Understanding resistance to oxysterol binding protein inhibitor fungicides

Downy mildews and late blight are important crop diseases caused by oomycetes which often require the use of fungicides with a cost of control estimated at $2 billion annually. In recent years, the crop protection industry has been rather successful in discovering and introducing plant protection products targeting oomycetes, however some species have developed resistance to nearly all fungicides with a single-site mode of action. Oxathiapiprolin and fluoxapiprolin are the most recently introduced oomycete fungicides and target oxysterol binding proteins (OSBP). The risk of resistance to OSBP inhibitors has been determined by combining risk values for the fungicides, pathogens, and agronomic systems using an assessment matrix. Resistance management guidelines for this mode of action include a strict limitation to the number of fungicide applications, preventative application timing and use exclusively in mixture with other fungicides having a different mode of action. In the context of regulatory pressure on older fungicides, especially those with multi-site activity, the observance of the FRAC OSBPI guidelines combined with Integrated Pest Management practices will be critical to protect the effectiveness of these valuable new tools.