ABSTRACT
Beet necrotic yellow vein virus (BNYVV) is the causal agent of rhizomania in sugar beet (Beta vulgaris). The virus is transmitted by the plasmodiophorid Polymyxa betae. The disease is controlled primarily by the use of partially resistant cultivars. During 2003 and 2004 in the Imperial Valley of California, partially resistant sugar beet cultivars with Rz1 allele seemed to be compromised. Field trials at Salinas, CA have confirmed that Rz1 has been defeated by resistance-breaking isolates. Distinct BNYVV isolates have been identified from these plants. Rhizomania-infested sugar beet fields throughout the United States were surveyed in 2004--05. Soil surveys indicated that the resistance-breaking isolates not only existed in the Imperial Valley and San Joaquin Valley of California but also in Colorado, Idaho, Minnesota, Nebraska, and Oregon. Of the soil samples tested by baited plant technique, 92.5% produced infection with BNYVV in ‘Beta 6600’ (rz1rz1rz1), 77.5% in ‘Beta 4430R’ (Rz1rz1), 45.0% in ‘Beta G017R’ (Rz2rz2), and 15.0% in ‘KWS Angelina’ (Rz1rz1+Rz2rz2). Analyses of the deduced amino acid sequence of coat protein and P-25 protein of resistance-breaking BNYVV isolates revealed the high percentage of identity with non-resistance-breaking BNYVV isolates (99.9 and >98.0%, respectively). The variable amino acids in P-25 proteins were located at the residues of 67 and 68. In the United States, the two amino acids found in the non-resistance-breaking isolates were conserved (AC). The resistance-breaking isolates were variable including, AF, AL, SY, VC, VL, and AC. The change of these two amino acids cannot be depended upon to differentiate resistance-breaking and non-resistance-breaking isolates of BNYVV.
