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Symptom expression on infected plants depends on the virus strain, host cultivar and environmental factors (McKinney and Greeley, 1965). Symptom expression is enhanced by warm temperatures (24-30°C) (McKinney, 1954).
Depending on the virus strain, symptoms vary from small streaks covering the whole leaf surface to elongated stripes. In barley, symptoms caused by mild strains of BSMV are scarcely visible. On different host cultivars symptoms vary from light-green or chlorotic striping or streaking to chlorosis and necrosis. Plants grown from infected seed may be severely stunted. Seeds from plants infected by BSMV are small and shrivelled.
Since seed transmission is necessary for the survival of BSMV in nature and this virus has no known vectors, a simple and extremely effective means of control involves the sowing of virus-free seed. Barley seed certification schemes in Montana (Carroll, 1983) and North Dakota, USA, have prevented significant yield losses since their inception. There are currently no known losses due to BSMV in the upper midwestern USA.
Certified seed programmes in both Montana and North Dakota involve production of foundation, registered and certified seed. In Montana, field inspections are supplemented with serological assays (Carroll et al., 1979a) on seed samples from foundation seed fields. In North Dakota, no field inspection is done; samples of 1,000 seeds per foundation seed lot are assayed by ELISA. In either programme, only virus-free seed lots are certified.
Resistance to BSMV has been identified in a number of barley cultivars in the USA National Small Grains Germplasm Collection (Timian and Sisler, 1955; Timian, 1975). There appears to be little interest among plant breeders to incorporate resistance into commercial cultivars, probably as a result of the effectiveness of clean seed programmes.
The inheritance of resistance to BSMV has been investigated in several studies which together identified up to five resistance loci (Timian and Sisler, 1955; Sisler and Timian, 1956; Vazquez et al., 1974; Timian and Franckowiak, 1987). A single gene for resistance was mapped to the centromeric region of the plus arm of barley chromosome 1 and was found to co-segregate with RFLP marker ABC455 (Edwards and Steffenson, 1996). A single recessive gene also conditions resistance to seed transmission of BSMV in Modjo barley (Carroll et al., 1979b).
Recently, Cui et al. (2012) reported the fine mapping of the Bsr1 BSMV resistance gene in the model grass Brachypodium distachyon.
In the USA, BSMV has been economically significant only in Montana and North Dakota (Carroll, 1983). Between 1953 and 1970 the total loss in barley caused by this virus was calculated at more than US$ 30 million. In 1964 the virus caused damage estimated at over US$ 3.1 million (Carroll, 1980). Yield reductions in barley were 24-46% when BSMV was inoculated mechanically (Chiko and Barker, 1978; Carroll, 1980). In most states of the USA and also in most other countries where it occurs, the economic significance of BSMV has decreased (Carroll, 1983). For further information, see Carroll (1986). The impact of mild strains of BSMV on barley yield is substantially lower (Jezewska, 2006).