Soybean mosaic virus (SMV) is a potyvirus that occurs worldwide in soybean-growing areas (Sinclair 1999). It is transmitted nonpersistantly by over 20 aphid species (Halbert 1981b). Aphid vectors found in the midwest include the green peach aphid (Myzus persicae (Sulzer)), corn leaf aphid (Rhopalosiphum maidis (Fitch)), bird-cherry oat aphid (R. padi), Aphis citricola (Van der Goot), spirea aphid (A. spiraecola), A. craccivora (Koch), cotton-melon aphid (A. gossypi), and the potato aphid (Macrosiphum euphorbiae (Thomas)) (Gunasinghe 1988, Halbert 1981b, Higley 1994). In the summer of 2000, the soybean aphid, A. glycines, was found colonizing soybeans for the first time in the United States. A. glycines has been shown to be a vector of soybean mosaic virus in China and in laboratory studies in the United States (Halbert 1986, Hill 2001). It is unknown what affect a colonizing aphid will have on the epidemiology of SMV in the United States. Epidemiological models for SMV use daily alate aphid flight data to predict the incidence of SMV (Irwin 2000, Ruesink 1986). The introduction of a colonizing aphid has the potential to make these models obsolete where A. glycines colonizes soybeans (Irwin 1992).

Soybean mosaic virus has been shown to decrease yield, germination, and oil content of seed (Ross 1969, Sinclair 1992, Suteri 1981). It has also been shown to increase mottling of soybean seed, which is caused by bleeding of anthocyanin pigments from the hilum (Ren 1997). Mottling of soybean seed decreases its value as food-grade seed (Wilcox 1987). The cause of mottling is not known, but mottling caused by SMV is increased by reduced temperature at early pod set and infection with bean pod mottle virus (Ross 1968, Ross 1970). Field data shows that inoculation of soybeans with SMV causes a significant increase in mottling (table 1). The presence of mottling does not necessarily correlate with seed transmission of SMV, however (Hill 1980, Kendrick 1924, Pacumbaba 1990, Porto 1975, Ross 1968, Ross 1970, Tu 1975, Tu 1989).