Foliar application of double-stranded RNA (dsRNA) as RNA interference (RNAi)-based biopesticides represents a sustainable alternative to transgenic, breeding or chemical-based crop protection strategies. A key feature of RNAi in plants is its ability to act non-cell autonomously, a process that plays a critical role in plant development and protection against pathogens. However, whether RNAi induced by foliar dsRNA application can act non-cell autonomously in whole plants remains debated and the potential mechanisms and implications of this movement largely unexplored. Here we show that upon foliar application, dsRNA enters the leaf vasculature and rapidly moves to vegetative, reproductive, and below ground tissue types in model plant and crop hosts. Unprocessed dsRNA was detected in the apoplast of distal tissue types and maintained in subsequent new growth, indicating apoplastic rather than symplastic transport. Furthermore, we show that mobile dsRNA is functional against root-infecting fungi. We found that mobile dsRNA can transfer to the fungus where it is processed and loaded by the fungal RNAi machinery to elicit gene silencing. Using a novel biochemical purification technique and small RNA sequencing we diagnosed functional siRNA species derived from foliar applied dsRNA that elicit effective silencing in both the applied and distal infected tissue types. Our mechanistic dissection of the uptake and maintained movement of dsRNA provides crucial insights into the mode of action of RNAi biopesticides and stands to add significant benefit to this emerging field of plant protection.