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Posted 20 September 2011. PMN Crop News.

Oh...That's What That Is

Source: University of Illinois Press Release.

Urbana, Illinois (September 12, 2011)--Several years ago, when the agricultural industry had serious concerns related to the potential impact of Asian soybean rust, Extension stumbled upon an unusual disease that made us very concerned. We had warned producers that the pustules associated with rust would tend to be brown in color and that they would appear on the bottom of the plant first. Extension also warned producers that those pustules might cluster near the main veins of the plant where moisture tended to be more prevalent. The disease that we found looked pretty serious. It made small, brown, somewhat angular lesions that clustered about the veins of the plant. However, the lesions appeared to be somewhat localized, infecting only portions of some leaves rather than entire leaves as would be expected with rust. Additionally, there were no pustules (spore-producing structures) associated with these lesions as would be expected with rust. We took solace in knowing that the disease was not rust, chalked the unusual symptoms up to sunscald or something bacterial, and thought little more about it. Yet, these strange symptoms continued to appear causing many to wonder what this "thing" was.


Fast forward to 2008, and one finds that plant pathologists in Arkansas and Tennessee were seeing the same unusual browning on beans to our south. In fact, they were seeing enough of the disease to generate some real concern over its potential yield impact. A little bit of laboratory work found the disease to be viral and further found the disease to be related to a nasty insect-transported pathogen that was already known to plague tomatoes, peppers, zucchini, and more than 60 other crops, vegetables and weeds. The disease was christened Soybean Vein Necrosis Virus (SVNV), agronomists in Illinois realized "oh…that's what that is," and subsequent field surveys found the disease to be the number one viral pathogen in Illinois.

So what do we know about SVNV? We know that SVNV belongs to a class of viruses termed "tospoviruses." Tospoviruses are composed of RNA, strands of chemical information that "tell" cells what materials they should produce. As is the case with other viruses, SVNV tricks cells into producing more copies of the virus instead of producing materials desperately needed by the plant. The "tospo" portion of the name refers to the first detected member of this "viral group" – tomato spotted wilt virus - the nasty vegetable disease noted earlier. In tomatoes and other crops, the tosposvirus causes plants to wilt, leaves to brown, and fruit to become mottled and bumpy. The yield of fruit is substantially reduced. Agronomists honestly do not know if SVNV creates similar yield issues in beans.

As with other tospoviruses, plant pathologists currently believe that SVNV is transported by small insects called "thrips." Thrips are about 1/16 inch long, are oblong, and can be white, yellow, yellow-green, or tan in color (sometimes with dark bands). The color and markings vary from species to species and from maturity stage to maturity stage. Thrips do not overwinter in Illinois and migrate back into our state during the middle part of the growing season. Multiple generations can be found each year. Thrips are perhaps the most prevalent insect species in soybeans but their feeding is typically not responsible for much yield loss. Pathologists currently believe that infected thrips remain SVNV vectors until they die which is atypical for most of the tospoviruses.

How do we manage SVNV or do we even need to? The question of "management need" remains open. In most cases, SVNV does not seem to eliminate large amounts of leaf area. However, we previously noted that similar pathogens can decrease yield in vegetables. Nobody knows if this disease does the same, and nobody knows if SVNV might decrease yield when present with other viral pathogens. Should management prove to be needed, comparable questions remain. In vegetables, Tomato Spotted Wilt Virus is partially managed by removing host plant material from the area (i.e. no ornamental plants too near the garden) while some tout the elimination of thrips via applying insecticide. Should SVNV prove to be more than a leaf oddity, pathologists will need to first determine where the pest picks up the virus (i.e. does thrip infection occur here or down south) before they settle upon possible management strategies.

Matt Montgomery