The relationships of some viruses causing necrotic diseases of the potato

Potato virus B, and some other viruses with reactions in potato varieties different from any previously described, are strains of virus X. All produce intracellular inclusions which vary with different hosts and virus strains. Except with virus B, the inclusions are larger and more frequent in potato than in tobacco or tomato. All give systemic infection when inoculated to tobacco, tomato and potato varieties in which they are carried or cause mosaic symptoms; some give systemic infection when inoculated to varieties in which they cause top-necrosis, whereas others give only local lesions. Potato virus C is a strain of virus Y: in tobacco and a few potato varieties both produce similar symptoms, but in those varieties in which Y causes leaf-drop streak, C causes top-necrosis. C causes systemic infection when inoculated to tobacco and to potato varieties in which it causes mosaic symptoms, but not when inoculated to potato varieties in which it causes top-necrosis. Virus C was not transmitted by M. persicae. Viruses C and Y produce a few small intracellular inclusions in potato and tobacco. Virus A is not related to Y or X: no inclusions were found in plants infected with A alone.