Specificity of the actinomycin-D-sensitive function of some RNA plant viruses

The actinomycin-D(AMD)-sensitive step of replication of three plant viruses - cowpea chlorotic mottle virus (CCMV), cowpea mosaic virus, and the cowpea strain of tobacco mosaic virus - in cowpea was specific for each virus. Different combinations of the viruses were examined in which leaves were inoculated with one virus and incubated for 48 h until multiplication was almost resistant to AMD. At that time, the same leaves were inoculated with a different virus and the ability of AMD to inhibit multiplication of the second virus was determined at intervals afterwards. In all combinations tested, the first virus had no effect on the sensitivity of the second virus to AMD. Also, resistance to AMD, which appeared to spread from cell to cell in advance of the virus in mechanically inoculated leaves, did not spread appreciably faster than CCMV into systemically infected upper leaves.