By: Tom Allen, Sead Sabanadzovic (MSU Plant Virologist), and Angus Catchot
Rarely are soybean viruses discussed. More often than not, during the normal year, soybean viruses go unnoticed in the field as they can be confused with some other pathogens or abiotic causes. For example: damages due to viruses can often be misinterpreted as herbicide injury early in the season and they can be difficult to observe later in the season as they can be masked by Cercospora blight.
In order to help identification of these problems in MS, we will focus on a brief description of three of the most common viruses observed during 2013 in MS (as well as in previous years). The presence of the particular viruses was verified in the lab at Mississippi State University.
In general, when speaking about all soybean diseases, affected plant tissue is more readily observed on the youngest leaves, in the top of the canopy. However, this will greatly depend on the particular virus. For example, in situations where Soybean vein necrosis virus is present the disease is more likely observed in the middle of the canopy rather than the top of the canopy. See associated images with each of the three viruses outlined below. In addition, mixed infections, a situation that can occur when a plant becomes infected with more than one virus, usually result in more dramatic effects such as severe stunting, or poor pod formation. Mixed virus reactions can also enhance negative effects of single viruses – a situation that is termed synergism – and increase the likelihood of a yield loss associated with the disease. Moreover, some soybean viruses can increase the severity of specific foliar fungal diseases in affected plants.
One other important piece of information regarding the three viruses outlined below. Specific symptoms attributed to each of the viruses can vary according to the strain of the virus, soybean variety, stage of the plant when infection occurs, and environmental conditions throughout the growing season.
In addition to the viruses outlined having the potential to be vectored by insects, the movement of infected/infested seed sources can also result in the virus occurring in soybean fields. Seed is an important method of movement of some viruses to areas that have not previously been observed to contain virus-infected plant material. This route of transmission is especially important for Soybean mosaic virus, reported to be transmitted at rates from 0 to 40%, depending on the virus isolate and soybean variety/genotype. Except for one report, seed transmission of Bean pod mottle virus (BPMV) is considered to not be as important. The importance of seed for Soybean vein necrosis virus (SVNaV) transmission is yet to be experimentally determined.
Bean pod mottle virus (BPMV)
The typical symptoms associated with BPMV include mottling (a coloration issue of affected leaves that appears as light and dark areas on the leaf tissue intermingled) of young leaf tissue. Furthermore, affected tissue can become puckered and assume a rough appearance, similar to Cercospora blight, but more pronounced. As plants mature the mottled leaf tissue may become more difficult to observe.
BPMV is spread from plant to plant by several leaf-feeding beetles, the most identifiable being the bean leaf beetle. Yield reductions can result from early infection since the virus can result in stunted plants. In addition to resulting in the potential for yield loss as well as producing leaf chlorosis, BPMV reportedly can also be responsible for the production of the “green stem” syndrome that results in leaves and stems remaining green on plants with physiologically mature pods and grain. However, the recent literature (and our experience in MS) does not support this notion as not all plants with green stem were infected with BPMV and several different issues may produce a green stem situation (e.g., environment, fungicide, herbicide, insects).
In addition to the mottling associated with the disease, leaf distortion and stunting of the plant can be observed.
Soybean mosaic virus (SMV)
Initial symptoms of the virus appear as mottled plant tissue in the upper plant canopy. A mottling of light and dark leaf tissue, or sometimes yellow, similar to the photos attached, can be the result of a SMV infection. In advanced stages of the disease, infected leaves can develop a rough appearance or appear blistered; however, this should not be confused with the thickening of leaves associated with Cercospora blight that is more noticeable late in the season. Affected leaves will remain green rather than developing the bronze, maroon, or yellowed appearance that can more readily be attributable to Cercospora blight.
In extreme cases plants can be stunted and will produce a reduced number of pods. SMV can also be responsible for producing smaller than normal grain as well as mottled grain that can reduce harvested seed quality at the elevator. However, infection early in the vegetative stages can be much more damaging than infection that occurs later in the growing season.
SMV is transmitted by aphids.
Soybean vein necrosis-associated virus (SVNaV)
Over the past few seasons the reports of SVNaV from MS as well as other soybean producing states have increased. SVNaV was positively confirmed in MS soybean fields during the past several years. At present, little is known regarding where this particular virus originated and how important yield loss is associated with the disease. It has been recently ascertained that the virus is transmitted by soybean thrips and can experimentally infect ivyleaf morning glory and several other plants (e.g., tobacco, cucurbits, vigna). Additional research is necessary to determine additional hosts that harbor the virus especially since kudzu is the closest botanical relative of soybean in MS and rarely has kudzu been as important as it is at this point in time.
Observationally speaking, the symptoms of the virus can be difficult to tell apart from other diseases and even insect feeding on leaves. The early symptoms of the disease can appear as small, yellow areas clustered near the main leaf veins. As the virus progresses the tissue around the affected area as well as the veins themselves can develop a necrotic (dead and brown) appearance. In some cases the veins of the leaves themselves can become brown in color due to the virus, but this is not always a symptom associated with the disease. In addition, this specific symptom can easily be confused with initial symptoms of Cercospora blight as well as with herbicide injury.
At present there are no management considerations for dealing with this particular virus.
In conclusion, viruses have been increasingly observed over the past several years in soybean production fields in MS. However, little is known regarding the potential yield losses resulting from infections by SMV, BPMV or SVNaV either alone or in combined infections. Additionally, there are still many unknowns concerning these viruses.
Research on virus-induced problems in MS has recently been funded by the Mississippi Soybean Promotion Board, which will further knowledge of these viruses in MS (and viruses yet to be discovered) and their impact on soybean production.