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soybean field
SCOUTING SOYBEANS: In midsummer, look for insect pests, weed escapes, disease problems and more.

Babysit those bean fields

Soybean Source: A weekly scouting trip through the fields is good exercise, as well as time well spent.

It has been another interesting spring and early summer in Iowa. Farmers fought cold temps and untimely rains during planting, and since then we’ve seen areas of flooding, storm damage from wind, hail and erosion in large areas of the state. Down in my region we registered abnormally dry to moderate, according to the drought monitor.  

Off and on through late May and into June, this question is common from many areas: Is it too hot to spray postemerge herbicides?

Closer to home folks wanted to discuss the implications of corn wilting and pastures burning up. Predicting the weather for the rest of the growing season is tough, so predicting the pest challenges our crops might face is a real guessing game. Keeping a close watch on fields (babysitting them) reduces the guesswork. Scouting to find issues early and taking action when needed are still the best methods for a lot of our pest issues, so here are a few things to keep an eye out for in soybean fields as we head through midsummer.

Check for these diseases
Diseases to watch for this summer:

 Foliar. Pathogens like septoria brown spot, cercospora and frogeye come to mind since they are more common, but there are others to watch for, so take your favorite scouting app or field guide with you. Often, if these foliar diseases stay in the lower third of the canopy, they don’t impact yield. If they head into the middle third of the canopy, then we start to take notice. If your scouting finds this happening, work with your local agronomist to positively identify the disease and determine if fungicide application is needed. Now that we have strobilurin fungicide-resistant frogeye isolates in Iowa, it becomes especially important to understand when to spray and what products to use for long-term control of this disease.

 Sudden death syndrome. One thing we’ve learned from outbreaks of SDS in recent years is this disease likes wet weather, especially in spring. Since significant parts of Iowa suffered through a pretty wet spring, we could see a lot of SDS showing up again. While we can’t manage SDS in-season, knowing where these areas are and the number of acres will be handy when ordering resistant genetics. Now that we have access to some fairly effective SDS seed treatments, having a record of where SDS showed up this year can better target the acres to spend the extra dollars on for the SDS seed treatments.

 White mold. Since our in-season white mold management applications are already on or will be applied soon, scouting for white mold is a similar concept as for SDS: planning ahead for future management.

These diseases are often what we talk about most, but make note of other diseases, too. It’s important to identify and keep track of where diseased spots are occurring in a field. That can help with variety selection and possible management of these diseases in the future.

Keeping track of escaped weeds
As we wrap up planned postemerge herbicide applications, scouting is critical in evaluating how well our weed management programs performed. Hopefully they worked well, but even in the best of conditions we can have weed escapes. With the weather challenges we had this year, locating potential weed escapes and identifying the offenders is just part of the equation.

When considering any rescue applications, read and follow the herbicide labels. Restrictions on weed size, crop height or stage, product use rates, application timing, and rotational restrictions are just a few of the issues we can run into with resprays. Scouting is the only way to gather some of that information.

This recent ICM news article illustrates some of the pratfalls of resprays, it is definitely worth reading.

Another issue to continue to keep an eye on is the spread of Palmer amaranth. It hasn’t overrun Iowa like it did some Southern states, but it is scattered across much of the state. Watching where it spreads and limiting seed production if possible is our best shot at keeping it at bay. While I hate to add to the workload, consider scouting both crop and non-crop areas for Palmer.

What’s bugging your beans?
Since 2000, soybean aphids have probably been the primary soybean insect pest in Iowa. They may not be an issue every summer, but aphids can cause significant yield loss in short order during periods of optimal reproduction.

Since we haven’t had a really big statewide outbreak for a few years now, it feels like we are overdue. Weekly scouting through mid-seed set (R5.5) is the key.

When the average number of aphids exceeds 250 per plant on about 80% of the soybeans, and populations are increasing prior to hitting R5.5, it typically justifies an insecticide application. We hear of other economic thresholds being used in the country, but a lot of work has been done in establishing the 250 number for Iowa.

Other insects, like bean leaf beetle, grasshoppers, spider mites, Japanese beetle, stinkbugs and green cloverworm can also wreak havoc individually or collectively in soybean fields. The economic thresholds vary among them, so rather than go into detail on each one, I recommend searching ISU’s Integrated Crop Management News website. Most pest management information, including thresholds and treatment advice, can be found there.

A question I get every year: What if we have a variety of insect pests in our soybeans, but each at sub-economic levels on their own? Is there a “cumulative threshold” for that scenario?

Unfortunately, we don’t have a cumulative pest economic threshold for this situation, so in these cases, we have to fall back on experience and good judgment. We have a lot of both out in the country, so if you need a second opinion to compare to your own, there are many qualified growers and agronomists that can offer insight.

Other benefits of timely scouting
We all agree that scouting is one of the most important, rewarding and glamorous tasks on the farm, right? OK, we are probably on the same page with “important” and agree that “glamorous” isn’t the right word. If “rewarding” is a stretch, don’t forget a few more things that midseason scouting can show us. Farmers who spend a lot of time in the field — and these are the folks that taught me more about scouting than any class or book — often look beyond bugs, weeds and diseases.

Looking at how different genetics are progressing and performing, observing planter performance, checking terraces and waterways for repairs or weed issues, digging roots looking for SCN while also observing soil conditions, and evaluating any erosion issues are also part of a laundry list of what many growers track during scouting trips. Good luck out there and have a safe summer!

McGrath is the on-farm research and Extension coordinator for the Iowa Soybean Research Center at ISU. Contact him at

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