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Corynespora leaf spot caused by <i>Corynespora cassiicola</i> is one of the major diseases in cucumber (<i>Cucumis sativus</i> L.). However, the resistance mechanisms and signals of cucumber to <i>C. cassiicola</i> are unclear. Here, we report that the mildew resistance locus O (MLO) genes, <i>CsMLO1</i> and <i>CsMLO2</i>, are both negative modulators of the cucumber defense response to <i>C. cassiicola</i>. Subcellular localization analysis showed that CsMLO1 and CsMLO2 are localized in the plasma membrane. Expression analysis indicated that the transcript levels of <i>CsMLO1</i> and <i>CsMLO2</i> are linked to the defense response to <i>C. cassiicola</i>. Transient overexpression of either <i>CsMLO1</i> or <i>CsMLO2</i> in cucumber cotyledons reduced resistance to <i>C. cassiicola</i>, whereas silencing of either <i>CsMLO1</i> or <i>CsMLO2</i> enhanced resistance to <i>C. cassiicola</i>. The relationships of pathogenesis-related proteins, reactive oxygen species (ROS)-associated genes, and abscisic acid (ABA)-related genes to the overexpression and silencing of <i>CsMLO1</i>/<i>CsMLO2</i> in non-infested cucumber plants were investigated. The results indicated that <i>CsMLO1</i> mediated resistance against <i>C. cassiicola</i> by regulating the expression of pathogenesis-related proteins and ROS-associated genes, as well as through ABA signaling pathway-associated genes. The <i>CsMLO2</i>-mediated resistance against <i>C. cassiicola</i> primarily involves regulation of the expression of pathogenesis-related proteins. Our findings will guide strategies to enhance the resistance of cucumber to corynespora leaf spot.