STRAINS of various virus diseases are now well recognized by repjlch workers. B. Kassanis and Ireson W. Selman(J. Pomol. Hort. Sci., 23, Nos. 3, 4 ; Dec 1947) now demonstrate two strains in a host plant (White Burley tobacco), which are barely distinguishable morphologically, but react differently to virus infection. Tomato aucuba mosaic produces necrotic local lesions or systemic necrosis in one host strain, and a yellow mottle in the other. Some strains of tobacco mosaic also react necrotically in the first -mentioned host strain. This unsuspected selective action of the host may possibly explain discrepancies in etiology reported by various workers. The behaviour of various strains of potato virus Y has also been studied by F. C. Bawden and B. Kassanis (Ann. Appl BioL., 34, No. 4, 503–516 ; Dec. 1947). These various strains cause diseases in the variety Majestic, ranging in severity from mild mosaic to leaf-drop streak. Symptoms of the strains are also compared with the serologically related potato virus (C, and possible mechanisms for the evolution of viruses C and Y are discussed.