A typical leaf symptom of BPMV is a yellow and green blotchy appearance called leaf mottle. Symptoms are most apparent during cool weather and periods of rapid growth. Sometimes symptoms resemble injury from herbicide drift. Click on photo to view larger version. Photo © Palle Pedersen.

BPMV is spread primarily by the bean leaf beetle, Cerotoma trifurcata, in Wisconsin. Showy tick trefoil infected with BPMV is a main source of BPMV outside of soybean fields. Seed transmission is also considered a major source of BPMV; transmission from seed to seedling is about 1%. The bean leaf beetle then spreads the virus from plant to plant.

Agronomic Impact of BPMV

• Reduced grain yield
• Reduced grain quality: protein and oil
• Poor seed appearance (mottled, moldy)
• Reduced seed performance: germination and seedling vigor
• Predisposition to other stresses?
• Delayed plant maturation (green stem)

Over a broad geographic range, yield reductions between 10% and 40% have been reported.  The impact of BPMV on yield depends upon the time of virus infection: early infection results in the highest yield reduction. Soybeans can be infected with more  than one virus at the same time. A synergistic reaction occurs when plants are infected with both BPMV and Soybean mosaic virus. Reduction in yield is much greater in mixed infections than either virus alone.

To determine if a virus problem exists in your field, look for the following signs: 

• Moderate to high insect activity
• Lower than expected yields
• Mottled seed
• Green stem plants at harvest maturity 
• Presence of leaf symptoms: mottled, crinkled leaf symptoms typical of virus infection. (Be aware that leaf symptoms are not a consistent indicator of virus infection. Varieties differ in the amount of leaf symptoms. Climatic conditions and growth stage can also mask leaf symptoms).

Virus symptoms will be most apparent during times of rapid growth and cooler weather. Mottled and crinkled leaves can also be caused by herbicide injury or nutrient deficiencies.	Presence of bean leaf beetles can indicate virus risk. Read more about bean leaf beetles »	Mottled seed can indicate virus infection. However mottling can also be caused by insect feeding and physiological causes.

To reduce risk of loss of yield and seed quality due to BPMV:

• Consider a later planting of soybean in 2007, especially if BPMV was a yield limiting factor in 2006. However, consider that late planting can result in an increased risk of soybean aphid activity at a sensitive growth stage.
• Plant soybean varieties with ability to yield in the presence of BPMV. Use good quality seed that is not mottled.
• Consider the field history of this problem before making decisions whether to spray for the bean leaf beetle. Sprays must be carefully timed to be effective.

Table 1. Virus incidence and soybean growth stage. Click on image to see larger version. Data from Craig Grau.

We have noted that varieties differ in the rate of BPMV infection. For example, Spansoy 201 had less incidence of BPMV throughout a study conducted in the 2002 growing season when compared to Spansoy 250 (Table 1). Less incidence can translate into slower disease progression, thus a higher yield at seasons end. One aspect of our virus screening is to identify those varieties with a lower incidence that would slow virus spread.

Mottled soybean seed can be a symptom of virus-infected plants. However, not all seed mottle is caused by viruses. Photo © Craig Grau.

Seed Mottle
Seed mottling results from pigments diffusing from the hilum. It can be a symptom associated with virus-infected plants, including BPMV and SMV. Some seed are entirely black or brown, but bicolored seed are by far more common. Hilum color will determine the color and intensity of seed coat discoloration.

Mottled seeds are an issue for grain grading, especially for food grade soybeans. Mottling is associated with poor germination and yield. All mottled seed is not caused by virus, however. Insect feeding and physiological stresses can also cause seed mottle.

Treatment thresholds for bean leaf beetle aimed at the prevention of bean pod mottle virus are not yet available. However, here are some guidelines:

Insecticide Timing Options for Bean Leaf Beetle and BPMV control. Click on table to view larger version. Data from Witkowski and Echtenkamp 1996.

• Were 2006 yields lower than expected and unexplainable? Were virus symptoms (leaf mottling, discolored seed, green stem) present in 2006? If so, then BPMV is likely a concern for 2007. 
• Synthetic pyrethroid insecticides tend to provide the most consistent bean leaf beetle control.
• To prevent transmission of BPMV, an insecticide application must be made in the very early stages (V-V2) of soybean development. 
• A second application may be necessary during the emergence of first generation beetles (late June or early July). 
• Delayed spraying at either crop stage can seriously increase incidence of BPMV.

Researchers at Iowa State University have constructed a flowchart to help growers with insect/virus management decisions.
View the chart in the article Revisiting an integrated approach to bean leaf beetle and bean pod mottle virus management»
(link to the Integrated Crop Management newsletter May 2007)

Reference
For a historical perspective of the BPMV-bean leaf beetle-soybean complex, and an overview of recent research on the agronomic impacts of the virus in the region, see Bean pod mottle virus. A threat to U.S. soybean production by L. J. Giesler, S.A. Ghabrial, T.E.Hunt, and J.H. Hill. 2002. Plant Disease 86:1280-1289 (view pdf»).