Tobacco ringspot virus (TRSV), or more commonly known as bud blight of soybeans, has been showing up across the Midwest in 2017. Producers on AgTalk have reported seeing symptoms of this disease in several states including Minnesota, Wisconsin, Illinois, Iowa, Nebraska and Ohio. These producers commented that the beans are very green and with minimal pod formation on them. It is not known whether the virus was confirmed in the lab, but the symptoms would lead to this conclusion. There are also other viruses cause similar symptoms, and in some cases more than one virus is causing the issue. The only way to verify it is to have a virus test run at a plant disease clinic. Iowa State University and the University of Nebraska plant disease clinics have received plant samples to have the test run on them, and they tested positive for the Tobacco ringspot virus. These pictures are from a field in western Iowa that had a viral test performed on it and it came back positive for both tobacco ringspot virus and Bean pod mottle virus.
Tobacco ringspot was first described in 1922, but it was not shown to infect soybeans until 1934 in experiments. It was first seen in fields in Indiana in 1941, Iowa in 1942, and Ohio in 1943. Soybeans are the most sensitive to this virus compared to the other diseases caused by TRSV. Yield losses have ranged from 25 to 100%. Temperature has an influence on the severity of the yield loss as soybeans, that are grown under 79 degrees showed the strongest bud blight symptoms and were severely stunted.
TRSV is a member of the nepovirus group of plant viruses and it is related to arabis mosaic virus, grapevine fanleaf virus, tomato blackring virus, and tomato ringspot virus. There several different strains of TRSV that can express a variation of visual symptoms. It has a wide range of woody and herbaceous host plants.
Early infected plants are stunted and will remain green after the non-infected plants mature. The terminal bud becomes curved forming a “shepherd’s hook”. As the disease advances, other buds will become brown, necrotic, and brittle. Random leaf and floral buds may reproduce rapidly. The pith of the stem or petioles may have a brown discoloration near the nodes at first and then throughout the stem. Leaflets are small when compared to normal and they tend to roll and cup. The blades become wrinkled and bronzed. Pods are usually underdeveloped or they abort. Pods that are developing before the infection develop dark blotches.
TRSV is a systemic infection in susceptible plants that starts at the leaflet and moves to the stem and into the roots. It is rare for the disease to move from the roots upward into the pant through the xylem but is easily moved in the sap. Several vectors have been identified as a way the disease moves in a field. Nymphs of thrips, aphids, tobacco flea beetle and grasshoppers have been studied on their ability to move the disease from plant to plant. It was found that these insects can translocate TRSV but were found to a have a low efficiency. Dagger nematodes have been identified as a vector but also have a low efficiency of the transmission of the virus. Nematode transmission remains in the root and seldom moves up in the plant as was mentioned earlier. Recent studies of honey bees have found that the virus can survive in their abdomen and may also be a contributing factor to colony collapse disorder.
Seed transmission has been found to be the way the disease has a long-range dissemination and survival from year to year. Reproductive portions of the soybean plant were studied in the 1950’s and the 1960’s and it was found that the virus moves within the embryotic tissues of the seed and over half of plants grown from infected seeds produce 100 percent infected seed. Studies also found that the virus remained infectious for at least 9 months under normal storage. This research also showed that the seed-transmitted virus had little to do with the effects on yields because soybean plants infected before flowering produce few or no seed, leading to the conclusion that seed is not the primary source of inoculum in a field. The other concern with this theory of the seed being the primary inoculum is the fact that the virus usually starts at the field edges and moves downwind into the field. This shows that more research needs to be conducted to better understand this plant disease.
There are soybean research cultivars that have resistance to a few strains of TRSV, and these are PI407287, PI92713, and PI154194. Some other management considerations would be to plant virus-free seed and treating the edges of fields with an insecticide.
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Download a copy of this Technical Bulletin: Tech_354 - Tobacco Ringspot Virus