Cross-protection and interference between electrophoretically distinct strains of cucumber mosaic virus in tomato

Cucumber mosaic virus (CMV) was partially purified by polyethylene glycol precipitation from extracts of tomato leaflets. The presence and quantity of two strains with different electrophoretic mobilities was analyzed by polyacrylamide gel (2.5%) electrophoresis of preparations from as little as 50 mg of tissue. This method was used to analyze interference and cross-protection between the strains. Coinoculation resulted in local and systemic mixed infections and reductions in the synthesis of both strains. Systemic symptoms and accumulation of the strain with the more severe symptoms were not detected in up to 13 newly formed leaves in five of six plants preinfected with the mild strain and challenged with the severe strain. Systemic accumulation of the mild strain was not detected in each of six plants preinfected with the severe strain and challenged with the mild strain. Both strains could accumulate in challenge-inoculated leaves. The level or absence of interference in these leaves did not affect systemic cross-protection. Cross-protection between these strains of CMV involves considerable inhibition of virus accumulation in addition to absence of symptoms.