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Virus Disease Risk in Early-Emerged Soybeans

May 11, 2001
In east-central Illinois, the earliest-planted soybeans are emerging well and are big enough that rows can be distinguished. The first pest customer for early-emerged soybeans, seedling blights aside, is the bean leaf beetle (BLB). The bean leaf beetle overwinters as an adult and has been waiting not so patiently for the soybeans to emerge. Why not just let the entomologists write about bean leaf beetle? Well, many pest species interact to cause larger problems than they would singly. BLB is one such pest. It not only causes physical damage to the plant by feasting on the leaves but can also transmit a viral plant disease called bean pod mottle virus (BPMV). First-emerged soybean fields are at a greater risk of BLB feeding than later-emerged fields.

Bean leaf beetle feeding and BPMV transmission. (Courtesy of Glen Hartman, USDA-ARS.)

Consequently these fields are at greater risk of being infected with BPMV.

The disease: 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 as well.

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.

Bean pod mottle virus foliar leaf symptoms. (Courtesy of Glen Hartman, USDA-ARS.)

Another symptom that can be exhibited by BPMV-infected plants is "green stems," after the plant matures. However, 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 very light purplish discoloration of their seed coat. BPMV natural and experimental host range is limited to three families of legumes. Its natural host range of concern to producers is 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, graft, and seed transmitted in a very low (0.1%) percentage.

Some added confusion: Well, at first glance, this seems to be a pretty straightforward disease. It has fairly recognizable leaf and seed symptoms, is transmitted by beetles that spread infective sap from plant to plant because of their messy eating habits, and seems to be increasing in frequency. So what's the confusion? Well, 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 any number of potential causes, including genetic mutants, BPMV infection, male sterility, and low potassium soils. 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.

Management: So what about management? First, determine 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 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. Stay tuned.--Suzanne Bissonnette

Author: Suzanne Bissonnette

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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