Virus Diseases of Soybean: the Usual Suspects and Two New Pretenders
USDA-ARS Dept. of Crop Sciences
Multiple viruses infect soybean in Illinois including, in decreasing order of prevalence, Bean pod mottle virus (BPMV), Alfalfa mosaic virus (AMV), Soybean dwarf virus (SbDV), Soybean mosaic virus (SMV), Tobacco streak virus (TSV) and Tobacco ringspot virus (TRSV). Virus diseases are most damaging when infection occurs early in the growing season. BPMV is transmitted to soybean primarily by bean leaf beetles, which migrate into soybean fields early each spring. The sources of inoculum for BPMV infections have not been clearly identified, but one candidate is BPMV-infected Showy ticktrefoil, a perennial legume. The incidence of BPMV infections is related to the sizes of bean leaf beetle populations. While BPMV incidence in soybean in Illinois in most years is less than 5%, in years when bean leaf beetle populations are large, greater than 50% of soybean plants can become infected with BPMV in some fields. Symptoms induced by BPMV are most obvious in plants infected early in the growing season, and include puckering and distortion of leaves and seed coat mottling. Currently, control options for BPMV are limited. Insecticide treatments to control bean leaf beetles have not been effective in reducing the percentage of soybean plants infected with BPMV, and no soybean varieties are resistant to the virus. However, some soybean cultivars are more tolerant and show lower yield losses and less seed coat mottling than others.
AMV, SbDV and SMV are transmitted to soybean by aphids. Their incidences are dependent upon the abundance of aphid vectors and the availability of virus-infected plants that can serve as sources of inoculum. While AMV and SMV are transmitted by several species of aphids that do not colonize soybean, SbDV is transmitted by a small number of colonizing aphid species. AMV and SbDV infect perennial legumes (for example, nearly 50% of the red clover plants in Illinois are infected with SbDV), which serve as virus reservoirs. However, SMV does not infect native plant species to a significant extent in North America and is almost completely dependent upon seed-borne infections to initiate new disease outbreaks. AMV and SMV infections can cause distortion and mosaic (often bright yellow for AMV) symptoms on leaves, stunting, reduced pod set and seed coat mottling. Seedling infections by SbDV induce severe stunting and reduced seed set. Resistance to AMV and SMV has been identified in soybean, but most soybean varieties are susceptible to both viruses. Resistance has not been found in soybean to SbDV, but some cultivars show only mild symptoms when infected by the virus. We are conducting research related to transmission of SbDV by aphids and resistance to transmission of SMV through seed.
In the last two years, two new soybean-infecting viruses have been discovered in the United States. The first virus, Soybean vein necrosis virus (SVNV), was detected in Tennessee in 2008 and in Illinois and Kentucky in 2009. SVNV infections initially induce vein clearing that becomes necrotic as leaves mature, which leads to large necrotic regions on leaves. The second virus, Soybean yellow mottle mosaic virus (SYMMV) was first reported from soybean plants in South Korea and then in southern Mississippi in 2009. Like SVNV, SYMMV initially induces vein necrosis shortly after inoculation, but then induces bright yellow mosaic on systemic leaves and eventually stunting and reduced growth of older leaves. Little is known about the economic impacts, insect vectors or sources of inoculum for the viruses. We are investigating the incidences of both viruses in Illinois soybean fields.