CPN 1003 — May 2015
Soybean farmers should be aware of a new disease caused by soybean vein necrosis virus (SVNV). The virus has likely been in soybeans for some time, but was probably overlooked or misdiagnosed before recent detections. The disease has been detected across the United States (including many states in the North Central region) and in Ontario, Canada.
SVNV is a Tospovirus, a group of viruses capable of causing serious damage in many different crops. The long-term implications of this disease are not yet known, and we are currently working to better understand the disease and potential for yield loss in soybean. Accurately diagnosing SVNV is important, especially because it can easily be confused with other soybean diseases, disorders, or damage.
This publication examines the symptoms of SVNV, describes how SVNV differs from several other plant problems that may look like SVNV, and provides scouting information.
SVNV symptoms are typically randomly distributed throughout the canopy. SVNV lesions start as a yellowing (chlorosis) along the leaf vein. Over time, yellowing becomes red-brown, irregular-shaped lesions, and eventually leads to tissue death (Figures 1 and 2). The yellowing around the lesion may begin to spread beyond the vein but will typically be limited to the area within other major leaf veins. The symptoms are generally not uniform across the leaf.
Leaf tissue will die following chlorosis. Lesions typically spread along or from the edge of a vein. The lesions range from ¼ to ½ inch (6 to 12 mm) but can be larger (Figure 3). In affected plants, the veins may appear clear, yellow, or dark brown. Vein discoloration may be especially noticeable on the undersides of leaves (Figure 4).
Although there is little information about soybean varietal response to SVNV, differences in symptoms may be caused by soybean thrips (the insect vector of this disease), which may prefer certain cultivars.
Several diseases can be confused with SVNV — we describe the seven most commonly confused diseases below. It is important to accurately diagnose SVNV to prevent unwarranted management decisions that will not work against this disease.
Brown spot causes dark brown spots that are scattered on the upper and lower surfaces of leaves in the lower canopy (Figure 5). Adjacent spots can join together to form dead blotches. Late in the season, affected leaves may turn yellow and drop prematurely (Figure 6). Brown spot is very common and is usually one of the first diseases to appear each year.
Brown spot lesions are mainly located in the lower canopy and are not associated with leaf veins like SVNV lesions.
Bacterial blight causes small, angular, yellow-to-brown spots that are surrounded by yellow halos (Figure 7). The angular spots enlarge and join together to produce large, irregular dead areas. The centers of these dead areas tear away so that infected leaves have a tattered appearance (Figure 8). Bacterial blight is seen on the leaves at the top of the plant. It is common after heavy rains, especially if temperatures remain cool.
Bacterial blight lesions have yellow halos and are smaller than those caused by SVNV. Leaves affected by bacterial blight appear tattered, unlike those affected by SVNV.
Bacterial pustule causes small, yellow-green spots with angular, reddish brown centers (Figure 9). You may observe small bumps (pustules) on the undersides of leaf surfaces (Figure 10). Symptoms can be found in the upper canopy. Favorable conditions are high temperatures and above average rainfall.
Bacterial pustule is the only common disease in the Midwest that has pustules on the leaf surface.
It is easy to confuse a younger SVNV lesion that has not yet formed a distinct lesion with Cercospora leaf blight. Cercospora leaf blight starts as a mottled, purple-to-orange discoloration that becomes orange or bronze (Figure 11). The leaves become leathery in texture. This disease usually occurs on the upper three to four trifoliate leaves and on the upper surfaces of leaves in warm, wet weather. Cercospora leaf blight typically occurs mid- to late season.
Cercospora leaf blight symptoms are present only on upper leaf surfaces.
Downy mildew causes light green to yellow, irregular-shaped spots on the upper surfaces of the topmost leaves (Figure 12). On the undersides of leaves, the fungus may be seen growing out of the center of the spots (Figure 13).
Downy mildew lesions may have fuzzy fungal growth on the undersides of lesions. Unlike SVNV lesions, downy mildew lesions may not be associated with leaf veins.
Phyllosticta leaf spot is a minor disease of soybean. Phyllosticta leaf spot results in circular, irregular- or V-shaped lesions that are gray or tan and have narrow, dark margins (Figure 14). In older lesions, numerous small, black specks may be visible.
The dark, black fungal structures found in Phyllosticta leaf spot lesions do not appear in SVNV lesions.
Sudden death syndrome (SDS) symptoms include yellow or dead leaf tissue between the veins of leaflets, while veins remain green (Figure 15). The early yellowing can be confused with SVNV (Figure 16). Leaflets infected with SDS will eventually curl or shrivel and drop off leaving only the petiole.
Symptoms of SDS occur between the veins rather than on or near the veins.
SVNV can also be confused with herbicide damage from ALS inhibitor and ACCase inhibitor herbicides.
ALS inhibitor herbicides are widely used for preemergence and postemergence control for a broad range of weed species in soybean. There are many different herbicide products with this mode of action. ALS inhibitor injury includes stunting, leaf yellowing, inhibition of terminal growth, red veins on the undersides of leaves, and shortened internodes (Figure 17). Damage from ALS inhibitors develops slowly, appearing first on new growth.
ALS inhibitor injury symptoms typically appear earlier in the season than SVNV symptoms. Review your herbicide program to determine if carryover from the previous season could occur.
Quizalofop (Assure II®, Targa®) is a postemergence ACCase inhibitor grass herbicide that is commonly used to control volunteer corn and grassy weeds in soybean. Quizalofop can cause damage along the leaf vein that is similar to SVNV symptoms (Figure 18).
Because quizalofop injury is so similar to the symptoms caused by SVNV, it is very important to review the field’s weed management program to rule out herbicide injury. Send leaf samples to a diagnostic laboratory to identify SVNV.
Tospoviruses are transmitted by a group of insects called thrips. To date, as many as 14 thrips species are known to be vectors of tospoviruses, but only a smaller subset of these are likely vectors of SVNV. Soybean thrips (Neohydatothrips variabilis) have recently been confirmed as a vector of SVNV and additional investigation may reveal other vector species.
Adult thrips are barely visible to the unaided eye (Figure 19). The insect feeds with a rasping-sucking mouthpart that leaves small, dash-like scars on leaves. Thrips development depends on temperature and adult females can survive for three to five weeks.
When nymphs emerge from eggs (Figure 20), they can acquire SVNV by feeding on an infected source plant. After pupation, infected adults spread the virus to additional plants. If the virus is acquired by adult thrips, transmission to additional plants is not possible as the virus can only complete its lifecycle when acquired by immature thrips.
Eggs from an infected adult do not carry the virus, but the virus may be capable of overwintering in living host weeds and within thrips migrating from the South.
Confirming virus infection is difficult since virus diseases often resemble other foliar diseases or disorders such as herbicide injury or nutrient imbalances. The only way to positively confirm a diagnosis of SVNV (or any virus) is through laboratory testing. The virus is easier to detect in leaf samples with early disease symptoms. Once lesions have turned from yellow to brown, it is difficult to detect the virus in symptomatic tissue. Contact your state’s land-grant university or private diagnostic lab for pricing and sampling procedures.
If SVNV will limit yields, growers may consider treating soybean with an insecticide to reduce thrips populations; however, we do not currently recommend insecticide applications in response to SVNV detection since vector control of other insect-transmitted diseases has not typically been successful.
Researchers will continue to monitor this disease and assess its potential impact in an effort to determine the best management options. Future recommendations will be developed as we learn more about this disease.
To learn more about soybean vein necrosis virus, visit the Crop Protection Network website (www.cropprotectionnetwork.org) or consult your land-grant institution. Other publications in the Soybean Disease Management series are available by visiting the CPN website or your land-grant institution’s website.
Chris Bloomingdale, University of Wisconsin
Carl Bradley, University of Illinois
Martin Chilvers, Michigan State University
Loren Giesler, University of Nebraska
Russ Groves, University of Wisconsin
Daren Mueller, Iowa State University
Damon Smith, University of Wisconsin
Albert Tenuta, Ontario Ministry of Agriculture and Food
Kiersten Wise, Purdue University
Emmanuel Byamukama, South Dakota State University
Anne Dorrance, Ohio State University
Doug Jardine, Kansas State University
Dean Malvick, University of Minnesota
Sam Markell, North Dakota State University
Punya Nachappa, Indiana University-Purdue University,
Adam Sisson, Iowa State University
Laura Sweets, University of Missouri
All photos were provided by and are the property of the authors and contributors except: Figure 10 by Alison Robertson, Iowa State University; Figure 18 by Paul Bachi, University of Kentucky Research and Education Center; and Figure 19 by John Obermeyer, Purdue University.
This content was originally published as Soybean Disease Management: Soybean Vein Necrosis Virus in 2014 as Purdue Extension publication BP-186.
The Soybean Disease Management series is a multi-state collaboration sponsored by the North Central Soybean Research Program (NCSRP). Learn more about the NCSRP at www.ncsrp.com.
This publication was developed by the Crop Protection Network, a multi-state and international collaboration of university/provincial extension specialists and public/ private professionals that provides unbiased, research-based information to farmers and agricultural personnel. More information available at www.cropprotectionnetwork.org.
This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaption Council assists in the delivery of GF2 in Ontario.
The authors thank the United Soybean Board and Grain Farmers of Ontario for their support.
Design and production by Purdue Agricultural Communication.
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or a part of an individual’s income is derived from any public assistance program. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination write to USDA, Director, Office of Civil Rights, 1400 Independence Avenue, S.W., Washington, D.C. 20250-9410 or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal opportunity provider and employer.
©2015 by the Crop Protection Network. All rights reserved.