Loss-of-function mutations in the EDR1 gene of Arabidopsis confer enhanced resistance to Golovinomyces cichoracearum (powdery mildew). Disease resistance mediated by the edr1 mutation is dependent on an intact salicylic acid (SA) signaling pathway, but edr1 mutant plants do not constitutively express the SA-inducible gene PR-1 and are not dwarfed. To identify other components of the EDR1 signaling network, we screened for mutations that enhanced the edr1 mutant phenotype. Here, we describe an enhancer of edr1 mutant, eed3, which forms spontaneous lesions in the absence of pathogen infection, constitutively expresses both SA- and methyl jasmonate (JA)–inducible defense genes, and is dwarfed. Positional cloning of eed3 revealed that the mutation causes a premature stop codon in GLUCAN SYNTHASE-LIKE 5 (GSL5, also known as POWDERY MILDEW RESISTANT 4), which encodes a callose synthase required for pathogen-induced callose production. Significantly, gsl5 single mutants do not constitutively express PR-1 or AtERF1 (a JA-inducible gene) and are not dwarfed. Thus, loss of both EDR1 and GSL5 function has a synergistic effect. Our data suggest that EDR1 and GSL5 negatively regulate SA and JA production or signaling by independent mechanisms and that negative regulation of defense signaling by GSL5 may be independent of callose production.