A disease is a physiological process. Hence, die study of plant disease is concerned with abnormal plant physiology (Whetzel, 1929). Consequently, the study of resistance examines how a plant can slow down or slop the disease process. Resistant plants are those which do not become infected, which do become infected but at a lower rate, or which develop less severe symptoms than other genotypes. The resistance to viruses may. hypothetically, interfere with any of the steps in the virus infection cycle: entry of the virus or disassembly, replication. movement within and between cells, and transport from the site of initial infection to other parts of the plant.