Botrytis cinerea is a notorious plant pathogen that can only be effectively controlled by chemical fungicides. With the aim to reduce fungicide application, new control strategies are in need. Cross-kingdom RNA interference (RNAi) is an emerging field in plant-pathogen research. Uncovering the key factors to understand the molecular mechanisms and functions in cross-kingdom RNAi is fundamental to develop innovative RNA-based strategies for crop protection. B. cinerea produces extracellular small RNAs to induce natural cross-kingdom RNAi in plant hosts for infection. In this study, we describe the B. cinerea BcRDR1 as a novel pathogenicity factor that is required for small RNA production and cross-kingdom RNAi. Establishing an advanced GFP-based switch-on reporter expressed transgenic plants allowed us to visualize and record the dynamics of cross-kingdom RNAi during infection. We transformed this newly acquired information to design pathogen small RNA-specific target mimics, so-called short tandem target mimics, to express in transgenic plants, which led to reduced disease severity. Our study not only expanded the knowledge of the molecular functions in cross-kingdom RNAi, but also showcased how such gained knowledge can be instrumental to interfere with pathogen cross-kingdom RNAi for a better pathogen control.