Evidence for interaction or genetic recombination between potato viruses Y and C in infected plants

When potato virus Y (PVY), which is aphid-transmitted, and a related virus, potato virus C (PVC), which is not aphid-transmitted, were jointly inoculated to Nicotiana glutinosa plants, mixed isolates, transmitted by aphids from systemically infected leaves had some of the properties of both. Like PVY they were transmitted by Myzus persicae, but unlike PVY they caused black necrotic lesions on inoculated leaves of Majestic, President, and Craig's Snow-White potatoes ( Solanum tuberosum L.). Like PVY and unlike PVC they caused systemic infection when manually inoculated to the potato varieties, but the symptoms that developed resembled those caused by PVC when it is transmitted to the same varieties by grafting. Many isolates from local lesions caused by the mixed virus isolates on potato were like PVC, and few resembled PVY. Isolates from systemically infected parts of potato less often gave symptoms like those caused by PVC, and many gave mild mottle with no necrosis. Three PVC-like viruses that resemble the isolates from mixed infection are: PVC n, an aphid-transmissible PVC-like virus described previously (Watson, 1956); PVC niab, found naturally occurring in International Kidney potatoes in Jersey, and a virus from India found in Solanum jasminoides (Badami and Kassanis, 1959). These viruses do not usually invade the potato varieties systemically but occasionally yield isolates that do so. The symptoms when they do invade resemble those caused by the isolates from mixed infections in N. glutinosa and often appear to be genetically unstable although the viruses from which they were derived either are stable in potato or, as with PVC n, revert to a form even more like PVC. The nature of the interaction between PVC and PVY is unknown, but a likely explanation is that particles are formed that carry genetic determinants from both viruses.