CROSS PROTECTION AND INTERFERENCE WITH SOME STONE-FRUIT VIRUSES

Abstract

One-year-old virus-free sour cherry trees of the variety Montmorency were inoculated singly with buds from different source trees infected with sour cherry yellows, necrotic ring spot, prune dwarf, and tatter leaf respectively. The infections thus established in the test trees were later challenged by inoculation with isolates from different sources. Conspicuous recurrent symptoms occurred when infection from a tatter leaf isolate was challenged by either of two necrotic ring spot virus strains, but not when the order of challenge was reversed. These symptoms are interpreted as a synergistic response to infection by two viruses, one from each source, within a limited range of interaction. Trees previously infected with necrotic ring spot when reinoculated with cherry yellows isolates showed symptoms of cherry yellows, usually preceded by secondary shock. The reciprocal challenges, on the other hand, induced no symptoms of necrotic ring spot, probably because each of the cherry yellows sources carried a strain of the necrotic ring spot virus. There were indications, however, of interference between the two viruses. Infection with the prune dwarf virus did not protect against subsequent infection with the cherry yellows virus. Mixed infections with the necrotic ring spot and prune dwarf viruses did not cause cherry yellows.