The Spread of Beet Necrotic Yellow Vein Virus from Point Source Inoculations as Influenced by Irrigation and Tillage
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A 3-year study was initiated in 1992 to map the spread of beet necrotic yellow vein virus (BNYVV) from known point sources of inoculum by irrigation and tillage practices. The experiment each year consisted of four plots, each containing 12-30-m rows, on 76-cm centers. Sugar beet seeds (cv. HH39) coated with BNYVV-infested Polymyxa betae were planted in the first 3 m of the two outside rows of each plot and constituted the point sources of inoculum. The remaining plot area was planted with uninfested seeds. Plots were furrow irrigated every 2 weeks. Plant and soil samples were collected from the point source areas and other predetermined locations in each plot before tillage operations. Plant roots were assayed by indirect double-antibody sandwich enzyme linked immunosorbent assay (DAS-ELISA) for BNYVV incidence. Soil samples were planted with sugar beet seeds in the greenhouse, and after 10 to 12 weeks, roots of bait plants were assayed for BNYVV. To further evaluate movement, soil samples were collected from the previous locations after tillage events and after a second sugar beet crop before being bioassayed. All plant samples taken from point sources exhibited typical symptoms of rhizomania, and both plant and soil samples tested positive for BNYVV in DAS ELISA tests. Out of 336 plants collected in the field from non point source areas, only 1 tested positive. Before tillage events, 2% of the soil samples were positive for BNYVV for the first experiment and 0.9% tested positive the second, suggesting negligible movement of viruliferous P. betae by irrigation. After tillage and harvest operations, 9 and 6% of the soil samples were infested for the first and second experiments, respectively. After a successive sugar beet crop, 15% of the soil samples were positive for BNYVV in the first experiment and 12% in the second. Our results show that physical movement of soil during tillage and harvest operations exert a much greater influence on spread of BNYVV than furrow irrigation, which contradicts the generally accepted concept that viruliferous P. betae is rapidly disseminated by irrigation.