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Scout for Soybean Pests

Here are five top SDS questions with answers from Daren Mueller, Iowa State University Extension plant pathologist.

It’s a familiar scene. Visions of plus-80-bushel-per-acre soybeans dance in your head at midsummer. Shortly afterward, though, sections of your field start dying. Literally. Once again, sudden death syndrome (SDS) has struck. So what gives?

Here are five top SDS questions with answers from Daren Mueller, Iowa State University Extension plant pathologist.  

1. Why does SDS seem to be expanding?

Mueller: Part of the problem is that the soilborne fungus has figured out the corn-soybean rotation. It doesn’t attack corn or impact yields, but it survives quite well on corn residue. Then, it is there for the next soybean crop. Most farmers use crop rotation as one way to manage SDS, but a two-year rotation of corn and soybeans isn’t controlling it. Rotating away from corn and soybeans helps.

We use a disease index (DX) to measure the level of SDS challenge, from 0 to 100. It’s a combination of disease incidence and severity. Every 10 points higher in the DX translates to a 7% reduction in yield. In some years, we’ve seen a DX of 40 or higher for SDS, and that indicates a significant yield loss.

What we normally see is a DX of 10 or higher with a two-year corn-soybean crop rotation. It’s usually under 10 if you go to a three-year rotation with a third crop. In a four-year rotation of something like corn-soybeans-oats-alfalfa, the DX drops to almost 0.

2. What are other cultural practices that can reduce the impact of SDS?

Mueller: You may think that if you plant soybeans early, like in April, you’ll have more SDS. We really haven’t seen that correlation in our research. In Wisconsin, there was some connection, so the overall research is a little inconsistent.

Even when there is a connection, it isn’t enough to offset the yield advantage of early planting. I would tell you to still plant early; don’t mess with that to control SDS.
As for tillage, we have a long-term study in Iowa looking at its effect on SDS, wondering about the role of soil compaction or if tillage actually spreads the fungus. We’re not seeing any connection.

3. Do cover crops help control or spread SDS?

Mueller: We know that other legumes can serve as hosts for the fungus. Grasses such as rye are not hosts. It doesn’t appear that cover crops make SDS better or worse.

4. Are there any other issues contributing to more SDS?

Mueller: SDS is known as a bigger problem in high-yield soybeans, along with white mold disease. The higher the yield potential, the greater the risk. Of course, you still want to strive for high yields – just know the risks and manage for them.

We also know there is an interaction between SDS and soybean cyst nematodes (SCN). When you have SCN, you have more SDS; they seem to spread together.

My number one suggestion is to plant soybean varieties that are resistant to both SDS and SCN. There are some that show promise and up to a 17.5% yield improvement. Eliminate the lines that are most susceptible from your selections.

5. What about chemical treatments?

Mueller: The visual effects of SDS usually occur in August. Some have tried foliar fungicide applications at that time. But we’ve done many research projects, including with the experimental products, and we haven’t seen much yield response from foliar treatments.

The fungus actually infects soybean plants shortly after planting. So, in theory, seed treatments should be effective at reducing SDS. Again, we’ve done lots of tests, and we haven’t found much response.

The exception to that is one of the newer fungicides, Ilevo. It has shown a 35% to 40% reduction in the disease index and a positive yield response of 8.5%, on average. The greater the risk of SDS in a field, the more yield response we’ve seen.

When there is no evidence of the SDS fungus present, we’ve not seen a yield response to Ilevo. If you’re not struggling with SDS, it probably doesn’t make economic sense to use it.

Watch for White Mold

One thing you can say about white mold: It’s not a bully. This fungal disease doesn’t target wimpy soybeans. Instead, it attacks fields you’d swear in midsummer would be bin-busters come fall.

“It’s a disease of high yield,” says Alan Scott, DuPont Pioneer technical product manager. “As we push (soybean) products northward, we are seeing more white mold."

Elite genetics that fuel excellent growing soybeans couple with several other factors to give white mold a foothold. They include:

  • Narrow rows. Row spacings of 15 inches or less can promote a stuffy air environment in which white mold thrives. Wider rows more conducive to airflow can discourage white mold development.
  • Manured fields. Manure keys rapidly growing fields prone to white mold.
  • Plant architecture. Rapidly branching, bushy soybeans quickly form a canopy that’s good for snuffing weeds like waterhemp. Unfortunately, this architecture also fuels a quick-forming canopy that enables the white mold fungus to thrive.
  • Crop rotation. Crops like sunflowers also host white mold. Thus, rotating to such crops won’t halt it.
  • Weather. Heavy rainfall when soybeans begin to flower (R1) is a prime time for the white mold infections to develop. Heavy dews — such as those that cause your pant legs to be wet well into the morning — also spur white mold infections. In the Upper Midwest, Scott says this period normally occurs from July 20 into August, with the crop damage often showing up at the end of August. The good news is that dry weather during this time can snuff white mold.

A fungicide applied around R1 followed by a subsequent application 14 to 21 days later can enable you to curb white mold in-season. That adds input costs, though. A better approach is to select varieties tolerant to white mold on suspect fields.

“The first line of defense is genetic,” says Steve Schnebly, DuPont Pioneer senior research manager.

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