Overview: Tobacco ringspot virus (TRSV) was first described in the United States in 1941. Of the many diseases caused by TRSV, bud blight of soybean is the most severe and causes the greatest losses. Yields may be reduced by 25 to 100%. In general, losses are greatest when young plants are infected or when seeds with a high percentage of TRSV are sown. Yields also are lowered through reduced pod set and seed formation on infected plants.
Symptoms: Plants infected while less than 5 weeks old are stunted. The most striking symptom is the curving of the terminal bud to form a crook. Later, other buds on the plant become brown, necrotic, and brittle. Adventitious leaf and floral buds may proliferate excessively. The pith of stems and branches may show a brown discoloration, first near the nodes and then throughout the stem. Leaflets are dwarfed and tend to cup or roll, and the blades become more or less rugose and bronzed.
Pods generally are underdeveloped or aborted. Those that set before infection often develop dark blotches. Such pods generally do not produce viable seeds and drop early. Maturity is delayed in infected plants; they remain green and often stunted until harvested or killed by frost.
Causal Organisms: Tobacco ringspot virus (TRSV) is the type member of the nepovirus group of plant viruses and is related to arabis mosaic virus, grapevine fanleaf virus, tomato black ring virus, and tomato ringspot virus. The genome of TRSV is bipartite and consists of two single-stranded positive sense polyadenylated RNA molecules. Several strains of TRSV naturally infect soybean. Indicator plants can differentiate these strains.
Disease Cycle: The virus causes systemic infection in susceptible cultivars, moving from infected leaves to the tips of stems and into roots. Movement from roots to leaves is uncommon. TRSV is easily sap-transmissible. Nymphs of Thrips tabaci transmit it to soybean at a low level of efficiency. The nymphs appear to retain the virus for at least 14 days after acquisition. The dagger nematode Xiphinema americanum also is a vector of TRSV, but its efficiency in transmitting the virus to soybean is low. Even when it does occur, nematode transmission of TRSV to roots of plants may be of no significance, because the infection generally remains confined to the roots.
Seed transmission is the most important mode of long-range dissemination and carry-over from season to season. Systemically infected plants often produce infected seeds, which give rise to diseased seedlings. The extent of seed transmission depends on the time infection takes place; plants infected before bloom produce few or no seeds.
Management: A few soybean cultivars have been noted to have resistance to a few strains of TRSV. One genotype (PI 407287) of the annual ancestor of soybean (Glycine soja) is resistant to the virus. Other resistant PIs are 92713 and 154194.
Virus-free soybean seeds should be used in commercial fields, and it may be desirable to avoid fields infested with dagger nematodes or treat them with an appropriate nematicide, if feasible.
Researchers: At the Laboratory for Soybean Disease Research, Dr. Houston Hobbs and Dr. Wang Yi are conducting research on this disease.
