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Soybean Mosaic Virus
Mosaic caused by soybean mosaic potyvirus (SMV) occurs wherever soybean is grown. SMV is the most common viral disease of soybean, and is transmitted by aphids and through infected seed and is also readily transmitted mechanically for experimental purposes. SMV infection results in significant yield losses, reduction in seed quality, decreased oil content and decreased nodulation. Losses in the United States of 90% have been reported, depending on the cultivar, incidence of primary inoculum, and aphid activity. Because soybean varieties with broad resistance to SMV are not available, and large-scale vector control is impractical, the most effective control measure is to plant SMV-free seed. There are eight recognized strains of SMV (G1-G7, C14) that are differentiated on the basis of reaction of differential host genotypes. Many soybean varieties have resistance to one or more strains, but resistance is often allelic, and genotypes resistant to one strain are often susceptible to other strains; commonly grown cultivars are susceptible to multiple strains. The resistance response may be expressed as absence of symptoms following inoculation with one strain of SMV, typical mosaic with other strains, or systemic necrosis following infection by an SMV strain to which the genotype is especially sensitive. Thus plants carrying the allele Rsv1-s are resistant (symptomless) when inoculated with strains G1-G4, and G7, but respond to strains G5 and G6 with systemic necrosis. The Rsv1-s allele is the only one so far identified at the Rsv1 locus conferring resistance to strain G7. There appear to be at least three loci conferring resistance to SMV, of which the Rsv2 gene confers resistance to all of G1-G7, but not C14. All three genes are completely dominant over the susceptibility alleles in cultivars Williams and Franklin.
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