The SDHI (succinate dehydrogenase inhibitor) fungicides belong to FRAC group 7 which have been on the market since the late 1960s. Third generation SDHIs have been available since 2003 with release of boscalid (penthiopyrad). Examples of FRAC group 7 fungicides used to control important vegetable diseases include: flutolanil ( year introduced,1986), boscalid (2003), penthiopyrad (2008), fluxapyroxad (2011), fluopyram (2013), and pydiflumetofen (2016). All fungicides in FRAC group 7 inhibit complex II of the fungal mitochondrial respiration by binding and blocking SDH-mediated electron transfer from succinate to ubiquinone. The SDHI fungicides work much like the FRAC group 11 fungicides, just at a different site in mitochondrial respiration. Much like FRAC group 11 fungicides, they are also at-risk for fungicide resistance development because of their specific modes of action. Research has shown there are numerous single point mutations that can lead to resistance development to FRAC group 7 fungicides. Although all SDHI fungicides share the same target site, sensitivity to the different fungicides within the FRAC group may vary. Thus, this variation in sensitivity among different SDHI fungicides leads to confusion on what the term “cross-resistance” means. With cross-resistance, once a pathogen develops resistance to one fungicide within the FRAC group, it becomes resistant to all others (e.g., strobilurin resistance in FRAC group 11 fungicides). However, in FRAC group 7 fungicides, there seems to be differences in sensitivity between fungicides within the group after resistance has been detected in one particular fungicide. The good news is that other FRAC group 7 fungicides may retain there efficacy even if resistance is detected in one particular fungicide. “The practical implications for resistance management would be the recommendation of mixtures of SDHIs, alternations of SDHIs, or even the substitution of members of the SDHI fungicide class. However, this would be counterproductive in protecting this mode of action.” (Klappach and Stammler, 2019). The important point, switching exclusively to another SDHI fungicide after resistance has been found in one is not a good resistance management strategy. As with all fungicide resistance management strategies, growers should focus on rotating as many different FRAC groups into their fungicide programs as possible, and limit the total amount of any one FRAC group during the production season.
Additional resources:
Theories on tank mixing and rotating fungicides in different FRAC groups
