That nasty virus is still here. We're talking about the bean pod mottle virus (BPMV) — the one with no sure prevention, host resistance or guaranteed treatment.

It's become the most common and widespread viral soybean pathogen in the north central U.S. Economic loss occurs at BPMV infection levels of between 20 and 40% of plants, experts say.

The virus is transmitted through the feeding of the bean leaf beetle, and possibly through infected soybean seed.

“It's difficult to know how much crop damage comes from the bean leaf beetle, and how much is from BPMV,” says John Hill, Iowa State University (ISU) plant pathologist and chairman of the USDA-APHIS Plant Virus committee. “The combination is pretty devastating.” Add to that the staining and mottling of soybean seed, and you have a real problem.

“Defoliation from the beetles can be 35-40%,” says Marlin Rice, ISU Extension entomologist.

“The transmission of BPMV earlier in the season results in great yield loss and reduced seed quality,” says Craig Grau, University of Wisconsin (UW) plant pathologist and BPMV researcher.

A prolonged winter cold snap may diminish beetle numbers, but a blanket of snow insulates beetles from the full punch of cold temperatures, says Palle Pedersen, ISU soybean Extension agronomist. “We need really cold temperatures without snow cover to knock down their populations.”

Last year, “bean leaf beetles returned with a vengeance” in Iowa, Hill says. “It was the highest level of bean leaf beetles here since 2002.”

These beetles overwinter in fencerows and woodlots. They first move to cultivated and wild legumes. As soybean cotyledons, unifoliate and first trifoliates emerge, overwintered bean leaf beetle adults move into soybean fields to continue feeding and lay eggs in mid to late April. They pick up BPMV from the legumes and then transmit it to emerging soybeans, Pedersen says. The greatest virus infection occurs early in the season from the overwintering generation. The first wave will show up in mid July, and the second wave of beetles emerges in mid to late August and feeds on pods.

Bean leaf beetles will concentrate in fields with the earliest-emerging seedlings. The beetles cause damages from feeding on the leaves, and the BPMV reduces yields by reducing grain yield and quality, diminished seed appearance, reduced seed germination and seedling vigor, delayed plant maturation, and vulnerability to other plant stresses.

You can identify BPMV by characteristic yellow to green blotchy leaves, called leaf mottle, on younger soybean leaves. Sometimes leaves have a raised or blistered appearance. BPMV-infected soybean seed can show staining, mottling or hilum bleeding. Symptoms vary considerably according to virus strain and soybean variety. Hill says, “In some varieties it's fairly low, in others it can be 60-70%. There's little correlation between the extent of visual damage and the true extent of the virus infestation.”

What can be done to control BPMV? The insect vector, bean leaf beetles, can sometimes be controlled with insecticides, but you can't always lower vector populations enough to reliably and consistently control the disease, Hill says.

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