Soybeans: Got Disease? It's Not a Good Thing

Suzanne Bissonnette

Extension Educator, Integrated Pest Management
Extension Coordinator, SCN Coalition

Office: Champaign Extension Center
801 N. Country Fair Dr., Suite E, Champaign, IL 61821
Phone: (217) 333-4901


Sudden Death Syndrome

Weather conditions were very favorable for sudden death syndrome (SDS) infection this season. In a state survey for the disease, researchers from the University of Illinois and Southern Illinois University noted and recorded levels in Illinois.

The soilborne fungus Fusarium solani f.sp. glycines causes SDS. The disease is favored by cool temperatures and by relatively high soil moisture levels through the first half of the growing season. When above-ground symptoms become evident, the seed will not mature any further. The disease typically shows up first in low spots, where other root rots may have been a problem earlier in the season, or in areas of the field where soybean cyst nematode (SCN) has been a problem. While neither SCN nor other root infections are necessary for SDS infection, they all share similar environmental factors for disease development.

Initial symptoms of SDS are the appearance of small, yellow spots on the upper leaves. These spots enlarge and the tissues between the veins turn brown while the vein tissues remain green. Leaflets may drop prematurely, leaving the petioles attached. Flowers may abort, and pods usually do not fill. The first set pods may have a few beans in them, which remain small. Later pods may not fill or may have immature green seed. Plants infected with SDS have reduced yield due to premature plant death, pod abortion, lack of pod fill, and low test weight.

A diagnostic symptom of SDS is that the interior of the stem remains white and the vascular tissue turns a grayish brown. Rotted roots may even show the presence of the blue fungus. The later in the season the above-ground symptoms appear, the less yield loss expected.

Bean Pod Mottle Virus

Certainly the subject of bean pod mottle virus has been broached this season. It can become confusing quickly because the talk across the state centered on four subjects that may or may not actually be related. First is the actual disease itself; next 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."

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 10-17 percent but can become even more significant if dually infected with soybean mosaic virus (SMV), where losses can approach 60 percent. 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 disease causes a mottling and distortion of leaves in the upper canopy of the plant during periods of rapid growth and cooler temperatures. Another symptom that can be exhibited by BPMV infected plants is "green stems" after the plant matures. Don't jump to any conclusions 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. BPMV natural and experimental host range is limited to three families of legumes. Within these three families, only two plants (soybeans and green beans) are of concern to producers.

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 can be seed transmitted in a very low (0.1 percent) percentage.

At first glance, this seems to be a 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? 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 a number of potential causes, including genetic mutants, BPMV infection, male sterility, or low potassium soils. Work in Wisconsin isolated a disease organism called a phytoplasma from 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 BPMV 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.

Cooperative research on green stem syndrome in Illinois is being done by the University of Illinois and 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 the bean leaf beetle and the northern and western corn rootworm beetle to the syndrome might be. The pattern exhibited in fields affected with green stem syndrome does seem to implicate a biological cause.

So, what about management? First, answer 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.

What should you do about the bean leaf beetle? Should you spray to reduce possibility of transmission of BPMV? Well, there is no definitive answer to this. However, drawing on experience with other virus diseases that have insect vectors present throughout the growing season (for example, barley yellow dwarf virus transmitted by aphids), one can make the observation that spraying for a vector that is present throughout the growing season in order 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 with 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 the bean leaf beetle and other beetles in the transmission of BPMV and the possible cause of green stem syndrome.

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