Scouting beans: Fun in the sun?
Welcome to mid-August, when we can count on some nearly universal constants every year: heat and humidity; kids and teachers who are coming to grips with a rapidly approaching new school year; and farmers and agronomists enjoying leisurely strolls through waist-high soybeans scouting for potential challenges.
OK, you make a good point: The words “enjoying” and “leisurely” aren’t often associated with scouting soybeans in August, but it’s a great workout and can pay huge dividends.
There’s a long list of things to watch for as you scout soybean fields. It can be hard to remember and identify the various issues, so we created a late-season soybean scouting card to outline diseases, insects and disorders that occur in soybeans during the R3 (pod fill) through R8 (maturity) growth stages.
There are also apps for phones and tablets that are great scouting tools. The common denominator making them all work well though is “boots on the ground.” This means you, fighting your way through your bean fields to find and quantify what is out there. In this month’s column, we won’t cover every issue that can pop up, but here are some of the most critical issues I watch for when I head out to take an August swim in the soybeans.
Watch for various diseases
August can hit us with diseases that we might still manage with foliar fungicides. These diseases are often what we talk about most, but make note of other diseases, too. It’s important to identify and to 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.
• Foliar diseases. With a lot of soybean acres planted later than usual, we could still have some fields in the foliar disease and fungicide window in early to mid-August. Diseases like septoria brown spot and frogeye come to mind since they are most common, but there are others to watch for that are on the scouting card. Often, if these 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.
Another way to approach the foliar fungicide issue is to check out the Iowa Soybean Association’s Fungicide Calculator, designed to help growers with the fungicide decision-making process. You put in your own price for beans, fungicide and application costs, as well as rainfall data.
• Sudden death syndrome. One thing we’ve learned from outbreaks of SDS in recent years is this disease likes wet weather, especially in the spring. Since much of the state suffered through a pretty wet (and miserable) spring, we could see a lot of SDS showing up.
Knowing where these areas are and the number of acres will be handy when ordering resistant genetics. Now that we have access to some pretty effective SDS seed treatments, having a record of where SDS showed up this year can better target what acres to spend the extra dollars on for the SDS seed treatments.
• White mold. By this time of year, we can’t help your field if it has white mold, but it’s still good to know if it is out there. If a soybean field is diagnosed with significant white mold, consider harvesting it last. This tactic, combined with thoroughly cleaning out the harvest equipment afterward, will help reduce the movement of the survival structures of the white mold fungus to fields that are not infested.
• Soybean cyst nematode. We won’t go too in-depth discussing these nasty critters since last month’s column is probably still giving you nightmares about SCN, but a gentle reminder won’t hurt. Digging roots in areas with typical SCN symptoms is a good idea, but keep in mind we can easily have yield loss with no visual symptoms. Plan some root digs, or better yet, wait for cooler weather and sample soil and have it tested for SCN.
Time for scouting
By this time of year, about the only management strategy for weed escapes is how I made some cash in high school and got dialed in for the heat and humidity of two-a-day football practices: walking beans.
• Weeds. For some weeds, such as Palmer amaranth or small patches of marestail, scouting might be worth the effort. But in general, these scouting trips are more about planning for future weeds. If you find weed escapes, identify them and work with your chemical supplier to see what the situation was that led to them. This may help fine-tune weed management programs moving forward.
• Soybean aphid. Since 2000, soybean aphids have 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. Weekly scouting through mid-seed set (R5.5) is the key, and it looks like this year could mean all through August and possibly into early September.
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. While soybean aphids are insect public enemy No. 1 for soybean growers over the long run, each year some of the other candidates can take a run at the title. 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 at .
Here’s a great question I get every year: What if we have a variety of insect pests in our soybeans, but each at subeconomic 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 a lot of qualified growers and agronomists who can offer their insight.
Stay hydrated, don’t get too much sun, and most of all, keep good records of your late-summer strolls through the soybean fields. They’ll pay off more often than not.
McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Contact him at firstname.lastname@example.org.
Photo by Erin Hodgson, ISU
This article published in the July, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.