ABSTRACT
When a plant already infected with one strain of a virus is inoculated with another strain of the same virus, symptoms due to the challenging strain usually fail to appear. Tobaccos inoculated with a strain of tobacco mosaic virus (McKinney, 1929) or potato virus X (Salaman, 1933) did not develop any additional symptoms when inoculated later with another strain of the respective virus. This phenomenon, known as cross-protection, is one of the biological properties of the viruses used to establish relationships among viruses and their strains.
