Fungal pathogens are significant plant-destroying microorganisms that present an increasing
threat to the world’s crop production. Chitin is a crucial component of fungal cell walls
and a conserved MAMP (microbe-associated molecular pattern) that can be recognized by specific
plant receptors, activating chitin-triggered immunity. The molecular mechanisms underlying the
perception of chitin by specific receptors are well known in plants such as rice and Arabidopsis thaliana
and are believed to function similarly in many other plants. To become a plant pathogen, fungi
have to suppress the activation of chitin-triggered immunity. Therefore, fungal pathogens have
evolved various strategies, such as prevention of chitin digestion or interference with plant chitin
receptors or chitin signaling, which involve the secretion of fungal proteins in most cases. Since
chitin immunity is a very effective defensive response, these fungal mechanisms are believed to work
in close coordination. In this review, we first provide an overview of the current understanding of
chitin-triggered immune signaling and the fungal proteins developed for its suppression. Second,
as an example, we discuss the mechanisms operating in fungal biotrophs such as powdery mildew
fungi, particularly in the model species Podosphaera xanthii, the main causal agent of powdery mildew
in cucurbits. The key role of fungal effector proteins involved in the modification, degradation, or
sequestration of immunogenic chitin oligomers is discussed in the context of fungal pathogenesis
and the promotion of powdery mildew disease. Finally, the use of this fundamental knowledge for
the development of intervention strategies against powdery mildew fungi is also discussed
