Strains of potato virus Y that: were not clearly differentiated from each other by their reaction on potato varieties were shown to be separate entities by the differing reactions that they caused in several other solanaceous species. One strain did not infect Physalis floridana, but this same strain caused necrosis and a severe distortion of the young leaves of Nicotiana sanderae. After infection with certain other strains, N. rustica showed leaf-drop streak symptoms; this observation has not been previously reported. An avirulent strain in all potato varieties and seedlings tested so far caused a severely necrotic disease in N. tabacum and other Nicotiana species.