QoI Fungicide Resistance Mutations in Alternaria solani and Alternaria alternata are Fully Established in Potato Growing Areas in Bavaria and Dual Resistance against SDHI Fungicides is Upcoming

Alternaria leaf spots caused by Alternaria solani and Alternaria alternata are a major foliar disease of potatoes. Reduced sensitivity of A. solani and A. alternata towards Quinone outside inhibitors (QoI) fungicides has been observed in Europe. QoI-insensitive A. alternata isolates carry a G143A amino acid exchange caused by a single nucleotide polymorphism (SNP) in the cytochrome b gene. A. solani evolved a similar F129L mutation. A shift from the predominant A. solani genotype I to genotype II, which was exclusively associated with the F129L mutation, was observed in Germany after QoI approval for Alternaria control. Here, we found QoI mutations to be highly abundant in A. solani and A. alternata field isolates, collected in 2016 in Bavaria, located in southeastern Germany. The frequency of the F129L mutation, but not of A. solani genotype II, was higher than in the last available study. This might be due to that we also found the F129L mutation in genotype I, indicating QoI resistance progression through the previously unaffected genotype. In contrast to previous data, QoI mutations were present in all examined areas. An analysis of SNP diversity pointed to the region of Lower Bavaria as a hotspot for F129L mutation evolution. Reduced sensitivity of Alternaria spp. towards succinate dehydrogenase inhibitors (SDHI) fungicides is an emerging issue in Europe. We observed the presence of SDHI mutations, but only in combination with QoI mutations, indicating a further adaption to applied fungicides and a selection for dual fungicide resistance.