Over the last week to ten days symptoms of a relatively new virus disease of soybean, soybean vein necrosis, have begun showing up in many soybean fields in various regions of the state. Initially, small light-green to yellow patches develop near main leaf veins. These patches then develop a mottled light green-yellow-brown pattern. The veins in these areas of the leaflet may become clear to almost translucent which is referred to as vein clearing. As the disease progresses these areas turn reddish-brown with a browning of the veins. The reddish-brown areas may have a scaly or scabby appearance. On more susceptible varieties the brown areas may expand killing larger areas of leaf tissue and giving a scorched appearance to the leaves.
Although these symptoms had been observed on soybeans for several years in Mid-South and Midwest states, it was not until 2008 that Dr. Ioannis Tzanetakis, University of Arkansas, discovered a new tospovirus in symptomatic leaf tissue. This new virus was named soybean vein necrosis virus (SVNV). The first identification of the virus was in a sample from Tennessee and in following years the virus was reported in Arkansas, Delaware, Kentucky, Kansas, Illinois, Maryland, Missouri Mississippi, New York, Pennsylvania, and Virginia. Until this season the reports of soybean vein necrosis in Missouri were primarily from seed production fields in southeast Missouri. But this season, symptoms suggestive of this virus disease can be found in fields throughout the state.
On August 23, I received digital images of soybean leaves with symptoms typical of those caused by this virus disease. August 24 and 25 I traveled through central and east central Missouri trying to stop in at least two soybean fields per county. Then August 27-29, as I traveled to field days in northwest Missouri, I stopped in fields in west central, northwest and northern Missouri. I was able to find plants with symptoms suggestive of soybean vein necrosis virus in Boone, Cole, Osage, Gasconade, Montgomery, Callaway, Howard, Chariton Carroll, Holt, Atchison, Nodaway and Harrison counties. I checked fields in Schuyler, Adair and Macon counties but was not able to find symptomatic leaves in those fields. I have also received digital images showing symptoms indicative of soybean vein necrosis virus from Ste. Genevieve county.
Samples have been submitted for virus assay but I have not yet received the results of those assays. So at this point, from visual symptoms it appears as though this virus may be quite widespread in Missouri this season. But until the assay results are available the diagnosis is unconfirmed.
Certainly other neighboring states (Illinois, Indiana, Kentucky, Tennessee and Arkansas) are reporting widespread occurrence of this virus disease this season.
The virus which causes soybean vein necrosis belongs to a group of viruses called tospoviruses which are spread by thrips. At this point the disease is still a relatively new disease and there are more questions about it than answers. It appears that the virus is spread from soybean to soybean by thrips but which species(s) of thrips is unknown. Other hosts, especially weed hosts, have not been confirmed. And there are many questions related to the disease cycle, possible yield losses and appropriate management strategies.
Varieties seem to vary in their susceptibility to this virus disease and symptoms may vary with varieties. In the fields I checked the incidence of infected plants (plants showing symptoms) ranged from a trace to almost 100% of the plants. The majority of the fields had less than 5% of plants showing symptoms. Symptoms were evident in the upper canopy or upper to mid-canopy. Of the plants showing symptoms, most had small, light-green to yellow blotchy patches near main veins, some vein clearing and some patches in which the veins had turned reddish-brown with a scaly or scabby appearance. Severity in all fields I checked was <10%. More severe symptoms may develop with larger areas of the leaflet turning reddish-brown and premature defoliation occurring.
At this time there isn’t enough known about the virus and disease to make effective management recommendations. As more information becomes available on this disease, management strategies can be formulated and recommendations made. Overall, disease symptoms of soybean vein necrosis have been low to moderate across fields in all states in which the disease has been reported this season. So for this season no control measures are recommended.
REVISED: October 1, 2015