Some receptor-like kinases (RLK) control plant development while others regulate immunity. The Arabidopsis ERECTA (ER) RLK regulates both biological processes. To discover specific components of ER-mediated immunity, a genetic screen was conducted to identify suppressors of erecta (ser) susceptibility to Plectosphaerella cucumerina fungus. The ser1 and ser2 mutations restored disease resistance to this pathogen to wild-type levels in the er-1 background but failed to suppress er-associated developmental phenotypes. The deposition of callose upon P. cucumerina inoculation, which was impaired in the er-1 plants, was also restored to near wild-type levels in the ser er-1 mutants. Analyses of er cell walls revealed that total neutral sugars were reduced and uronic acids increased relative to those of wild-type walls. Interestingly, in the ser er-1 walls, neutral sugars were elevated and uronic acids were reduced relative to both er-1 and wild-type plants. The cell-wall changes found in er-1 and the ser er-1 mutants are unlikely to contribute to their developmental alterations. However, they may influence disease resistance, as a positive correlation was found between uronic acids content and resistance to P. cucumerina. We propose a specific function for ER in regulating cell wall-mediated disease resistance that is distinct from its role in development.