By Carolyn King
There’s some good news on the disease front for Ontario soybean growers. Based on findings so far, soybean vein necrosis appears to pose a relatively low economic risk in the province at present. But understanding of this recently discovered viral disease is still quite limited. So plant pathologists are seeking answers to important questions about the disease, its management and its longer-term implications.
Soybean vein necrosis is a leaf disease in soybeans. Typically the symptoms start as yellowish patches along the leaf’s main veins. The patches spread along the main veins and sometimes between the veins, and become reddish brown areas of necrotic (dead) tissue. Infected veins may be clear, yellow or dark brown, and vein discoloration may be more noticeable on the underside of the leaf.
Dr. Ioannis Tzanetakis, a plant virologist at the University of Arkansas, first identified the disease in 2008. “I was new in Arkansas, and I was looking to see what soybean viruses were found in the state. I collected samples of plants with virus-like symptoms, and I was seeing the symptoms [of what is now called soybean vein necrosis disease]. I tested the samples, but none of those plants were infected by any of the 14 viruses that I was testing for,” says Tzanetakis.
“At almost the same time, Dr. John Rupe was visiting Tennessee, and he brought back some samples with exactly the same symptoms. He told me they had tested the samples in Tennessee for 18 viruses and they couldn’t find any that were associated with those symptoms. So I looked at the possibility of a new virus.”
By sequencing its genome, Tzanetakis determined that the virus belongs to group of viruses called tospoviruses. Some viruses in this genus, such as tomato spotted wilt virus, can cause devastating losses in their crop hosts.
Tzanetakis developed a test to detect the new virus. Then he and his lab examined soybean leaf samples with and without the vein necrosis symptoms. They confirmed that the symptoms were always associated with the new virus. He says, “We collected more than 700 isolates of the virus from 11 states, and every single sample that had the soybean vein necrosis symptoms was infected with the virus [and none of the symptomless samples had the virus].”
As of November 2013, the soybean vein necrosis virus (SVNV) had been detected in soybean crops in 16 U.S. states – in the south, the Midwest and the Great Lakes states – and in Ontario. Tzanetakis says, “It is now considered the most widespread soybean virus in the United States.”
Learning about SVNV
Since the 2008 discovery, Tzanetakis and his collaborators have been carrying out various studies to learn more about SVNV, its transmission and its impacts on soybeans.
Tospoviruses are vectored (transmitted) by tiny, winged insects called thrips. Knowing which particular thrips species – among the thousands of them – are SVNV vectors could help in managing the disease. Since the virus’s genome has some unusual characteristics compared to the other tospoviruses, Tzanetakis thought it might be vectored by a thrips species that was different from those known to vector other tospoviruses.
So he and his PhD student, Jing Zhou, looked into soybean thrips as a possible SVNV vector because neither soybean thrips nor any of the other species in the same subfamily of thrips has ever before been proven to be a virus vector. Their study proved their hypothesis: soybean thrips is an SVNV vector.
Soybean thrips are common in soybean fields. The insects acquire SVNV when feeding on an infected plant and then transmit it to other plants by feeding on them. The thrips feed mainly along the veins of the leaves, which is why the initial symptoms appear there. It takes some time for the thrips to become infected and transmit the virus to new plants, and time for the visual symptoms to appear on the infected plants. In Arkansas the symptoms of the disease start to appear in soybean crops in about early June.
By using thrips inoculated with SVNV, the researchers have proven that the virus is definitely causing the disease.
The weather plays a big part in levels of the disease because the thrips population explodes in hot, dry conditions. “A hot, dry spring helps the soybean thrips to populate newly emerging soybean plants, and the newer the plant, the more the damage. When you have a hot, dry spring, the disease is everywhere. When you have a cool, wet spring, the disease is not a big problem,” says Tzanetakis.
He adds, “I was in a meeting in February 2013 with extension pathologists from all over the U.S. In 2012, it was dry and hot, so there weren’t really major issues with fungal diseases, but I was truly surprised that 12 out of the 14 people in that room mentioned that vein necrosis was the major [soybean] disease in their state. In 2013, the spring was cool and wet, and the disease was not a major problem.”
Other SVNV studies are investigating such questions as whether other thrips species also transmit the virus, how soybean planting date relates to the effects of the thrips and the virus on plant growth, and how the virus impacts different soybean cultivars.
Tzanetakis is involved in studies to evaluate if and when it makes economic sense to use insecticides to control thrips as a way to manage SVNV. “Dr. Les Domier, a collaborator in the project, is studying how different cultivars react to the disease; some cultivars are hammered by the disease, while some others are tolerant,” Tzanetakis explains. “And we’re evaluating how economical it is to eliminate the disease by spraying for the thrips, or whether it’s more economical to just let it go and accept that you’ll have a certain loss.”
His research team is also testing various plant species to identify alternative hosts of SVNV. The virus is an obligate parasite, which means it must always have a host, including the times of year when no soybean plants are growing. So controlling SVNV’s weed hosts might help in controlling the virus.
The researchers have already confirmed one weed host – morning glory, a common weed in many soybean-growing regions in the United States. And they will soon be publishing their results for several other weeds frequently found in soybean fields in the south-central U.S.
In another current project, Tzanetakis is exploring what happens when a soybean plant is infected with several viruses including SVNV. Previous research on seed-borne viruses, such as soybean mosaic virus, has shown that when a plant is infected with multiple viruses, the whole plant can be killed. “Vein necrosis appears to be localized where the thrips feed. But if you have a second virus infecting that particular plant, it may be that the vein necrosis virus can get outside that restricted area and then potentially it could kill the whole plant,” he notes.
SVNV in Ontario
In Ontario, soybean vein necrosis was first confirmed in 2012; at that time it was present in much of southwestern Ontario.
“We had been seeing these odd symptoms in the field for a few years and had been trying to get an idea of what was causing them through discussions and sampling with colleagues in the U.S. midwest. Then soybean vein necrosis virus was confirmed as the causal agent of the symptoms,” says Albert Tenuta, field crop plant pathologist with the Ontario Ministry of Agriculture and Food (OMAF).
He notes, “With the dry, hot conditions in 2012, the thrips populations in Ontario were up. So we had greater spread of the virus from plant to plant. [With the cooler, moister conditions] in 2013, the disease was still present, but it was definitely less – you could find it in most soybean fields, but it was at trace levels.”
In Ontario, symptoms of the disease tend to appear later in the growing season when the plants are more mature and better able to withstand the infection. This later onset may be because soybean thrips don’t overwinter in northern soybean-growing areas; they migrate into the area from further south.
“There is a build-up period in each growing season of both the thrips population and the virus within that population, and that takes time. In most cases, the worst-case scenario would be that we would see the symptoms showing up in about July,” explains Tenuta.
He adds, “As well, the injury is often confined to the vein area and maybe some of the surrounding tissue along the veins. So soybean vein necrosis is not like some of the fungal leaf diseases, where the leaves’ photosynthetic capability is decreased so much that the plant’s ability to develop pods and seeds is impacted. And it’s not like some viral diseases, such as soybean mosaic virus or bean pod mottle virus, that impact seed quality and are definitely a concern, especially for food-grade soybean production.”
Compared to these other diseases, Tenuta believes the risk from soybean vein necrosis to Ontario soybean production, and northern soybean production in general, is relatively low at present. “We’re not seeing any significant economic injury from it,” says Tenuta. “The injuries to the leaves do not seem to be affecting soybean yields, seed quality or seed germination.”
Given the low impact in Ontario, Tenuta does not recommend an insecticide application to control the thrips vector of SVNV.
For now, the key for Ontario growers is accurate identification of the disease to avoid unwarranted control measures. Soybean vein necrosis symptoms may be mistaken for other diseases, such as cercospora leaf blight and bacterial blight, and other problems, like herbicide injury. Fortunately, Tenuta and his American colleagues have produced a new soybean vein necrosis information sheet that includes tips for identifying the disease and differentiating it from other soybean leaf problems.
The only way to definitely confirm any viral infection is through laboratory testing. You can contact the University of Guelph’s Pest Diagnostic Clinic or private diagnostic labs for information on pricing and sampling procedures.
Tenuta and his American colleagues are continuing to monitor the disease and evaluate its risk to soybean production to determine the best course of action for dealing with it. And they’ll keep growers up to date as more is learned about soybean vein necrosis.
Tenuta adds, “And it’s not just soybean vein necrosis virus – one of our goals every year is to get better, more accurate and more up-to-date information on all crop diseases. There are always new diseases coming along, like soybean vein necrosis, and diseases that are already here that are changing, like soybean cyst nematode and northern leaf blight in corn. So it’s a never-ending battle to manage them in an economically and environmentally safe way.”