Interactions Between Beet necrotic yellow vein virus and Beet soilborne mosaic virus in Sugar Beet

Beet necrotic yellow vein virus (BNYVV) and Beet soilborne mosaic virus (BSBMV) are members of the genus Benyvirus (9). Both viruses infect members of the family Chenopodiaceae, most notably sugar beet (Beta vulgaris L.), and are transmitted by the plasmodiophorid, Polymyxa betae (1,2). BNYVV infection causes rhizomania disease in sugar beet (4,21); however, BSBMV infection does not (28). BSBMV was first described in the United States in 1988 (14), has a genome organization similar to BNYVV, and a genomic sequence similarity to BNYVV ranging from 35 to 77% at the nucleotide level (10). Based on these and other studies (26), and the fact that BSBMV does not cause rhizomania, BSBMV is considered a distinct member of the genus Benyvirus (9,24). BNYVV was introduced into California (5) most likely from Europe. Based on enzyme-linked immunosorbent assay (ELISA) tests conducted at the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) in Salinas, CA, BNYVV has spread throughout most states where sugar beet is grown (Idaho, Nebraska, Colorado, Oregon, Minnesota, North Dakota, Texas, Washington, Wyoming, and Michigan; 6,23,26,28). In contrast, BSBMV is not established in California, and is found primarily on the eastern slope of the Rocky Mountains and in the Midwestern beet growing states (26,27). In previous studies, isolates of BNYVV in the United States were virtually identical based on immunodiffusion, ELISA, host reactions, and results from molecular comparisons. Variability exists, however, among isolates of BSBMV from Texas, Idaho, Nebraska, and Colorado (26). The distribution and lack of variability of BNYVV in the United States (26,27) is consistent with the epidemiology of a virus that has been recently introduced. In contrast, the distribution and variability of BSBMV is indicative of a virus that is endemic, has been spread to other areas, and has evolved over a long period of time. BSBMV has not been reported in Europe. As a result of the international significance and economic losses caused by BNYVV on sugar beet production, a breeding program was established and has been maintained in the United States since 1984 (3,11–13). A gene that induces resistance to rhizomania in sugar beet (Rz) was identified in 1983 from sugar beet and subsequently from wild beet (B. maritima; 3,13). Breeding programs have improved rhizomania resistance to the point where, even under conditions of high inoculum (i.e., the San Joaquin Valley, CA), yields and sugar production are equal to those of high-yielding, susceptible sugar beet cultivars in the absence of rhizomania (R. T. Lewellen, unpublished data). Furthermore, the level of resistance is significantly correlated with the dose of the Rz allele as measured by (i) root weight, (ii) the rhizomania disease index (DI) rating (from 1 to 9, where a low DI indicates resistance and lack of symptoms, and a high DI indicates susceptibility and the presence of hairy roots, a wine glass-shaped root, and internal necrosis), (iii) sugar yield, and (iv) ELISA absorbance values (25). For example, a strong negative correlation was shown between a decreasing dosage of the Rz allele, and both absorbance and the DI rating (Rzrz < Rzrzrz < rzrzrz). However, a positive correlation was shown between a decreasing dosage of the Rz allele and root weight (Rzrz > Rzrzrz > rzrzrz) (25). A diploid rhizomania-resistant cultivar (Rzrz) has a lower virus titer and DI rating but a higher root weight than a triploid resistant cultivar (Rzrzrz), and a triploid susceptible cultivar (rzrzrz). Thus, a clear relationship was obtained between virus concentrations in BNYVV-infected plants and absorbance values with ELISA (16,22). In contrast to BNYVV, little is known about the effect of BSBMV on yield and sugar content in sugar beet. In the past 5 to ABSTRACT Wisler, G. C., Lewellen, R. T., Sears, J. L., Wasson, J. W., Liu, H.-Y., and Wintermantel, W. M. 2003. Interactions between Beet necrotic yellow vein virus and Beet soilborne mosaic virus in sugar beet. Plant Dis. 87:1170-1175.