Nancy Gregory of the University of Delaware Plant Clinic sent out an alert through the NPDN (National Plant Diagnostic Network) that there might be a new virus on soybeans showing up in our region. It has not yet been confirmed, however, the pictures of the symptoms are consistent with samples or pictures I have seen from around Maryland. I have to admit I initially considered the symptoms to be either signs of feeding injury from insects, or early symptoms of Cercospora leaf blight. So far the presumptive diagnosis based on symptoms alone is of soybean vein necrosis virus (SVNV). This is a relatively new problem first identified in 2008 by Dr. Ioannis Tzanetakis of the University of Arkansas. Since then it has been detected and confirmed in Arkansas, Illinois, Kansas, Kentucky, Missouri and Tennessee. We have samples or pictures of samples from Caroline, St. Mary’s and Washington counties. Since the alert, New York has also indicated that they have samples with similar symptoms. If these all turn out to be SVNV then this new disease has spread rather rapidly, under the radar.
What is known about SVNV is that it is related to what are known as tospoviruses.
These are thrips‐transmitted viruses. The best‐known example belonging to this group is tomato spotted wilt virus, which is a very serious problem. One characteristic of this group is that the virus is persistently transmitted, meaning the virus can be transmitted by the thrips for a long time. Many plant viruses are only picked up by their vectors and are able to be transmitted for a short period, but this group of viruses replicates in the vector as well as the plant and thus the vectors can transmit the virus for long periods. This facilitates moving the virus over long distances and could explain the rapid spread. It is not yet known how serious this disease can be with regard to affecting yield. However, the pictures and descriptions from Arkansas and Illinois indicate that significant leaf necrosis and possible early defoliation can occur. The advanced stages of this disease look a lot like a leaf spot caused by Phyllositicta, a fungal disease that is generally considered a minor problem in our area. It is even possible that as the necrotic leaf spots develop as a result of SVNV that organisms like Phyllosticta are secondary invaders that colonize the dead or dying tissue. This could mean we had SVNV for a few years and missed it because we found Phyllosticta associated with the lesions.
It would be very useful to establish whether or not we actually have SVNV, how extensive it is, and whether or not we are getting the advance stages of the disease that would indicate possible losses due to the disease. We have verbal agreement with Dr. Tzanetakis that he would be willing to run some samples to confirm the presence of the virus. There is no commercially available test for this virus that we know of. Please examine some soybean fields for symptoms. Send a sample to the Plant Clinic or contact me. See if you can estimate the percentage of plants that display symptoms and typically how extensive it is in the canopy, for example, is it on the upper leaves only, or just the middle or lower canopy leaves, or is it throughout the canopy. Lastly if you can determine the cultivar this would be useful. There is some indication that there are varietal differences. A couple of good websites with information are:
http://arkansasagnews.uark.edu/5423.htm
http://agronomyday.cropsci.illinois.edu/tour-soybeannecrosis.html
If the MD and DE samples turn out to be SVNV, they appear to be earlier stages of infection.
Arv Grybauskas
U MD Extension Field Crops Plant Pathologist