Bean leaf beetle feeding

Overwintering adults feed on emerging seedlings. Their eggs become the first generation of bean leaf beetles.
Click on image to view a larger version. Photo: University of Illinois.

BEAN POD MOTTLE VIRUS

Bean pod mottle is a viral disease of soybean, snap bean and other legumes caused by the bean pod mottle virus (BPMV).

Like many plant viruses, BPMV is spread by an insect. In the North Central region, the most important insect vector is the bean leaf beetle, Cerotoma trifurcata. The bean leaf beetle feeds on soybeans or other legumes infected with the virus and transmits the virus particles to the next plant on which it feeds.

How the virus spreads

There are three potential sources of BPMV:

  • virus in overwintered bean leaf beetles

  • virus in perennial host species

  • virus in infected seed

Although the level of virus transmission by overwintered beetles is low, beetle populations in the upper Midwest are at historically high levels, and thus even low rates of transmission through beetles may be important. In a two-year study reported in the journal Plant Disease, bean leaf beetle population densities were linearly related to BPMV incidence in both years, indicating that BPMV epidemics were greatly influenced by the bean leaf beetle.

Similarly, the high beetle populations may make low rates of transmission through seed (less than 0.1%) important in the development of virus epidemics.

Finding out how BPMV spreads is an active area of research. It is possible that all three sources of the virus play important roles, depending on the specific location and environment.

Reference

Quantifying the Within-Field Temporal and Spatial Dynamics of Bean pod mottle virus in Soybean.
Byamukama, E., Robertson, A. E., and Nutter, F. W., Jr. 2011. Plant Disease 95:126-136

  • Agronomic impact

    Infection by viruses and damage to the seed pod can cause a discoloration of soybean seed, also called seed coat mottling (right). Photo: Iowa State University
    Click to view enlargement

    BPMV has been reported to reduce soybean yields of southern cultivars from 3% to 52%, with the most severe reductions occuring when virus infection occurs in early growth stages. Yield reductions between 10% and 40% have been reported over a broad geographic range

    Infection by BPMV decreases pod formation and reduces seed size, weight, and number.

    Infection with more than one virus, such as soybean mosaic virus, is not uncommon, and yield loss can be much greater in mixed infections than either virus alone.

    Mottled seed
    Virus infections can cause seed coat mottling, a problematic symptom for soybean growers and the soybean industry.

    A study in the journal Plant Disease reported that plants infected with BPMV and SMV, alone or in combination, produced seed coat mottling, whereas noninoculated plants produced little or no mottled seed. BPMV and SMV inoculated on the same plants did not always result in higher percentages of mottled seed compared with BPMV or SMV alone. There was significant virus, line, and virus-line interaction for seed coat mottling.

    BPMV can also predispose the plant to infection of seed by the fungal pathogen Phomopsis, which can lower seed germination and seedling vigor. Mottled seed can be a sign of poor seed quality, resulting in financial penalties to producers.

    Refernce: Occurrence of seed coat mottling in soybean plants inoculated with Bean pod mottle virus and Soybean mosaic virus. Plant Disiease 87:1333-1336.
  • Scouting

    Foliar symptoms may be apparant on young, fast-growing leaves during cool weather. Photo: Iowa State University - Click to view enlargement

    Scouting for signs of virus infection.

    Check for leaf symptoms on young leaves, which show symptoms more readily than older leaves.

    Scout for plant symptoms of virus during periods of rapid growth and cool temperatures. Symptoms will be most apparant under these conditions than during dry, hot weather.

    Scouting for bean leaf beetles.

    Scout for the first-generation peak in late V or early R growth stage - around early July.

    Scout for the second-generation peak during pod-fill stage in August. Read more on scouting for bean leaf beetles

  • Symptoms

    BPMV

    Leaf mottling is a typical symptom of BPMV infection, although it is not always present. Click on image to view enlargement. Photo credit: Iowa State University

    A typical leaf symptom of BPMV is a yellow and green blotchy appearance called leaf mottle. Young leaves show symptoms more severely than older leaves, sometimes with a raised or blistered appearance and distortion of leaves in the upper canopy.

    Symptoms are most obvious during periods of rapid growth and cool temperatures. Sometimes symptoms resemble injury from herbicide drift.

    Symptoms go into remission during hot weather and later during the reproductive stages.

    Accurate diagnosis is the key to virus management.
    Soybean viruses are spread by different vectors and have very different management requirements. Therefore, it's important to know which viruses are most prevalent. Unfortunately, the symptoms caused by BPMV are similar to those caused by other viruses. This makes it difficult to diagnose BPMV and most other viruses based on symptoms alone.

    Serological tests can accurately detect and distinguish among suspected viruses. Most diagnostic clinics in the North Central region are able to perform these tests.
    Contact your state university diagnostic clinic
    .

  • Risk Assessment

    The bean leaf beetle can transmit bean pod mottle virus when it feeds on soybean. Photo: University of Wisconsin

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

    • Moderate to high feedling actitivity by bean leaf beetles
    • Lower than expected yields
    • Mottled seed
    • Presence of leaf symptoms: mottled, crinkled leaf symptoms typical of virus infection
  • Management

    Researchers at Iowa State University have constructed a flowchart to help growers with insect/virus management decisions. View flowchart (pdf)

    Although bean leaf beetles transmit BPMV all season long, soybeans are most affected when infection occurs in the seedling stage.

    Current management options for managing bean pod mottle virus are:

    • plant virus-free, certified soybean seed
    • consider early-season insecticide treatments if BPMV has been confirmed in your fields in previous years and bean leaf beetles are present
    • time soybean planting to avoid the highest populations of the bean leaf beetle. This can be done using a degree day model. The method is described in detail in the article Predicting first-generation bean leaf beetles.

    Development of BPMV-resistant soybean varieties is in progress
    The development of virus-resistant or tolerant soybean varieties is an active area of research. Soybean varieties have been observed to differ in their tolerance to BPMV, and some progress has been made in quantifying those differences. BPMV resistance has been detected in a closely related wild soybean species, Glycine soja.

    A recent two-year study looked at BPMV-infection in soybean genotypes that included two insect-feeding-susceptible genotypes, two insect-feeding-resistant, semidwarf genotypes, and an insect-feeding-susceptible, semidwarf genotype. Although the results indicate that BPMV infection varied by genotype, they do not support the hypothesis that insect-feeding resistance is sufficient to reduce the incidence and spread of BPMV.

    Managing the bean leaf beetle for virus control
    Researchers at Iowa State University have constructed a flowchart to help growers with insect/virus management decisions. The flowchart is a dynamic, two-pronged decision guide based on field history. If bean pod mottle virus has not been present in your fields, then the only reason to consider early-season bean leaf beetle management is if populations reach extremely high levels (more than 2.5 beetles per plant) and economic feeding damage can occur. Insecticide treatment will not be cost-effective if the virus is not present.

    Right now, most of the information for predicting economically important variables for beetle damage is available. More information is still needed regarding bean pod mottle virus. As more research is conducted, the results can be added to the decision guide.

    Accurate diagnosis is the key to virus management
    Soybean viruses are spread by different vectors and have very different management requirements. Therefore, it's important to know which viruses are most prevalent. Unfortunately, the symptoms caused by BPMV are similar to those caused by other viruses. This makes it difficult to diagnose BPMV and most other viruses based on symptoms alone.

    Serological tests can accurately detect and distinguish among suspected viruses. Most diagnostic clinics in the North Central region are able to perform these tests.

  • Resources

  • Photo Gallery

    Overwintering adults feed on emerging seedlings. Their eggs become the first generation of bean leaf beetles. - Photo: Kevin Black, University of Illinois

    Mosaic symptoms - Photo: Iowa State University

    Leaf chlorosis and mosaic - Photo: Iowa State University

    Rugosity (rough leaves) caused by virus infection - Photo: Iowa State University