Suppression of Resistance-Breaking Beet necrotic yellow vein virus Isolates by Beet oak-leaf virus in Sugar Beet

Beet necrotic yellow vein virus (BNYVV) is a member of the genus Benyvirus (21,22) and causes the disease known as rhizomania. It often is reported to be the most important sugar beet (Beta vulgaris L.) virus (17). BNYVV is transmitted by the plasmodiophorid, Polymyxa betae Keskin (1,2,6). In the United States, the virus first was identified in California in 1984 (5) but now occurs in every major sugar beet production region in the country (17). Rhizomania has caused major reductions in sugar beet root yield and sugar content. As soon as rhizomania was identified in North America, the United States Department of Agriculture–Agricultural Research Service (USDA-ARS) in Salinas, CA began extensively screening genetic resources to identify potential sources of resistance to BNYVV and incorporate resistance into sugar beet germplasm (3,10). Rz1 is a single dominant resistant gene for BNYVV (12) and is the only major resistance gene identified within sugar beet germplasm (3,20). A second resistance gene, derived from wild beet (WB42) (9) and designated as Rz2 (18,19), was shown to be different from Rz1 and conferred a higher level of resistance (16,19). Recently, a third resistance gene, Rz3, has been reported from WB41 (9), which is linked to Rz1 and Rz2 on chromosome III (8). Plants with combined Rz1 and Rz2 or Rz3 in a heterozygous condition have lower titer for BNYVV than with Rz1 alone (15). At the present time, planting partially resistant cultivars is the only economical way to control this devastating disease. In 2002–03, severe symptoms of rhizomania were observed in the rhizomaniaresistant Rz1 sugar beet cultivars planted in the Imperial Valley of California. We soon verified that certain isolates of BNYVV from the Imperial Valley had overcome the Rz1 resistance gene (15). Since 2003, the resistance-breaking BNYVV (RB-BNYVV) appeared not only in the Imperial Valley but also in other sugar-beet-growing regions in the United States including Colorado, Idaho, Minnesota, Nebraska, and Oregon (13). During the survey for RB-BNYVV isolates in the United States, Beet oak-leaf virus (BOLV) (14) was frequently found co-infected with BNYVV in the same field and sometimes in the same sugar beet plant. BOLV first was isolated from rhizomania-infested fields in California. Infected sugar beet leaves showed oak-leaf pattern symptoms different from rhizomania. BOLV is serologically distinct from BNYVV, Beet soil-borne mosaic benyvirus (BSBMV), and Beet soilborne pomovirus (BSBV) (14). The host range of BOLV, which is confined largely to Chenopodiaceous plants, is similar to BNYVV and BSBMV. BOLV has been purified from spinach (Spinacia oleracea). Virus particles are 20 nm wide and range from 80 to 640 nm in length. BOLV also is transmitted by P. betae. The molecular mass of the capsid protein was estimated as 46.0 kDa. The taxonomic status of BOLV has not yet been determined. A polyclonal antibody from rabbit has been produced against BOLV and can be used in enzyme-linked immunosorbent assay (ELISA), Western blot, and immunogold labeling tests. BOLV appears to be spread widely in the United States. It has been found in California, Colorado, Michigan, Minnesota, Nebraska, and Wyoming. In contrast to BNYVV, little is known about the effect of BOLV on yield and sugar content in sugar beet. The objectives of this study were to determine if the Rz BNYVV resistance genes confer resistance to BOLV and to determine the effects of P. betae, RB-BNYVV isolates, and BOLV alone and in combination on growth and relative ELISA values in sugar beet.