Resistance evolution and local adaptation of Venturia inaequalis to old and new generation SDHI fungicides

The use of succinate dehydrogenase inhibitor (SDHI) fungicides is widespread in controlling apple scab disease caused by Venturia inaequalis. However, resistance in populations develops and new SDHIs are generated commercially, thus evaluating this arms-race in local populations is important for appropriate disease management. In this study, in vitro mycelial growth effects of five commercial fungicides, including old and new generation SDHIs on a large V. inaequalis population in Türkiye were investigated. Overall, the entire population was resistant to Boscalid (Cantus®), and the most effective fungicides tested were Isopyrazam (REFLECT®) and Pydiflumetofen (Candidate fungicide, Miravis®). Although the results show the effectiveness of new generation SDHIs, remarkable differences in pathogen resistance responses were detected at local level. There were also differences in colony growth rates among isolates, but this was not associated with fungicide response traits. Furthermore, the intraspecies genetic diversities obtained from the sequence data of the four selected gene regions were depicted with the Neigbour_Joining (NJ) trees. There appeared to be no relationship between NJ-based branching and fungicide response phenotypes. As a result of mating type determinationof the isolates with newly designed primers, 1:1 ratio indicated possible recombination in almost all local groups, with one exception. Additionally, detection of both mating type genes in some isolates was an interesting finding about the reproductive structure of the pathogen. This study, which is the first comprehensive study of V. inaequalis in Türkiye, presents new findings about the pathogen from different perspectives.