Barley cultivation is commonly challenged by powdery mildew epidemics caused by the biotrophic fungus Blumeria graminis f. sp. hordei (Bgh). Immunity to Bgh in domesticated barley is traditionally conferred by the introgression of allelic Mildew locus A (Mla) resistance specificities encoding coiled-coil domain-containing NLR (CNL) immune receptors from wild barley. MLAs are activated upon the recognition of strain-specific Bgh effector proteins (AVRAs) that are delivered into host cells and are mainly recognised by the leucine rich repeat (LRR) domain of the receptor. This apparently direct receptor-effector association triggers immunity to Bgh, which is intimately associated with a host cell death response. AVRa effector genes in the pathogen population undergo rapid evolutionary sequence diversification, leading to a rapid collapse of strain-specific Mla-mediated immunity through population-level Bgh selection of variants that escape receptor recognition but retain a conserved structural effector scaffold. Resolving the structures of multiple MLA-AVRA heterocomplexes by cryo-electron microscopy is expected to reveal receptor residues necessary for AVRA recognition by their cognate MLAs and might provide a blueprint to design synthetic receptors capable of recognising effectors that are undetected and conserved between barley and wheat-infecting powdery mildew. I will present the cryo-EM structure of a MLA13-AVRA13 heterocomplex and show by structure-directed mutagenesis that only very few polymorphic receptor residues are required for cell death activation. This offers new opportunities for the rational development of MLAs designed for broad and durable resistance to Bgh in cereal crops.