Obligate biotrophic pathogens of plants must circumvent or counteract defences to guarantee accommodation inside the host. To do so, they secrete a variety of effectors, which regulate host immunity and facilitate the establishment of pathogen feeding structures called haustoria. The barley powdery mildew fungus, Blumeria graminis f. sp. hordei, produces a large number of proteins predicted to be secreted from haustoria. Fifty of these Blumeria Effector Candidates (BECs) were screened by Host Induced Gene Silencing (HIGS), and eight were identified that contribute to infection. One shows similarity with β-1,3 glucosyltransferases, one with metallo-proteases, and two with microbial secreted ribonucleases; the remainder have no similarity to proteins of known function. Transcript abundance of all eight BECs increases dramatically in the early stages of infection and establishment of haustoria, consistent with a role in that process. Complementation analysis using silencing-insensitive synthetic cDNAs demonstrated that the ribonuclease-like BECs, 1011 and 1054 are bona fide effectors that function within the plant cell. BEC1011 specifically interferes with pathogen-induced host cell death. Both are part of a gene superfamily unique to the powdery mildew fungi. Structural modelling was consistent with BEC1054 adopting a ribonuclease-like fold, a scaffold not previously associated with effector function.