A 1986 to 1995 survey of beet western yellows luteovirus (BWYV) in western Oregon crop and weed species demonstrated that BWYV was endemic in at least 30 plant species. Because sugarbeet-seed crops (Beta vulgaris L.) were sometimes perceived as the primary western Oregon source of BWYV inoculum, we sought to examine all components of the inoculum reservoir. The array of BWYV hosts in western Oregon provided a year-round supply of BWYV inoculum comprising three interactive sub-reservoirs : (a) widespread annual, winter annual, and perennial weeds, (b) BWYV-susceptible vegetable crops, including garden beet, lettuce, spinach, Swiss chard, and turnip, and (c) sugarbeet-seed crops. We defined zones of high and low BWYV incidence in sugarbeet-seed fields within western Oregon, and monitored incidence of BWYV in a common weed species, Brassica campestris L. (field mustard). We demonstrated abrupt variations in BWYV incidence in sugarbeet-seed plantings in the vicinity of Corvallis near the Willamette River. BWYV incidence in sugarbeet-seed fields increased in localities where sugarbeet-seed crops were grown successively. Although marked differences in seed yields were observed, no reduction in sugarbeet seed yield was attributable to BWYV infection.