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What to ‘watch out’ for in midsummer soybeansWhat to ‘watch out’ for in midsummer soybeans

Soybean Source: It’s been a challenging growing season to date; you can expect a few more surprises throughout the rest of the summer.

Clarke McGrath

June 22, 2017

6 Min Read
SCOUTING: Enjoying a weekly scouting trip through the fields is good exercise, as well as time well-spent. In midsummer, look for insect pests, weed escapes, disease problems and more.

Knowing what “surprises” may be in your fields is a big step toward managing them and protecting crops. Scouting is a fun (sometimes) and lucrative pastime. There’s a long list of things to look for, so we’ll hit the highlights.

Which diseases are present?

Check for these diseases while scouting:

• Foliar diseases. 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 a fungicide application is needed.

• 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 much of Iowa suffered through a pretty wet spring, we could see a lot of SDS showing up. 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.

Evaluate weed control results
Most of our planned postemerge applications are done or going on soon, so we’ll be scouting to see how they performed. Hopefully, they were successful for the most part, but Mother Nature doesn’t always cooperate; good scouting will tell us how things went. In some cases, we’ll be transitioning into “rescue” application mode.

Scouting to find out what weeds escaped, to what extent and what stage the crop and the weeds are in is critical information for deciding if and how to give a respray a shot. If you are considering any rescue applications, double-check the label for issues like crop stage restrictions and rotational restrictions. As an example of what can happen, this spring we had some cornfields with herbicide carryover injury from last summer’s rescue applications in soybeans; it wasn’t pretty.

Another issue to watch for is the spread of Palmer amaranth. It’s in many counties across Iowa after contamination of conservation seed mixtures; 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.

Are insects bugging your beans?
Here’s some pests to be on the lookout for:

• Soybean aphid. Since 2000, soybean aphids have probably been the primary soybean insect pest in Iowa. They may not be an issue every summer, but they have the ability to 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. In my experience, if soybean aphid is the only pest out there, the 250 threshold is a solid number.

A lot of us “estimate” the aphid numbers, rather than sit and count to 250 (patience isn’t one of my strong suits). You can “calibrate” your estimating skills by counting small colonies to establish what 100 aphids looks like, and then use this as a guide to make your estimate. As another point of reference, a colony that completely covers all sides of a stem for 1 inch will have around 250 to 300 aphids.

• Other insects. Bean leaf beetle, grasshoppers, spider mites, Japanese beetle, stinkbugs, green cloverworm and other insects can 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, qualified growers and agronomists can offer insight.

Other things to think about
It’s nice to have a spade handy when scouting so you can look for things like root development and soil structure. With the proliferation of smartphones, getting images isn’t a problem; but sometimes getting “good” images is. Take plenty of pictures of anything you want to pass along for another opinion, and share them via a method that allows sending enough resolution. A good knife, a tape measure, a hand lens or magnifying glass, baggies for samples, bug spray, sunscreen and plenty of liquids are nice to have along, too.


New scouting toys (I mean tools)

A recent run-in with an unmanned aerial vehicle reminded me that (when used properly) a UAV has a great role in crop scouting. Almost anything done on foot or on an ATV can be done faster: assessing stands; looking for foliar diseases, nutrient deficiencies, weed escapes, insects, spray drift and erosion; and discovering and evaluating other abnormalities the camera shows. We can also add thermal or multispectral imaging to assess crops, not so easy to do from ground level. Sure, many of these things will need some “ground-truthing,” but UAV’s can speed up the process and help us find more issues when combined with terrestrial scouting.

My interaction with a UAV wasn’t in a field, but rather in a kayak, and it was getting a little close as it buzzed back and forth across the water around several groups of kayakers and fishermen. It was tempting to head to shore, grab a shotgun and paddle back out to do some drone hunting. Luckily for me, my wife pointed out that while it might make for some great video, a tippy kayak, 12-gauge recoil and a husband who sinks like a rock are not so good for drone hunting. On a related note, it’s great to be able to say that the growers I know who have UAVs are responsible and safe operators. They have also mentioned that while they can do a lot of scouting with their UAVs, they aren’t worth a darn at running a spade or collecting samples. In other words, they are a great tool, but nothing replaces boots on the ground.

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at ISU. Contact him at [email protected].

About the Author(s)

Clarke McGrath

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

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