Key message CRIB motif-containing barley RIC157 is a novel ROP scaffold protein that interacts directly with barley RACB, promotes susceptibility to fungal penetration, and colocalizes with RACB at the haustorial neck. Abstract Successful obligate pathogens benefit from host cellular processes. For the biotrophic ascomycete fungus Blumeria hordei ( Bh ) it has been shown that barley RACB, a small monomeric G-protein (ROP, Rho...