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Soybean diseases kicking up

08/27/2013 @ 8:49am

Now, 100-degree temperatures don't make scouting soybeans the most enjoyable task in the world. But, like salt on an open wound, the heat and dryness right now in much of the Corn Belt is being accompanied by growth in a few key soybean diseases, making it important to get into the field to see if you're going to incur more damage than that caused by Mother Nature.

August is typically the most critical month for soybean crop development. That's changed this year with the tardiness of planting and slow development of the crop, making September the most important month for the crop. And, it's not starting out on the best note, with pest, disease, and weather pressures mounting.

Weather stress can open the door to a lot of pest and disease prospects, especially when that summer heat and drought stress is preceded by a cool, wet spring, experts say. And right now, three diseases are starting to come on strong in spots around the Corn Belt.

Sudden death syndrome (SDS)

Some fields in the central and western Corn Belt continue to incur damage from SDS. With this disease, look for yellow and/or brown patches on soybean leaves, "often at the edges of fields or low-lying areas," says Iowa State University plant pathologist and microbiologist Daren Mueller.

"Remember to split the stem to differentiate from brown stem rot, which will have the characteristically brown discolored pith," he adds.

Other stem and root rot

This includes brown stem rot, phytophthora root and stem rot, and white mold (sclerotinia stem rot) in addition to SDS. All of these diseases start in either individual plants or "small pockets of dead or dying plants," says University of Nebraska Extension plant pathologist Loren Giesler. Though visible stem damage from sclerotinia rot can be either black or white in color, this summer's dryness makes it easier to scout for the disease.

"Upon close inspection, you will see a white cottony fungal growth on the stems. There also may be dark black bodies (sclerotia) of the fungus on the stems. If it is drier and plants are dead, the stems will be very light (bleached) in color," Giesler says. "When dead stems are split, often you’ll see sclerotia of the fungus inside."

Control isn't easy. Fungicides can help, but if these diseases are coming on now, it's likely past the window of efficacy for current products.

"The best action at this time is to make sure you identify this and other problems correctly so that you can use plant resistance to manage the problem in the future. In general, fungicide applications during flowering to control Sclerotina are often unsuccessful," Giesler says. "Soybean genetics will be more consistent; however, no varieties are fully resistant to Sclerotinia. Narrow-row spacing also has been shown to increase disease incidence in a field."

Soybean vein necrosis virus (SVNV)

Mueller says farmers in Iowa are reporting SVNV in the southern half of that state, but thus far, it's in low levels. The virus was first found earlier this month -- earlier than when it's normally found -- and still has pathologists wondering if its early arrival, albeit in low levels, will cause economic damage before fall. The virus did not show up in Iowa's fields last year, Mueller says.

"Symptoms often begin as chlorotic (light green to yellow) patches near the main veins, which may enlarge, eventually becoming necrotic (brown) areas. The veins may appear clear, yellow, or dark brown. The browning of the veins may be especially noticeable on the lower leaf surface, but this may not always occur," he adds. "Currently, there are no management recommendations for this disease. Because of the newness of this disease, there are no known sources of resistance. Insecticide application only should be considered in fields with a known risk of yield loss."

Soybean rust

In the last few years, although soybean rust has appeared in fields in the southeastern U.S., it's never made its way into the Corn Belt. That will likely be the case this year, too, as the spores will be too far south and east to make their way into the Midwest. But there is one major wildcard this year, Mueller says.

"It is certainly worth keeping an eye on over the next several weeks because we do have late-planted soybeans that could be affected," he adds.