Technical Abstract: Disease resistance genes most commonly used in breeding programs are single, dominant, resistance (R) genes with relative effectiveness influenced by plant developmental stage. Knowing the developmental stages at which an R gene is functional is important for disease management. In rice, resistance at the seedling stage is crucial, as wounding during transplanting increases the potential for bacterial blight (BB) disease, and not all BB R genes are effective in seedlings. Effectiveness of the BB R genes Xa7, xa5, and Xa4 at different developmental ages was evaluated by measuring lesion length and bacterial numbers after inoculation with the bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). The xa5 or Xa4 genes controlled disease at all growth stages. In contrast, Xa7 was not fully functional in very young seedlings, but was completely effective by 21 days after sowing (das). Thus, Xa4, xa5, and Xa7 will be effective at the transplant stage (>21das).