A method of non-covalently loading a plant picornavirus is described. The method includes contacting a plant picornavirus in solution with a molar excess of a cargo molecule to load the plant picornavirus with the cargo molecule, and then purifying the loaded plant picornavirus. Examples of cargo molecules include imaging agents, antitumor agents, and antiviral agents. Loaded plant picornaviruses prepared in this manner can be used to delivering cargo molecule to cells.