field trials identified treatment strategies most favorable for the selection of each mechanism and thus allowed for the design of specific resistance management strategies with regard to SDHI use.