Abstract
Alternaria alternata is the causal agent of Alternaria leaf blotch and Alternaria fruit spots on apple (Malus × domestica Borkh.) in South Tyrol (Northern Italy). Depending on climatic conditions, cultivation area and variety, the symptoms may lead to a substantial quality loss of apples. The active substances boscalid and fluxapyroxad are applied in the plant protection strategy against the Alternaria diseases. However, these fungicides belong to the succinate dehydrogenase inhibitors (SDHI) and according to FRAC, both are at medium to high risk for the development of fungicide resistance. Therefore, several isolates from symptomatic apples sampled in the years 2002 and 2005 (baseline) as well as in 2018 (monitoring) were analysed in vitro for their sensitivity to these actives by using mycelial growth assays. In comparison to the baseline isolates, isolates from 2018 exhibited a statistically significant reduction of sensitivity. Although the genetic characterisation of the sdhB gene, coding for one subunit of the target protein of the SDHI fungicides, showed an increased number of single nucleotide polymorphisms (SNPs), only a few of these mutations were previously reported to reduce the sensitivity of A. alternata against SDHIs. In the present study, only one isolate with reduced sensitivity presented a known amino acid substitution that results in resistance. Thus, other regions of the succinate dehydrogenases subunit genes might be the target for the examined active substances in A. alternata. Additionally, a phylogenetic analysis based on a region of the pgs1 gene revealed the occurrence of two different A. alternata subspecies on the symptomatic fruits, namely A. alternata ssp. tenuissima and A. alternata ssp. arborescens.