The hook: Well, since the subject of bean pod mottle virus has been broached this season, let's shed a bit of light on the subject. It can get confusing fast because the talk across the state centers on four subjects that may or may not actually be related. First is the actual disease itself; second is what the vector of the disease might be; third is the relationship of the curious "green-stem" symptom to the disease; and fourth is something called "green-stem syndrome." |
The line: Bean pod mottle virus (BPMV) is not a new disease to Illinois and has been present in many of the southern soybean-growing states for many years. Infection by BPMV can cause losses of 10% to 17% but can become even more significant if dually infected with soybean mosaic virus (SMV), where losses can approach 60%. Losses are greater when the plants are infected with BPMV in the seedling stage. Plants infected with BPMV have a higher incidence of other seed diseases.
The symptoms: The disease causes a mottling and distortion of leaves in the upper canopy of the plant during periods of rapid growth and cooler temperatures.
Leaf mottling. (Courtesy of NCSU.)
Another symptom that can be exhibited by BPM-infected plants is "green stems," after the plant matures. Don't jump to any conclusions yet, though, because not all BPMV-infected plants exhibit the green-stem symptom. Plants may also exhibit death of new terminal leaf growth. Seeds of BPMV-infected plants may have a light purplish discoloration of their seed coat.
Seed discoloration from soybean mosaic virus on left; bean pod mottle virus on right. (Courtesy of X. B. Yang, ISU.)
BPMV's natural and experimental host range is limited to three families of legumes. Its natural host range of concern includes soybean and green bean.
The transmission: BPMV is a sap-transmitted virus. Several beetles can move the infective sap around to spread the virus disease, the most prevalent being Cerotoma trifurcata (bean leaf beetle). Other beetles can transmit the virus, including Colaspis brunnea (grape colaspis), C. lata, Diabrotica balteata (banded cucumber beetle), D. umdecimpunctata howardi (southern corn rootworm beetle), and Epicauta vittata (striped blister beetle). It can be mechanically transmitted, graft transmitted, and seed transmitted in a very low (0.1) percentage.
The confusion: At first glance, this seems to be a pretty straightforward disease. It has fairly recognizable leaf and seed symptoms; it is transmitted by beetles that spread infective sap from plant to plant because of their messy eating habits; and it seems to be increasing in frequency. So what's the confusion? The confusion is introduced because of the green-stem symptom that can be exhibited by this disease. There is a syndrome in soybean called green-stem syndrome. The syndrome has been accredited to a number of potential causes, including genetic mutants, BPMV infection, male sterility, and low-potassium soils. Work in Wisconsin isolated a disease organism called a phytoplasma from some green-stem symptomatic plants. So the message on green-stem syndrome is that at this point we don't have a complete explanation of what may actually cause it. Research so far indicates that while BMPV can cause a green-stem symptom, it doesn't always. Also, while it is known that the bean leaf beetle can transmit BPMV, it is not the only vector, and the association is not thoroughly understood.
The answers: Research on green-stem syndrome in Illinois is being done by Glen Hartman, Wayne Pedersen, Les Domier, Darrin Eastburn, Eli Levine, Scott Isard, and Joe Spencer of the University of Illinois and John Russin of Southern Illinois University, funded by a grant from the Illinois Soybean Producers Operating Board. They are looking at BPMV as one possible cause and studying what the relationship of bean leaf beetle and northern and western corn rootworm beetles to the syndrome might be. The pattern exhibited in fields affected with green-stem syndrome does seem to implicate a biological cause.
The sinker: So what about management? The first answer depends on what you are trying to manage. You'll find this isn't easy. Are you trying to manage BPMV? Green-stem syndrome? Bean leaf beetle? Some or all of these things?
If you think you have virus infection, do you even know that's what may be causing the foliar or stem symptoms? You won't know for sure unless you have the tissue tested. As with most of our field viruses, you can send a sample to Agdia for virus testing to find out.
What should you do about the bean leaf beetle? Should you spray to reduce the possibility of transmission of BPMV? Well, there is no definitive answer to this. However, I can draw on experience with other virus diseases that have insect vectors that are present throughout the growing season (for example, barley yellow dwarf virus transmitted by aphids) and make the observation that spraying for a vector that is present throughout the growing season to reduce virus transmission is a very ineffective method of reducing virus disease. And, of course, if the only symptom you get is green stem and no foliar symptoms, the question of spraying is moot because the season is over. If you want to spray for bean leaf beetle, do it because the percent defoliation from the beetle has reached the threshold for treatment.
Many questions remain to be answered both about the role of bean leaf beetle and other beetles in the transmission of BPMV and what the cause of green-stem syndrome might be. Your observations about this increasing situation are very valuable to us. Stay tuned for an update on beetle isolations later in the season.--Suzanne Bissonnette