In 1999, soybean producers from around the state reported problems with soybean green stem and mottled or discolored soybean seed. Many of these reports of poor soybean quality are suspected to be caused by the spread of a new soybean disease in Iowa, bean pod mottle virus. Bean pod mottle virus was identified in Iowa as early as 1968, but it hasn't been widespread or implicated in causing significant yield losses. Last year bean pod mottle virus was confirmed in soybeans in several central and western Iowa counties (Dallas, Ida, Marshall, Polk, Story, and Woodbury). Unconfirmed reports suggest that it may be present in all but the northern two tiers of Iowa counties. The virus has been a problem in soybeans in the southern United States.
||Discolored soybeans from an Iowa field heavily infected with bean pod mottle virus.
What is bean pod mottle virus?
Bean pod mottle virus is a disease that can infect soybeans and other legume species. Symptoms of bean pod mottle virus may resemble injury from herbicide drift or symptoms of soybean mosaic virus. Infected soybean plants may have mottled or crinkled leaves and plants may be stunted. As soybeans mature, foliar symptoms may not be obvious. At harvest, infected plants may have mottled soybean seeds. The effect of bean pod mottle virus on yield has been shown to vary by soybean variety and growing conditions, but losses of more than 50 percent were found in some studies. Bean pod mottle virus also can occur in combination with soybean mosaic virus, resulting in even greater yield reductions. Potential problems with bean pod mottle virus are twofold: reductions in soybean quality and reductions in yield.
Where does bean pod mottle virus come from?
The main path for bean pod mottle virus entry into soybean plants is through insect feeding. The main vector for this virus is the bean leaf beetle. When beetles feed on soybean leaves they produce a small amount of regurgitated plant material. If this regurgitant is from a plant that was infected with virus, it can infect a healthy plant on which the beetle is feeding. Other vectors for this virus have been reported, including blister beetles and southern corn rootworm; however, the bean leaf beetle is considered the most important vector in Iowa because of its abundance and statewide distribution. Seed transmission of bean pod mottle virus has been documented but is minimal so far (less than 1 percent).
||The bean leaf beetle transmits bean pod mottle virus to soybeans.
When does the bean leaf beetle transmit the virus?
One study found that the virus could overwinter in adult bean leaf beetles. Thus, beetles could pick up the virus from infected soybean plants in the fall and spread it when they come out of overwintering sites and enter soybean fields the following spring. However, other work has shown that bean pod mottle virus does not stay infective in beetles throughout the winter, so there seems to be some uncertainty as to whether beetles coming out of hibernation remain infected with the virus. Beetles also may obtain the virus by feeding on infected wild legumes in the early spring and then transmit the virus to soybeans when they emerge. Although we don't know exactly when beetles may be transmitting bean pod mottle virus, we do know that the earlier soybeans are infected, the greater the potential reduction in yield.
How will you know if your soybeans have bean pod mottle virus?
The only way to be sure is to test soybeans from your field. Soybean plants that have the above-mentioned symptoms would be good candidates for testing. If you are concerned about disease symptoms in your fields this summer, plants can be sent to the Plant Disease Clinic at Iowa State University for analysis. Before sending plant samples, contact Paula Flynn, Plant Disease Clinic diagnostician, at 515-294-0581 or view the clinic's Web site at www.exnet.iastate.edu/Pages/plantpath/pdcintro.html for instructions on preparing and sending plant samples. We hope to have a technique developed this summer that can rapidly evaluate bean leaf beetles for the presence of bean pod mottle virus. This work will be done in cooperation with John Hill, Department of Plant Pathology.
What can you do if your soybeans are infected?
At this point, the best way to manage the virus is to manage the bean leaf beetle. If bean leaf beetles were abundant in your fields last year, you will probably have them again this year because of the mild winter (see next week's ICM newsletter article on overwintered beetles). If bean pod mottle virus symptoms were observed in your fields, you may want to consider an early-season insecticide treatment when the beetles first invade the field. However, this management strategy has not been tested yet for its ability to prevent viral infection of the crop. The reason this strategy may not work is that timing an insecticide application for early-season beetles is difficult because beetles leave overwintering sites and colonize soybean fields over a several-week period. Because there is so much that we do not understand about the virus-beetle-soybean relationship, we cannot state confidently that this is the best management tactic. We will be evaluating this management strategy in field trials this summer.
Another option for managing early-season beetles is to plant soybeans later in the season to deter colonization of fields by bean leaf beetles. Even delaying planting until mid-to-late May can reduce bean leaf beetle densities in a field and thereby possibly reduce incidence of the disease in the crop. Plant resistance to bean pod mottle virus might be another option of managing the disease, but currently there are no funds available to support this work.
The next step: Research.
There is still a tremendous amount to learn about bean pod mottle virus. This summer a cooperative project between plant pathology and entomology will begin exploring many of the basic questions about the disease and possibly how it can be controlled. This research will be supported in part by a grant from the Iowa Soybean Promotion Board.
This article originally appeared on pages 43-44 of the IC-484 (6) -- April 24, 2000 issue.