Cross-kingdom RNA trafficking and environmental RNAi for powerful innovative pre- and post-harvest plant protection

•Small RNAs can transfer from pathogens or parasites into their interacting hosts.•Plant and animal hosts can deliver small RNAs into interacting pathogens and parasites.•External RNAs can be taken up by fungal cells and plant cells and induce RNAi.•Spray-induced gene silencing represents an innovative disease control tool. Small RNA (sRNA) induces RNA interference (RNAi) in almost all eukaryotes. While sRNAs can move within an organism, they can also move between interacting organisms to induce gene silencing, a phenomenon called ‘cross-kingdom RNAi’. Some sRNAs from pathogens or pests move into host cells and suppress host immunity in both plants and animals; whereas some host sRNAs travel into pathogen/pest cells to inhibit their virulence. Moreover, uptake of exogenous RNAs from the environment was recently discovered in certain fungal pathogens, which makes it possible to suppress fungal diseases by directly applying pathogen–targeting RNAs on crops and post-harvest products. This new-generation of RNA-based fungicides is powerful, environmentally friendly, and can be easily adapted to control multiple diseases simultaneously.