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Scout for bean stressors

09/30/2011 @ 1:35pm

After Roundup Ready soybeans debuted in 1996, growing soybeans seemed like a breeze. Just plant, spray, harvest, and you were done.

Well, maybe it wasn't that easy. Still, raising soybeans is more difficult these days. Pests that weren't around in the 1990s – such as soybean aphids – now attack your soybeans. Diseases like white mold and Sudden Death Syndrome (SDS) strike with increasing ferocity.

Meanwhile, the multiple glyphosate applications that controlled weeds so well in Roundup Ready soybeans have spurred a number of weed biotypes that resist glyphosate. Some of these biotypes also resist other herbicide modes of action.

These maladies and others make summer scouting a must. Although you may not be able to halt all pests, scouting can help you halt or curtail them in future years. Here are some diseases and insects to watch for during the growing season.

Sudden death syndrome

True to its name, SDS suddenly kills soybeans in August and September. This damage is rooted earlier in the year, due to a fungal infestation of the plant's xylem system.

By the time you confirm SDS, there is nothing you can do to salvage your soybeans. Ditto for future moves that work with other pests and diseases. Fungicides don't work. Nor does crop rotation. Since the SDS fungus survives in crop residue, rotating to corn or other crops doesn't deter SDS.

One tool you can use in the future is SDS-resistant soybean varieties. They won't completely nix SDS, but these varieties planted in high-risk fields can alleviate yield damage.

High-risk fields include slow-draining soils in flat river bottoms. These fields often are slow to warm in the spring. Cool soil temperatures below 70°F. heighten SDS risk.

“The longer soybean seed stays in cooler soils, the more chance that infection will occur,” says X.B. Yang, Iowa State University (ISU) Extension plant pathologist. “When the soil is saturated in summer, the fungus will survive.”

A complicating factor is white mold may occur simultaneously with SDS. Some control methods for both diseases conflict. For example, tillage slices SDS risk, but it heightens the odds of white mold. In these cases, Yang advises planting a reliable SDS-resistant variety and using a fungicide to control white mold infestations.

Even if soybeans are infested with the fungus earlier in the year, dry weather can halt SDS. That's what happened in mid-August in Iowa in 2010.

“The chlorotic and dead leaf tissue on SDS-infected plants are due to a toxin produced by a fungus growing on the roots,” says Paul Stevens, senior research director for Pioneer Hi-Bred. “Reduced rainfall limits the amount of the toxin moved to the upper leaves, thereby reducing the impact on the plant.”

Glyphosate-resistant weeds

Southern soybeans have been rifled by glyphosate-resistant weeds, particularly Palmer amaranth. In the Midwest, though, glyphosate still controls a majority of weeds.

That's changing, though, as pockets of glyphosate-resistant weeds continue to surface. This is compounded by weed biotypes that also resist herbicide modes of action like ALS inhibitors and PPO inhibitors.

There are various ways to control glyphosate-resistant weeds, including rotating herbicide modes of action and using preemergence residual herbicides.

Ditto for full-use rates. Herbicides applied below label rates can enable weeds prone to resist herbicides to get a foothold, to survive, and to spread.

“We see problems with herbicide-resistant weed populations when growers shave label rates,” says Carroll Moseley, herbicide brand manager for Syngenta Crop Protection.“That's why we are so adamant about applying label rates.”

Another weed that's on the increase may not seem like a weed. Nevertheless, that's the case with volunteer corn.

“Volunteer corn is highly competitive in soybeans,” says Moseley.

A 2008 Purdue University study showed that just two plants per square meter can significantly reduce soybean yields. Densities as high as 16 plants per square meter can slice yields 40% to 45%.

Fortunately, there are several fop and dim postemergence herbicides that can be sprayed on volunteer corn in soybeans, Moseley says.

Soybean aphids

Foliar insecticides halt soybean aphids from clipping soybean yields. It's important, though, that levels reach economic threshold levels.

In 2010, ISU researchers examined various treatment types. One was conducted under Integrated Pest Management (IPM) criteria of infestation levels reaching 250 aphids per plant on 80% or more of plants. Insecticide treatments with fungicides were applied between R1 (bloom) and R5.5 (beginning to full seed) under IPM criteria.

In 2008 and 2009, treatments made at IPM levels and R3 (beginning pod) applications of the insecticide alone (or tankmixed with fungicides) yielded the best. In 2010, aphid levels did not reach IPM criteria for applications to occur.

“There are years when pesticide applications are not necessary,” reminds Rebekah Ritson, an ISU entomology graduate student who worked on the trials. Unnecessary applications can eliminate beneficial insects and heighten aphid resistance to insecticides.

Red-banded stink bug

Midwestern soybean growers could be in for a big stink that Southern growers have been wrestling with in recent years. The red-banded stink bug is moving northward.

“Right now, it is in two southern counties in Missouri, coming out of Louisiana and other Southern states,” says Wayne Bailey, University of Missouri (MU) Extension entomologist.

In Louisiana, it has sliced soybean yields from 60 bushels per acre down to 21 bushels per acre.“It can displace other stink bugs,” says Bailey. “It is more aggressive, and it can withstand more chemicals than other stink bugs.”

On the other hand, colder climates may slow or restrict the pest's movement northward. MU entomologists are monitoring its movement in Missouri.