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Scout for bean stressors

Gil Gullickson 09/30/2011 @ 1:36pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

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.

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