Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 15, 1997
Publication Date: N/A
Interpretive Summary: Many dairy farmers of the northeastern U.S. rely on rotational grazing in meeting nutritional and energy needs of the milking herd from May through October. White clover is the principal legume species in rotationally grazed pastures. The present survey was undertaken to determine the presence and incidence of viruses in white clover in rotationally grazed pastures of Pennsylvania (PA), New York (NY), and Vermont (VT). The surve covered 24 pastures in 16 PA counties, 5 pastures in 5 NY counties, and 7 pastures in 5 VT counties sampled in spring and fall for up to 2 years. About 2555 white clover plants were tested by sensitive serological assays for six viruses. Season had little effect on incidence. Three viruses were very prevalent in the northeast. They were identified as white clover mosaic virus, red clover vein mosaic virus, and alfalfa mosaic virus. They infected 10% to 100% of the white clover samples from most pastures. Previous greenhouse and field plot studies indicated these three viruses may reduce growth and longevity of white clover. There are no resistant cultivars or control measures for the northeast. An assessment of the economic impact of these viruses on the northeastern grazier is the needed next step before determining the benefit from developing control measures.
Technical Abstract: The distribution and incidence of six viruses in white clover was determined in 36 rotationally-grazed pastures on 33 dairy farms of Pennsylvania (PA), New York (NY), and Vermont (VT). Seventeen PA pastures were sampled fall 1994, spring 1995, fall 1995, and spring 1996. Five NY and seven VT pastures were sampled spring and fall 1995, and seven PA pastures were sampled one to three times. Enzyme-linked immunosorbent assays (ELISA) were conducted for alfalfa mosaic virus (AMV), red clover vein mosaic virus (RCVMV), white clover mosaic virus (WCMV), peanut stunt virus (PSV), clover yellow mosaic virus (CYMV), and the potyvirus group (POTY) in 2555 plants. Occurrence and incidence varied widely among pastures, and was more consistent among seasons than among pastures. Over the entire survey, RCVMV, WCMV, and AMV occurred in 36, 34, and 35 of the 36 pastures, and in 67%, 32%, and 27% of the plants, respectively. Maximum mlevels of infection by AMV, WCMV, and RCVMV were reached about 1, 2, and 3 years after stand establishment, respectively. POTY and PSV were detected in several pastures and in about 5% of the plants. PSV was never found in VT and was rare in NY. CYMV was not detected.