Wallaces Farmer

Identifying late-season soybean issues

Soybean Source: While crop scouting tends to slow down at the end of the growing season, this year has brought more issue-related questions.

Clarke McGrath

August 20, 2021

3 Min Read
charcoal rot in soybean field
DRY-YEAR PROBLEM: While not a disease we typically worry about in Iowa, charcoal rot can become an issue in years like this, with hot, dry conditions when soybeans reach reproductive stages and need more water. Daren Mueller

Summer is about over when we hit state fair time, meaning the combines could be rolling before we know it. Crop scouting tends to slow down toward the end of the growing season, but this year I've had more calls asking, "What is going on with my beans?" than usual. Considering the weather we had this season and the increased number of tough-looking patches in bean fields, the number of questions will likely continue to increase.

The list of potential issues we'll find as we head toward harvest is pretty long, so we'll hit on just a couple of things that are already on a lot of folks’ radar, based on the calls I've been getting. While we might not be able to do much to correct these issues this season, identifying them will may a long way towards mitigating their impact in the future.

Charcoal rot

You're right, charcoal rot isn't a disease we typically worry about a lot in Iowa. It is often thought of as more of a Southern disease, although it is a pretty common challenge for states as close to us as Kansas. Caused by a soilborne fungus, infection of plants occurs early in the season, when soil moisture levels are high. If we don't have drought conditions, we usually don't see symptoms. In years like this where we've been hot and dry in many areas, when beans get into reproductive stages and need more water — that's when we see issues.

Areas of the field may show some stunting and wilting in the midday heat at first; then over the next few days or weeks, infected plants often suffer premature yellowing of the top leaves and premature leaf drop. I know lots of other issues can cause similar symptoms; so as much as I hate to ask, you'll need to head out to those areas to find out what is going on. Rather than get too far into the weeds, here is a great summary of the disease, pics to help with identification, and management strategies: soybeanresearchinfo.com/soybean-disease/charcoal-rot.

Soybean cyst nematode 

We talk about SCN being a huge issue every season, but in a drought year it really kicks us in the backsides. Not only does SCN tend to exacerbate yield losses in dry years, but the darn things also tend to ramp up reproduction. ISU's renowned nematologist Greg Tylka lays it out for us: "Every time drought conditions develop, for some reason that we haven't figured out, reproduction is increased. So in a normal year where we might end up with a final egg count of 5,000 eggs per half-cup of soil, in a drought year that would be 10[000] or 15,000 eggs per half-cup of soil."

It's not a pretty picture for yields in SCN-infested fields this fall; and depending on your rotation, there could be a lingering impact on future bean crops. In drought-plagued areas with a traditional corn-soybean rotation, many of this year's bean fields that would rotate back to beans in 2023 are likely to have higher levels of SCN and increased yield loss.

I think this is probably a good time to say, "Take the test and beat the pest" and send you off for more information. I am a big fan of the short informative videos in the "Let's Talk Todes" series from the SCN Coalition: thescncoalition.com/lets-talk-todes/all-videos.

The bottom line on charcoal rot and SCN — and many other things we'll find toward the end of the season — is that while we may not be able to take action to help this year's bean crop, knowing what we are dealing with will definitely help improve our odds in fighting off yield losses in the future.

McGrath is an Iowa State University extension field agronomist and On-farm Research and Extension coordinator for ISU's Iowa Soybean Research Center.

 

About the Author

Clarke McGrath

Clarke McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU.

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