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Historical Significance

Early history of SBWMV research
Harold H. McKinney first described soil-borne wheat mosaic, which he initially called “rosette,” in Illinois in 1919 and through a series of experiments concluded that the causal agent of “rosette” was a virus (Figure 17). Soon McKinney realized that the virus was soil-borne; that some cultivars expressed the rosette symptoms, some the mosaic, and some were resistant to both symptoms; and that, although the disease occurred predominantly in winter wheat and winter rye, during prolonged cool weather in the spring even spring grains could show mild symptoms. In 1925, SBWM was identified at the Arlington Experiment Farm in Virginia, and in the following year McKinney was transferred to the Arlington farm to lead research on viral diseases of cereals. The land that was then Arlington Experiment Farm (Figure 18) is currently the site of the Pentagon.

Figure 17

Figure 18

Early history of Polymyxa graminis research
Polymyxa graminis was first discovered in 1929 and was thoroughly described by G. A. Ledingham in 1939 as part of a doctoral thesis submitted to the University of Toronto. M. K. Brakke and co-workers first correlated vector (P. graminis) and viral (SBWMV) infections in the 1960s, and provided some evidence that SBWMV is internalized by its vector.

Economic significance
Early in the history of SBWM research, host genotypes susceptible to rosette stunting (Figure 15) were common, and the resulting yield loss was potentially greater than 50%. As a result of the exclusion of cultivars exhibiting rosette symptoms, the devastating rosette phenotype is seldom seen today. However, many contemporary cultivars still exhibit the mosaic phenotype and may experience significant yield losses. In many grain producing regions, SBWM results in significant annual yield losses, but, since SBWM symptoms are short-lived and mimic nutritional deficiencies, its economic significance is often underestimated. SBWMV infection causes a reduction in kernel weight, tiller number, and test weight, cumulatively leading to lower grain yields.

World-wide distribution
SBWMV is currently distributed throughout most of the eastern and central U.S. Since the first European report in 1960, SBWMV has spread rapidly on the continent and is now widely distributed in France, Germany, and Italy. In the U.K., SBWMV was first found affecting winter wheat crops on a farm in Wiltshire in 1999, and subsequently on two farms in Kent in 2000. SBWMV is also found in Argentina and Brazil. The Japanese isolate of SBWMV differs significantly from the type isolate, but is still considered to be the same virus.

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