A number of viruses affect soybean production in the southeast on an annual basis. Although yield loss from viruses is generally relatively low, individual fields may suffer significant losses in any given year. Viruses are infectious submicroscopic particles made up of DNA or RNA enclosed by a protein coat. Soybean mosaic virus (SMV) and bean pod mottle virus (BPMV) are the most common viruses infecting soybean in North Carolina. Soybean viruses are generally transmitted by insect vectors such as aphids or beetles, although Tobacco ringspot virus is also transmitted by the dagger nematode. Symptoms of the two most common viruses, SMV and BPMV, may overlap and soybean plants may be infected with both viruses. Tests are available to accurately identify each virus, but are used only rarely because taking corrective action is rarely possible.
Symptons, Virus Transmission, Epidemiology, and Management of Specific Viruses.
SOYBEAN MOSAIC VIRUS – SMV
Soybean mosaic virus (Fig. 1) is the most common virus encountered in North Carolina. It may be seedborne or transmitted by aphids. SMV causes raised areas or puckuring on the leaf surface, stunting of the plant, and mottling of the seed (Fig. 2). Seed mottling, however, may be caused by other factors. The severity of symptoms is related to the virus strain, soybean variety, and how early the plant is infected. Yield loss to SMV is generally related to time of infection. Virus transmission through seed may be as high as 30%. When infected seed is planted aphids may spread the disease from infected plants to the remainder of the soybean crop. Wild hosts of SMV are relatively rare. Many soybean varieties are resistant to this virus including most varieties developed in North Carolina or Virginia, but strains of SMV that attack resistant varieties are fairly common. Symptom expression is also influenced by temperature. High temperatures limit symptom expression, whereas cool temperatures enhance development of leaf symptoms. Frequently, the crop may appear healthy until several days of cool weather in late summer or early fall, when the entire crop appears to be affected.
BEAN POD MOTTLE VIRUS – BPMV
Bean pod mottle virus (Fig. 3.) though generally less common than SMV can severely lower soybean yield in infected fields. BPMV causes mottling of the leaves, leaf distortion (Fig. 3.), stunting of the plant, and mottled seed (Fig. 2). Bean pod mottle virus has also been implicated in “green stem syndrome”, and this can result in additional yield losses. Green stem syndrome is the condition where the stems of mature plants remain green and leathery making harvest difficult. The severity of symptoms is related to the virus strain, soybean variety, and how early the plant is infected. Yield loss to BPMV is generally related to time of infection; early infection can result in severe losses. Virus transmission through seed is very low, generally less than 0.01 %. Like SMV, high temperatures limit BPMV symptom expression, whereas cool temperatures enhance development of leaf symptoms. BPMV is transmitted by several species of leaf feeding beetles, including bean leaf beetle. Overwintering beetles probably acquire virus from wild legume weed species. No varieties with resistance are available, but varieties do vary with regard to tolerance. Mixed infections of BPMV and SMV can result in severe stunting or death of the plant.
TOBACCO RINGSPOT VIRUS – TRSV
Tobacco ringspot virus (TRSV) is fairly common in North Carolina but yield losses in production fields are generally insignificant. Principal symptoms include stunting, leaf distortion, and characteristic browning and curling of the terminal branch. The most obvious symptoms are the proliferation of buds and flowers, and lack of pods or poorly formed pods (Fig. 4). Stems remain green and petioles may remain attached (Figs. 5 and 6) with black lesions. The stems remain green because the plants are sterile as a result of pod abortion. Infected plants will stand out in a mature soybean field because of the green stems. If there are large numbers of these green plants harvest may be more difficult. The primary impact of this disease is that soybean grown for seed from fields with this virus cannot be shipped to certain countries. Although the virus may be seedborne, this is probably not an important means of transmission since most infected plants are sterile. The dagger nematode can transmit this virus, but the virus does not move from the roots to the shoots and leaves of the soybean plant. Spread is probably by thrips, though no efficient vector has been identified.
COWPEA CHLOROTIC MOTTLE VIRUS – CCMV
The cowpea chlorotic mottle virus (CCMV) is occasionally found in North and South Carolina as well as Georgia. Symptoms are a distinct mosaic (Fig. 7) and stunting. Yield losses from this virus are probably not important since it generally appears on only scattered plants in a field. Bean leaf beetle and the spotted cucumber beetle are vectors for this virus. The virus probably survives in various weed species.
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.