Ten Japanese field isolates of beet necrotic yellow vein virus (BNYVV) were transmitted to by inoculation with sap from rootlets of sugar-beet seedlings, to which the virus had been transmitted by the fungus . RNA extracted from BNYVV particles obtained from the leaves was analysed by agarose gel electrophoresis. Some isolates contained RNA-1 (7.1 kb), RNA-2 (4.8 kb), RNA-3 (1.85 kb) and RNA-4 (1.5 kb) and the others contained, in addition, RNA-5 (1.4 kb). Further isolates, derived from single lesions produced by these isolates, had a variety of RNA compositions. Some contained only RNA-1 and RNA-2. Others contained, in addition, RNA-3, RNA-4, RNA-5 or RNA-6 (1.0 kb), or combinations of two or three of these components. Such isolates generally maintained their RNA composition on further subculture, and their particles had length distributions corresponding to their RNA components. Isolates containing RNA-1 + 2 + 3 caused yellow or strongly chlorotic local lesions in and , and caused systemic stunting and yellow mosaic in and, occasionally, in . In contrast, isolates containing RNA-1 + 2 + 4 or 1 + 2 + 5 induced chlorotic lesions, those containing RNA-1 + 2 + 6 or 1 + 2 induced faint chlorotic lesions, and none of these isolates easily infected systemically. Isolates containing different combinations of RNA-3,-4 and -5 induced more severe symptoms than those containing a single RNA. Such synergistic effects occurred between RNA-3 and RNA-4 or RNA-5, or between RNA-4 and RNA-5 or RNA-6, but not between RNA-3 and RNA-6, or between RNA-5 and RNA-6. These small RNA species therefore contain the genetic determinant(s) for lesion type and for ability to infect and systemically. RNA-1 and RNA-2 are viral genome components. The other RNA components have some characteristics of viral satellite nucleic acids but they may not all be dispensable if the BNYVV isolates are to survive in nature.


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