Although we hear much about fungi and bacteria that cause disease of soybeans, we have not heard as much about viruses that cause soybean diseases--at least not until recently. In contrast to many fungi and bacteria, viruses often cause mild or even hidden effects on soybean. Unlike fungal and bacterial pathogens, viral pathogens cannot survive without living cells, such as those in certain seeds, perennial plant roots, or some insects. Much is known about soybean viruses, but much remains unknown, and there are several ongoing research projects in Illinois and neighboring states that will provide new information about these pathogens in the near future.|
Symptoms of soybean viruses include a mosaic pattern of light green/yellow and dark patches, distorted and wrinkled leaves,
Mosaic symptoms caused by soybean mosaic virus.
stunted plants, seed mottling,
death of stems and petioles, stems that remain green after pods have matured, and bud blight. Impacts of viruses can include reduced grain yield and quality, reduced seed germination and seedling vigor, and predisposition to other stresses. The symptoms can vary depending on type and strain of virus, environmental conditions, time of infection, and soybean variety. It is important to remember that plants can be infected and show no virus symptoms.
Unfortunately, some of the symptoms caused by viruses are similar to some types of nutrient deficiencies and injuries caused by benzoic acid or phenoxy herbicides. The similarity between some virus symptoms and herbicide injury is a complex issue at times, and the true problem can best be determined by observing the pattern of symptoms in a field, the timing and history of herbicide applications, and a laboratory diagnosis to detect specific viruses.
Although systematic surveys of viruses that infect soybean in Illinois have not yet been completed, some of the key soybean viruses in Illinois are known. The primary viruses in Illinois appear to be bean pod mottle virus (BPMV) and soybean mosaic virus (SMV). Other viruses that are known to occur less frequently are peanut stunt virus, cowpea severe mosaic virus, tobacco ringspot virus, and bean yellow mosaic virus.
BPMV currently appears to be the most common virus affecting soybeans in Illinois and is widespread in the United States. It is transmitted primarily by bean leaf beetles (BLB) or other leaf-feeding beetles. Recent studies suggest that overwintering BLBs can carry the virus but do not readily transmit the virus to soybean seedlings until they feed on infected legume plants in the spring. This virus can also be transmitted at a low rate by infected seed (0.1%). Symptoms of BPMV include mottling of leaves, puckering and distortion of leaves, death of new tip leaf growth, and perhaps stems that remain green after pods have matured ("green stem"). Management of BPMV is based on planting virus-free seed, controlling perennial weeds that may be alternative hosts for the virus, and delayed planting to avoid overwintering beetles; in some cases insecticides may be warranted. Commercial resistant varieties are not available.
SMV is also common in Illinois and many areas of the world where soybeans are grown. SMV and BPMV can cause similar symptoms and can occur together in a single plant. Many different aphids are known to transmit SMV. The discovery of the soybean aphid in Illinois and surrounding states last year and its reappearance this summer may indicate a potential for increased problems with aphid-transmitted viruses such as SMV. Seeds are an important means by which this virus is transmitted, and transmission rates can vary from very low to quite high (<5 to 75%), depending in part on the soybean variety. Symptoms of SMV include mottling/mosaic on the leaves, stunted plants, reduced pod number, and mottled seed. Other factors can also cause seed mottling. SMV is best managed by using virus-free seed, weed control, planting resistant varieties, and perhaps using insecticides in some cases to control aphid vectors.
New information will be developed in the near future to shed more light on the soybean virus situation in Illinois. For more information, the following Web site may be of value: http://www.soydiseases.uiuc.edu/. We will keep you posted as new information on soybean viruses in Illinois is developed.--Dean Malvick