ABSTRACT The barley powdery mildew fungus, Blumeria hordei ( Bh ), secretes hundreds of candidate secreted effector proteins (CSEPs) to facilitate pathogen infection and colonization. One of these, CSEP0008, is directly recognized by the barley nucleotide-binding leucine-rich-repeat (NLR) receptor, MLA1, and therefore designated AVR A1 . Here we show that AVR A1 and the sequence-unrelated Bh effector BEC1016 (CSEP0491) suppress immunity in barley. We used yeast two-hybrid next-generation interaction screens (Y2H-NGIS), followed by binary Y2H and in planta protein-protein interactions studies, and identified a common barley target of AVR A1 and BEC1016, the endoplasmic reticulum (ER)-localized J-domain protein, Hv ERdj3B. Silencing of this ER quality control (ERQC) protein increased the Bh penetration. Hv ERdj3B is ER luminal, and we showed using split GFP that AVR A1 and BEC1016 translocate into the ER - signal peptide-independently. Silencing of Hv ERdj3B and expression the two effectors hampered trafficking of a vacuolar marker through the ER as a shared cellular phenotype, agreeing with the effectors targeting this ERQC component. Together, these results suggest that the barley innate immunity, preventing Bh entry into epidermal cells, is dependent on ERQC, which in turn requires the J-domain protein, Hv ERdj3B, regulated by AVR A1 and BEC1016. Plant disease resistance often occurs upon direct or indirect recognition of pathogen effectors by host NLR receptors. Previous work has shown that AVR A1 is directly recognized in the cytosol by the immune receptor, MLA1. We speculate that the AVR A1 J-domain target being inside the ER, where it is inapproachable by NLRs, has forced the plant to evolve this challenging direct recognition. SIGNIFICANCE The complex plant immune system is highly dependent on fundamental cellular machineries, such as the endomembrane system and the ER quality control (ERQC), essential for delivery of immunity-associated membrane-bound and endomembrane soluble proteins to their destinations. We now find that pathogen effectors can interact with an ERQC component and suppress immunity, thereby adding to the molecular insight in plant-pathogen interactions.