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Plants trigger immune responses upon recognition of fungal cell wall chitin, followed by the release of various antimicrobials, including chitinase enzymes that hydrolyze chitin. In turn, many fungal pathogens secrete LysM effectors that prevent chitin recognition by the host through scavenging of chitin oligomers. We previously showed that intrachain LysM dimerization of the Cladosporium fulvum effector Ecp6 confers an ultrahigh-affinity binding groove that competitively sequesters chitin oligomers from host immune receptors. Additionally, particular LysM effectors are found to protect fungal hyphae against chitinase hydrolysis during host colonization. However, the molecular basis for the protection of fungal cell walls against hydrolysis remained unclear. Here, we determined a crystal structure of the single LysM domain-containing effector Mg1LysM of the wheat pathogen Zymoseptoria tritici and reveal that Mg1LysM is involved in the formation of two kinds of dimers; a chitin-dependent dimer as well as a chitin-independent homodimer. In this manner, Mg1LysM gains the capacity to form a supramolecular structure by chitin-induced oligomerization of chitin-independent Mg1LysM homodimers, a property that confers protection to fungal cell walls against host chitinases. Author summary: Chitin plays a central role in plant-fungi interactions, since it is a major component of the fungal cell wall that is targeted by host hydrolytic enzymes to inhibit the growth of fungal pathogens on the one hand, and release chitin fragments that are recognized by host immune receptors to activate further immune responses on the other hand. In turn, many fungal pathogens secrete chitin binding LysM effectors to which currently two functions have been assigned. Most LysM effectors that were functionally characterized to date function to prevent chitin recognition by host immune receptors through chitin sequestration. Additionally, some LysM effectors were shown to protect fungal hyphae against hydrolysis by host chitinases. The crystal structure of Mg1LysM from the Septoria blotch pathogen of wheat, Zymoseptoria tritici, revealed that chitin-induced dimerization of two Mg1LysM protomers through high affinity binding is required for hyphal protection against chitinases. Since Mg1LysM also forms ligand-independent homodimers, a supramolecular structure can be formed in which chitin-induced oligomerization of Mg1LysM ligand-independent homodimers form a contiguous Mg1LysM higher ordered structure that is anchored to the chitin in the fungal cell wall to prevent hydrolysis by host chitinases. [ABSTRACT FROM AUTHOR]