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Corn+Soybean Digest

New Soybean Disease Concerns Pathologists

DNA technology, the same high-tech science used in criminal cases, has identified a possible new and potentially serious worry for soybean growers.

The technology has allowed the detection of two previously unreported parasites of soybeans.

They are new strains of the clover proliferation phytoplasma and/or the aster yellow phytoplasma, often found together, explains Craig Grau, University of Wisconsin plant pathologist.

The new strains were discovered and identified by a USDA scientist, Ing-Ming Lee, in collaboration with University of Wisconsin scientists. Lee also confirmed an old phytoplasma strain submitted by a University of Arkansas scientist in 1996.

There's somewhat of a dilemma for the scientists at this early point. They know that phytoplasmas are usually associated with plants that express abnormal growth characteristics like they are now seeing.

However, Grau cautions that they haven't been proven to be pathogenic to soybeans - that is, causing disease and yield loss. They look suspicious, though.

"I feel it is one of the more promising leads we have to explain these soybean plant abnormalities," Grau explains.

How serious is this threat?

"If phytoplasmas do prove to be causing disease, I doubt they will be a problem that is going to wipe out the soybean crop in this country, or anything close to it," declares Grau. "But the situation has a lot of people worried."

These new strains obviously have been seen in Wisconsin and elsewhere in recent years. But no one knew what they were seeing, acknowledges Grau, whose team is working hard on the new challenge.

Previously, Grau suspects, the symptoms on soybeans caused by the clover proliferation phytoplasma or the aster yellow phytoplasma were likely blamed on herbicide damage or possibly quirky weather.

So where do they come from, and how do they get to soybeans?

"We don't know for sure how they're transmitted," Grau says. "However, all phytoplasmas usually have some type of insect as a vector that moves them from a source to the crop in question - and it's usually a leafhopper.

"We have found both of these phytoplasmas in alfalfa and clovers, which is where the name of one of them comes from," the scientist adds. "So most likely plants in ditches, waterways and a field next to soybeans could certainly all be sources of the phytoplasmas."

Like many diseases, this one doesn't settle on a whole field like fog. It starts with individually infected plants, usually along a field edge, then becomes a scattering of plants and finally pockets of infection of various sizes - much as something like soybean cyst nematodes begin in a field. Check first along field edges, Grau advises.

"In these pockets of activity, I think we're talking 20 bu per acre yield losses, in some cases even 50% or more," Grau estimates. "On a whole-field basis, if the disease spreads, it could translate into substantial yield losses."

What are the symptoms to look for?

Some affected plants samples were coming into the university's disease clinic in late June. The plants were bushy with short nodes and had more stems than normal.

Plants in midseason have a darker-than-normal color. That's caused by the purple anthocyanins, or pigments, starting to come through and mixing with the normal green pigments. It produces kind of a blue-looking plant, Grau says. A yellow pigment may be present, too, which may be because the aster yellow organism is also present. So there sometimes may also be a yellowing of plants involved.

"That blue-looking plant is one of the most common symptoms," Grau explains. "This, many times, is the whole plant.

"But sometimes the purpling may be on only one side of the stem. Many times we will also see more stems than normal; sometimes stunting; many, many buds at nodes; and frequently there are no pods, or fewer pods. Or, if there are pods, they are misshapen and don't have seed, or they can be relatively normal."

In his experience in the fall, however, one of the most apparent symptoms is that stems stay green when leaves are yellow or brown. The only thing that will eliminate that green color is killing frost.

As with many plant diseases or pests, it appears weather can be a big factor in its severity. In this case, add leafhopper populations.

"In our lab tests, symptoms appear to be more severe if temperatures are in the 60s than in the 70s or 80s," Grau notes. "So under hot weather conditions, I suspect the disease will go into remission.

"I suspect that disease incidence will be very reliant on the type of year we have weatherwise or the leafhopper activity. Last year we had one of the most severe leafhopper years we ever witnessed. Their activity lasted long into the growing season."

If you see symptoms that appear to match those described in this article, what should be your next step?

"I would have them get their seed company agronomist out there, because there can be many causes of some of these symptoms I've described," Grau advises. "The agronomist will likely send samples to the university's plant disease clinic. As our experience grows, there will be more and more of these disease clinics aware of this potential problem."

There's one other bit of good news concerning this new disease challenge. It appears there may be differences among varieties in how severely they are affected.

In short, resistant or tolerant varieties may become effective weapons to combat this new profit robber.

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