Xanthomonas albilineans (Xal) and Xylella fastidiosa (Xf) are xylem-limited bacteria causing leaf scald of sugarcane and Pierce’s disease in grapevines, respectively. Hydrogen peroxide (H₂O₂) is essential for protoxylem differentiation and cell wall lignification during xylem maturation, resulting in an oxidative environment in differentiated (dead) xylem vessels. We hypothesize that xylem-limited pathogens such as Xal and Xf must therefore breach an intrinsic oxidative burden for successful colonization. Genes encoding a bifunctional heme-dependent catalase and broad spectrum peroxidase (E. coli katG homologs) are annotated in both Xal and Xf genomes. The Xal katG (VB87_RS05200) was replaced by an enhanced Green florescent protein gene (gfp) via marker exchange. Xal ΔkatG/gfp mutant strains exhibited significantly reduced pathogenicity on newly emerged sugarcane leaves and compromised viability in planta as compared to wild type Xal. Similarly, Xf katG (PD_RS06770) was replaced by nptII via marker exchange. Challenge inoculations of Vitis vinifera grapevines with Xf ΔkatG/nptII mutant strains exhibited only limited disease symptoms near the inoculation sites. The pathogenicity of the Xal ΔkatG/gfp and Xf ΔkatG/nptII mutant strains were completely restored by complementation with full-length Xal katG (cloned in pUFR047) and Xf katG (cloned in pBBR1MCS-5). Since Xf has only a single peroxidase gene, it is being evaluated as a target for chemical control.