Abstract Phytophthora infestans is a hemibiotrophic oomycete pathogen that causes late blight in important crops in the Solanaceae family worldwide, including potato. Infections by this pathogen can lead to substantial yield losses during cultivation and storage. To prevent epidemics, oomyceticides are widely used. In recent years, various researchers reported strains, which were adapted to selection pressures from commonly used carboxylic acid amide (CAA) or oxysterol binding protein inhibitor (OSBPI) oomyceticides. In this study, 75 P. infestans isolates were sampled from six regions in Germany and the Netherlands, where loss of field efficacy of OSBPIs was reported in 2023. Sequencing the osbp -gene from those isolates identified mutations causing relevant amino acid changes (N837F, G770V, N837F + G770V). Such isolates were found at a high cumulative frequency (73%) in the samples analyzed. Additionally, many CAA-adapted isolates with relevant number of copies with the G1105S mutation in the cesA3 -gene were detected (45%). Most worryingly, simultaneous occurrence of mutations in both cesA3 - and osbp -genes were found in nearly all those isolates (44%). The mutations correlated with high resistance factors measured in vitro, confirming the loss of sensitivity to the respective oomyceticide. Moreover, single- and double-resistant isolates could not be controlled by CAA and OSBPI oomyceticides at their recommended dose rates in greenhouse experiments. Thus, only the mixing partners at their respective full dose rate had the potential to control such isolates. Most importantly, farmers and seed potato producers need to implement strict resistance management strategies to prevent the spread of double-resistant isolates of P. infestans to further potato-growing regions in Europe.
Derpmann et al. (Sun,) studied this question.