Powdery mildew (Golovinomyces cichoracearum), one of the most prolific obligate biotrophic fungal pathogens worldwide, infects its host by penetrating the plant cell wall while avoiding the activation of the plant’s innate immune system. The Arabidopsis mutant powdery mildew resistant5 (pmr5) carries a mutation in a putative pectin acetyltransferase gene, which confers enhanced resistance to powdery mildew. Here, we show that heterologously expressed PMR5 protein transfers acetyl groups from [(14) C]- acetyl-CoA to oligogalacturonides. Through site-directed mutagenesis, we show that three amino acids within a highly conserved esterase domain in putative PMR5 orthologs are necessary for PMR5 function. A suppressor screen of mutagenized pmr5 seed selecting for increased powdery mildew susceptibility identified two previously characterized genes affecting acetylation of plant cell wall polysaccharides, RWA2 and TBR. The rwa2 and tbr mutants also suppress powdery mildew disease resistance in pmr6, a mutant defective in a putative pectate lyase gene. Cell wall analysis of pmr5, pmr6, and their rwa2 and tbr suppressor mutants, demonstrates minor shifts in cellulose and pectin composition. In direct contrast to their increased powdery mildew resistance, both pmr5 and pmr6 plants are highly susceptibility to multiple strains of the generalist necrotroph Botrytis cinerea and have decreased camalexin production upon B. cinerea infection. These results illustrate that cell wall composition is intimately connected to fungal disease resistance, and outline a potential route for engineering powdery mildew resistance into susceptible crop species. This article is protected by copyright. All rights reserved.