Cereal powdery mildew effectors: a complex toolbox for an obligate pathogen

[Display omitted] •Cereal powdery mildews have evolved a large repertoire of candidate effector proteins.•These effectors are encoded within size variable and highly diversified gene families.•Mildew effectors can act as virulence factors, Avr genes and Avr suppressors.•Their mode of action provides a molecular basis for the Zig-Zag evolutionary model. Cereal powdery mildews are major pathogens of cultivated monocot crops, and all are obligate biotrophic fungi that can only grow and reproduce on living hosts. This lifestyle is combined with extreme host specialization where every mildew subspecies (referred to as forma specialis) can only infect one plant species. Recently there has been much progress in our understanding of the possible roles effectors play in this complex host–pathogen interaction. Here, we review current knowledge on the origin, evolution, and mode of action of cereal mildew effectors, with a particular focus on recent advances in the identification of bona fide effectors and avirulence effector proteins from wheat and barley powdery mildews.