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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“Managing the beetles does a poor job of managing the virus,” Grau says. Controlling spring colonizing beetles is the most successful approach since there are no resistant varieties.
Scouting newly emerged soybeans is recommended for early planted soybeans in late April and throughout May for overwintered bean leaf beetle feeding, says Eileen Cullen, University of Wisconsin Extension entomologist. Make sure the soybean defoliation thresh-olds are not reached. “Defoliation from overwintered bean leaf beetles on emerging soybeans at V1 (unifoliate) to V2 (1st trifoliate) is noticeable and can look dramatic, especially given how small the plants are at this time,” she says. “Keep in mind, however, that in terms of economic damage due to defoliation, the plant is very resilient.
“The cultural control of late planting will permit fields to escape peak overwintered bean leaf beetle feeding,” Cullen says.
Early season insecticide application should be based on “yes” answers to the following three questions before spraying, says ISU's Rice:
Are bean leaf beetles present in the field?
Are plants in very early growth stages (VC-V2)?
Were BPMV symptoms confirmed or strongly suspected (i.e., green stems at harvest) in your soybean fields last fall?
“Some growers will want to wait to spray the beetles at the same time they apply a burndown herbicide to soybeans,” Rice says. “If soybean seedlings are beyond the V2 leaf stage, an insecticide application will probably be too late to be effective against the overwintering population.”
Test plots showed that Cruiser and Warrior did the best job of decreasing bean leaf beetle numbers when their populations were low or medium, Rice says. When beetle populations were high, seed treatments didn't help.
The insecticide Warrior applied after soybean emergence and again at first generation beetle emergence significantly reduced beetle densities through midseason in ISU research.
“Insecticides applied to control overwintering bean leaf beetles may cause higher soybean aphid populations because it kills aphids' predators, insidious flower bugs,” Rice says.
Spraying only reduces the number of beetles present to spread the virus, but doesn't attack the BPMV. The only way to completely manage BPMV is with host resistance, which has yet to be developed, Pedersen explains. “Insecticide manages the vectors (beetles) but doesn't eliminate the problem,” he says.
The economic threshold of just the bean leaf beetle leaf feeding (excluding BPMV losses) is 16 beetles per foot of row in the early seedling stage, or 39 per foot of row at stage V2+, according to research at the University of Nebraska.
Compounding identification of BPMV is soybean mosaic virus (SMV), whose leaf mottle symptoms and stained seeds resemble those caused by BPMV.
“SMV, efficiently transmitted by the soybean aphid and not the bean leaf beetle, has grown with the introduction of the soybean aphid,” Hill says. And infection by both viruses can cause synergism, meaning damage caused by both viruses in the same plant is more than additive. You can't base your diagnosis upon visual symptoms; the only way is through a laboratory test,” Hill says.
Infection by both viruses can cause yield drops by up to 85%.
SMV is transmitted by aphids, and BPMV is transmitted by bean leaf beetles. Research shows that spraying for aphids and bean leaf beetles cannot be combined.
Because BPMV can appear with SMV, it's wise to select SMV-resistant soybean varieties where BPMV is a problem, Hill says.
“I think in the next five to 10 years you'll see varieties in the marketplace with claims of tolerance,” Grau says. “Complete resistance has never been found.”