Some of you might have seen soybean seeds having discoloration or brown-black pigments diffusing from the hilum. It is called soybean mottling and can be a symptom associated with viruses, including bean pod mottle virus (BPMV) and soybean mosaic virus (SMV). Some seed is entirely black or brown, but it is more common to see bicolored soybean seeds. Usually, it is a hilum color that determines the color and intensity of seed coat discoloration of the soybean.
Some farmers and agronomists may face a problem when they cannot determine the cause of the seed mottling because these two most common soybean viruses, SMV and BPMV, may overlap and soybean plants become infected with both viruses at the same time. So, in this article, we will talk about these viruses and try to help to determine the real cause of the seed mottling and diverse ways of dealing with this problem.
Soybean mosaic virus – SMV
Soybean mosaic virus is a seed-borne virus that affects the leaf and seed surface of the soybean causing yield losses and bad seed quality due to its smaller size and mottles on the seeds. Virus transmission through seed may be as high as 30%. Once the virus is in the field, aphids can spread it from plant to plant as they feed infecting the rest of the crops.
SMV infection is most obvious in the youngest leaves. Leaf symptoms include a mosaic of light and dark green areas and rough curly leaves. SMV also affects pods and seeds. Infected pods are small, flat, have less hair, and are more curved than pods that are not infected with the virus. Infected seeds have discoloration and pigmented mottles of black or brown color.
Symptom expression is also influenced by temperature – elevated temperatures limit symptom expression, whereas cool temperatures increase the development of virus symptoms. Usually, the crop may look healthy until several days of cool weather in late summer or early fall, when the entire crop becomes affected.
It’s very important to keep in mind that:
Soybeans infested with aphids are at a higher risk of virus infection.
Infection in the early growth stages has the greatest risk of yield loss and reduced seed quality, compared to infection later in the season.
Mixed infections with SMV or other viruses can result in significantly more severe symptoms and yield loss.
One of the few practical ways of managing this virus is the selection and use of resistant varieties of soybeans. It would be the most effective way because the main source of infection is introducing already infected seeds into your fields. It is not a promising idea to plant in the later stages due to the higher populations of the soybean aphid at this time of the season. The probability of transmission to young seedlings in the initial stages is higher and most likely will cause the greatest risk of yield loss and reduced seed quality, compared to infection later in the season.
A study in Iowa and Wisconsin was conducted to determine if a foliar application of insecticide recommended for decreasing aphid populations would also reduce disease caused by soybean mosaic virus. The study concluded that insecticide does not eliminate the impact of aphid species immigrating into the soybean field from transmitting the virus.
Bean pod mottle virus – BPMV
Bean pod mottle virus is generally less common than SMV but can severely lower soybean yield in infected fields. BPMV causes a mottling of the leaves, leaf distortion, and mottled seed. Virus transmission through seed is very low, generally less than 0.01 %. Several species of leaf-feeding beetles, including bean leaf beetles, can transmit BPMV.
Like SMV, hot temperatures limit BPMV symptom expression, whereas cool temperatures enhance the development of leaf symptoms. The presence of leaf symptoms is usually associated with mottled and crinkled leaves. However, be aware that leaf symptoms are not a consistent indicator of virus infection. Mottled and crinkled leaves can also be caused by herbicide injury or nutrient deficiencies.
It is important to know how to separate the symptoms of herbicide injury from the bean pod mottle virus:
Rugosity (rough leaves), cupping, twisting, and distortion of leaves are symptoms of both. In case you didn’t apply herbicide these symptoms may indicate a virus.
Herbicide injury may follow a pattern, such as a spray swath, while virus-infected plants appear in patches that are not consistent in size.
Soybeans with herbicide injury outgrow the symptoms, while virus-infected plants may continue to show symptoms throughout the growing season. Note, however, that virus symptoms may be masked in hot dry weather.
BPMV also causes lower than expected yields, mottled seed, poor germination and delayed maturity, reduced grain quality, such as protein and oil. The impact of BPMV on yield depends upon the time of virus infection. Early infection results in the highest yield reduction.
Like with SMV, the only practical way to manage this virus is the selection and use of tolerant soybean varieties. Complete resistance is not yet available. But there are some soybean varieties with partial resistance to BPMV with acceptable yield and low seed mottling.
It is also advisable to consider a later planting of soybean. Especially if BPMV was a yield-limiting factor in the previous season. It is important because early soybean planting coincides with high populations of overwintered bean leaf beetles. So they move into soybeans to feed and lay eggs and transmit the BPMV. However, consider that late planting can result in an increased risk of soybean aphid activity and SMV appearance.
Treatment thresholds for bean leaf beetles aimed at the prevention of bean pod mottle virus are not yet available. However, synthetic pyrethroid insecticides tend to provide the most consistent bean leaf beetle control. To prevent transmission of BPMV, you must apply insecticide in the very early stages (V-V2) of soybean development. A second application may be necessary during the emergence of first-generation beetles (late June or early July). Delayed spraying at either crop stage can seriously increase the incidence of BPMV.
BPMV vs. SMV
It is common that more than one virus at the same time can infect soybeans. A synergistic reaction occurs when both BPMV and SMV infect the plant. Mixed infections of two or more viruses in a single plant result in more severe symptoms than single infections. Reduction in yield is much greater in mixed infections than either virus alone.
Multiple virus infections also increase the level of seed transmission of SMV, depending on how early in the season the plant is infected. With the recent increase in the incidence of bean pod mottle virus (BPMV), the potential for synergism between SMV and BPMV is increasing more and more every day. Moreover, symptoms in plants infected with both SMV and BPMV can be very severe and can lead to terminal death.
As we can see it is very difficult to establish the real cause of soybean seed mottling. Since both SMV and BPMV can provoke brown-black pigmentation on your soybean seeds. As both these viruses started to “work” together it is hard for farmers and agronomists to tell which virus they have and what they must do. A lot of laboratories offer their services in testing your seeds for the determination of the virus you have.
So you can choose the proper management for your crop. Mottled soybean seeds are an issue that can cause a harmful effect on your soybean production and yield. The virus is not the only cause for mottled seed, however. Insect feeding and physiological stresses can also cause seed mottle.
Understanding the differences between both viruses and good data collection gives you the knowledge to start preventative measures against these viruses.