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Sources and Dissemination of Potato Viruses in the Columbia Basin of the Northwestern United States. P. E. Thomas, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Irrigated Agriculture Research and Extension Center, Prosser, WA 99350. Plant Dis. 67:744-747. Accepted for publication 4 January 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/PD-67-744.

The incidence of efficiently aphid-transmitted potato leafroll virus (PLRV) and potato virus Y (PVY) increased 1,600 and 2,000%, respectively, during the growing season in potato fields of the Columbia Basin, whereas the incidence of viruses more dependent upon mechanical transmission, potato viruses X (PVX) and S (PVS), increased only 600 and 350%, respectively. Volunteer potato plants were implicated as the chief sources of virus vectors and of inoculum, accounting for the increases in incidence of PLRV and PVY. The volunteers arose from tubers missed during harvest of the previous year, often growing profusely in fields planted to wheat or corn following potatoes in the cropping rotation and had the same virus incidence as fall-harvested tubers. Large numbers of aphids were present on volunteers in wheat and cornfields but not in potato fields by mid-June. Yet, an abrupt increase in PLRV infection began 5 or 6 wk later, a period equal to the incubation period for PLRV in mature potato plants. Only a massive influx of viruliferous aphids supplied through the midsummer aphid migration that characteristically occurs in the Columbia Basin could account for such an increase in PLRV infection, and volunteer plants in neighboring grain fields were the only outside source of aphids and virus. Most PLRV infection was concentrated in a few fields and data indicate the difference between good and poor control may be attributable to a single management practice. The midsummer influx of vectors was also necessary for the increase in PVY infection, but seed tubers may have been a more important source of inoculum for PVY. Most seed lots were totally infected or had about 50% PVS, but a few were PVS-free. Most seed lots were free of PVX or had about 25% PVX, but a few were totally PVX-infected.