Phytophthora infestans causes late blight disease on potato and is controlled by heavy fungicide spraying, raising environmental concerns. Thus, there is an urgent need to develop alternative means for late blight control. RNA interference (RNAi) is a conserved cellular defense mechanism mediated by double-stranded RNA (dsRNA) and small RNAs (sRNA) that target specific messenger RNAs for destruction, thereby regulating protein expression. This project builds on our recent discovery that spraying plant surfaces with dsRNAs that target essential P. infestans genes can confer efficient protection against late blight disease. Also called spray-induced gene silencing (SIGS), this strategy is environmentally friendly. We aim to understand how small RNAs are transported during P. infestans infection of potato and to use this knowledge to improve SIGS to control late blight disease. The research question addressed in this proposal is novel and will significantly increase our knowledge of the role of RNAi gene-silencing in plant disease.