Oomycete phytopathogens such as Phytophthora capsici cause destructive crop diseases yet remain poorly addressed by current fungicides. Here, we present copper-doped carbon dots (Cu-CDs) as a multifunctional nanomaterial that couples catalytic singlet oxygen (1O2) generation with plant immune activation for effective oomycete control. Cu-CDs, synthesized by a one-step hydrothermal route from salicylic acid, urea, and copper chloride, exhibit an ultrasmall average diameter of 2.14 nm and strong 1O2 -generating activity. Cu-CDs displayed potent anti-oomycete activity against P. capsica, completely inhibiting mycelial growth at 200 μg mL−1 and disrupting multiple developmental stages of the pathogen. In planta, post-infection application of Cu-CDs reduced detached-leaf lesion area to 5.2% of the infected control and lowered whole-plant disease incidence to 16.6%. Beyond direct pathogen inhibition, Cu-CDs also activated host immunity, including a 211.3-fold induction of CaPR1, consistent with strong systemic acquired resistance. Importantly, these protective effects were achieved without detectable photosynthetic penalty. This work establishes Cu-CDs as a dual-action nano-oomycide that integrates catalytic pathogen eradication with host immunity priming, highlighting a nanotechnology-enabled strategy for sustainable crop protection.