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Corn+Soybean Digest

Mottle Virus Becomes New Threat

If you're a Midwestern soybean grower, and seed quality has taken a nosedive the past couple of years, there's a likely reason. It's probably bean pod mottle virus, which can whack yields 10-30%.

Actually, the disease, which is seed-borne at a very low transmission rate, has been around for years at extremely low levels in the Midwest. That's changing.

"It used to be a virus disease primarily of the South and Southeast," explains John Hill, Iowa State University plant pathologist. "We don't know whether there has been a change in the virus, so maybe the seed transmission rate is higher, or there has been a change in the germplasm that produces a higher level of seed transmission by inadvertent selection.

"In any case, it has gotten heavier in the Midwest somehow. And it's being efficiently transmitted by the bean leaf beetle, which is definitely a new threat, especially if the virus is overwintering in the beetles."

Scouting for this disease won't be easy because growing season symptoms can be subtle. Plants may be malformed and have green to yellow mottled or "blistered" leaves.

"On the basis of leaf and plant symptoms, I can't tell it apart from soybean mosaic virus in the field," says Hill. "Symptom severity can be highly variable, depending on the soybean variety, but damage can still occur in fields with mild symptoms." Therefore, if you notice unexplained abnormalities in plants while scouting, send samples to your state university for diagnosis.

Like the mosaic virus, it can cause "scuzzy-looking seed" with brown or black seed coat mottling and "bleeding" hilums, says Hill. "We've had an increasing problem with poor seed quality the past two or three years. The problem has surfaced primarily in northwestern, west central and central Iowa, and it appears to be moving east."

There have been reports of the problem in several other Midwestern states, too. It was confirmed in Wisconsin in '99 for the first time.

Mother Nature, with its recent mild winters, is thought to be a key player in the increasing bean pod mottle virus problem because of its connection with greatly increasing bean leaf beetle populations.

"I think the milder recent winters are probably a primary factor," says Marlin Rice, Iowa State University entomologist, who is working with Hill.

"Anytime we have good snow cover, or an absence of extremely cold temperatures for an extended period, those beetles are going to make it through the winter and do quite well. So we start with a big population in the spring, and it snowballs from there."

Bean leaf beetles are 3/16" about long, with either a red or yellow background and rectangular black spots. The beetles overwinter as adults in heavy vegetational cover, such as in nearby woods with a lot of leaf cover on the ground.

Beetles emerge in April and early May and move to an alfalfa field if they can find one. They'll feed there until the early soybeans emerge, then pounce on them and lay their eggs. The insect has two generations, with the second generation feeding on soybean pods.

"Those early planted fields kind of act like bug magnets," Rice explains. "And they're going to pull in the bean leaf beetles. So you don't want to have the first soybean field out of the ground in an area. If you can delay your planting until mid-May, you greatly reduce your chances of bean leaf beetles showing up."

Besides the recent trend toward earlier planting, and and its agronomic promise of higher yields most years, Rice thinks another reason for resistance toward planting later is that the bean leaf beetle alone has not been perceived as a serious problem. That has been correct. Entomologists have normally recommended ignoring that early feeding because it doesn't hurt yield. But the game rules may be about to change.

"If the beetles are transmitting this disease, which it appears they are, we may be wrong in that recommendation," says Rice. "I think if we tie the beetles and the disease together and have this double whammy, that's going to get some people's attention. And they're going to have to start rethinking how to get around this problem."

Unless Mother Nature steps in with a winter such as 1993, with little or no snow cover and extreme cold for an extended period, the bean leaf beetle problem will likely continue to multiply. It might even get to the point where the beetles themselves create enough damage to justify spraying.

If that hard winter hits, however, both the beetle population and the bean pod mottle virus could become essentially a non-problem again, at least for a time.

When Iowa grower David Linn discovered the mottle virus, he couldn't get anybody to believe he had a real problem.

"I've been fighting this thing for two or three years," says Linn, of Correctionville. "At first, nobody could tell me what it was. So I contacted John Hill at Iowa State, who knew about the disease. I finally got someone to realize that, yes, it sure is a problem out here."

The best longer-term defense against bean pod mottle virus, Hill believes, is through transgenic resistance, much like the promising-looking one Iowa scientists have developed against soybean mosaic virus.

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