Increased Virulence of Barley Stripe Mosaic Virus for Wild Oats: Evidence of Strain Selection by Host Passage

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Abstract
Only one of 4 isolates (C4) of barley stripe mosaic virus (BSMV) was transmitted (airbrush method of inoculation) from barley (Hordeum vulgare ''Black Hulless'') to wild oats (A. fatua). Initially, the proportion of inoculated wild oat plants systemically infected by isolate C4 was low, and in some plants only localized infections developed in inoculated leaves. However, when isolate C4 was subsequently transferred from infected to healthy wild oats, the proportion of inoculated plants infected was high and the infection was invariably systemic. These observations and several additional lines of evidence indicated that this pattern of transmission was due to strain selection during systemic passage of isolate C4 through wild oats. Complete separation of strains comprising this isolate, however, apparently did not occur until 3 successive passages of the virus through wild oats. After this, the selected strain from barley systemically infected almost all wild oat plants that were inoculated. In immunodiffusion tests with crude extracts from infected barley, no antigenic differences were detected among the 4 isolates of BSMV or between isolate C4 and the selected strain. The possible significance of some of these findings in the epidemiology of barley stripe mosaic is discussed.