Fungal diseases collectively termed “powdery mildew” afflict a broad range of plant species, including agriculturally relevant cereals such as barley, and result in significant reductions in crop yield. Fungi that cause powdery mildew deliver so-called effector molecules inside plant cells, where they manipulate the host’s physiology and immune system. In response, some plants have developed Resistance (R) genes, usually intracellular immune receptors, which recognize the infection by detecting the fungus’ effectors, often leading to plant cell death at the site of attempted infection to limit the spread of the fungus. In the prevailing view, direct recognition of effectors by immune receptors is rather a rare event in plant-pathogen interactions, however, and it has been thought instead that in most cases recognition proceeds via other host proteins that are modified by the pathogen.