Cross-protection is a phenomenon in which a plant that is infected with a virus becomes immune to a secondary infection by the same or related viruses. Although molecular mechanisms underlying this phenomenon are not completely understood, cross-protection induced by an attenuated strain with mild symptoms has been successfully used to prevent damage by more severe strains. In the development and selection of an effective attenuated strain among candidate isolates, evaluating their infectivity and efficiency of cross-protection is important. We describe two protocols to check the infection efficiency and distribution in a plant based on immunostaining results. In addition, a practical inoculation method that uses a spray gun to apply attenuated viruses to a large number of seedlings is presented.
