Soybean mosaic is a viral disease of soybean and other legumes caused by Soybean mosaic virus (SMV). SMV is moved from plant to plant or vectored by aphids. Soybean mosaic does not generally affect yield; although infected plants may produce fewer and smaller seeds, the major concern with SMV is reduced seed quality due to mottled soybeans. There may be clusters of symptomatic plants or just single plants with symptoms in a field.
Foliar symptoms include a mosaic of light and dark green areas, chlorosis and puckered or curled leaves. The youngest and most rapidly growing leaves show the most symptoms. Symptoms are most obvious at cooler temperatures and often disappear when it is hot. Symptoms can resemble injury from herbicide drift and are similar to those caused by other viruses. This makes it difficult to diagnose SMV, like most other viruses, based on symptoms alone. Laboratory tests can be done at diagnostic clinics to distinguish among soybean viruses. Infected plants can be stunted with shortened petioles and internodes. They also can have reduced pod numbers, and infected pods are smaller, flatter, more curved and with hair than non-infected pods. Seed mottling may occur and seed germination may be reduced.
Infected seed is the most important way SMV is introduced into a soybean field. Seed transmission depends on the variety planted and ranges from 0 to 75 percent. In most modern soybean varieties, rates of seed transmission are usually between 0 to 5 percent. Once the virus is in the field, aphids can spread it from plant to plant as they feed. More than 30 species of aphids transmit SMV worldwide, including the soybean aphids (Aphis glycines). Symptoms of virus infection are frequently associated with specific fields, especially those with moderate to high insect activity. Infection with more than one virus is common, and the risk of yield loss or reduced seed quality is much greater with dual or multiple infections than with only one virus.
The best time to scout for this virus is anytime during the season; however, the most practical way to check for virus infection is to look at the seed. Check plants near the edge of fields.
Variety selection: Although most commercial soybean varieties are susceptible to SMV, resistance has been identified in soybean genotypes and varieties.
Planting date: Later planted soybeans may have higher risk of SMV infection and spread by insect vectors.
Foliar insecticides: These are not effective in reducing transmission of SMV by aphids. While aphid populations may be reduced, SMV infection is unaffected.
Photo by Craig Grau