Characterization of CAA-, OSBPI- and Double Resistant Field Isolates of Phytophthora infestans and their Impact on Late Blight Control in Potatoes

Phytophthora infestans is a hemibiotrophic oomycete pathogen that causes Late Blight disease in important Solanaceae crops worldwide, including potatoes. Infections by this pathogen can lead to substantial yield losses during cultivation and storage. To prevent local infections leading to epidemics, oomyceticides are widely used. In recent years, various researchers reported about 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 generated from regions in Germany and the Netherlands, from which loss of field efficacy was reported in 2023. Sequencing the osbp -gene from those isolates identified mutations causing relevant amino acids changes (N837FL, G770V, N837F + G770V). Such isolates were found at high cumulative frequencies in the samples analyzed with 73%. Additionally, many isolates with relevant copies of the G1105S in cesA3 -gene were detected (45%). Most worryingly, simultaneous occurrence of mutations in both cesA3 - and osbp -gene were found in nearly all those isolates (44%). Mutations correlated with high resistance factors measured in vitro confirming the loss of sensitivity towards 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 mixing partners at full dose rate have the potential to control such isolates. In conclusion, farmers and seed potato producers need to implement strict resistance management strategies to prevent spread of double resistant isolates of P. infestans to further potato growing regions in Europe.