Insect pests and crop diseases still bugging Iowa fields

Insect pests and crop diseases still bugging Iowa fields

Soybean aphid and SDS showing up in some soybean fields, corn diseases are also present.

Is it too late to spray soybeans for soybean aphid? Not if the infestation of this insect pest is at the economic threshold to justify spraying an insecticide, says Clarke McGrath, an Iowa State University Extension field agronomist in western Iowa. He and his ISU colleagues around the state are fielding a number of crop questions here in the final two weeks of August. Now that it's beyond the stage for spraying fungicide on corn to control gray leaf spot and northern corn leaf blight, he's answering questions from growers concerned about some issues in soybean fields.

SDS SYMPTOMS: Other diseases may be confused with sudden death syndrome, such as brown stem rot and stem canker. Look for lesions on the outside (stem canker) and browning in the pith (brown stem rot) to distinguish from SDS.

It's been a wet summer in most of Iowa, setting corn and soybeans up for invasion by foliar diseases, notes McGrath, who writes a monthly agronomy column for Wallaces Farmer magazine. Other ISU Extension agronomists also take turns writing the Corn Source, Soybean Source and Cropping Systems Insights (CSI) columns.

Sudden Death Syndrome showing up in Iowa soybeans
"We were afraid this disease would show up this summer," says McGrath. "SDS likes a lot of spring rainfall, so it was just a matter of time before it started to appear in late summer. While there isn't anything we can do for this year's soybean crop if it has SDS, it is good to know if SDS is indeed the culprit in your field so you can plan ahead for the next soybean crop and take management steps to avoid problems."

Occasionally SDS can be mistaken for a few other diseases, so McGrath offers these keys to help you correctly identify SDS:

• Soybean roots will appear rotted and plants will be easily pulled from the soil.

SOYBEAN APHID: Soybean aphids prefer to feed on the undersides of leaves and will colonize the newest leaves. If a large colony develops and leaves are crowded, the aphids will feed on stems.

• The fungus that causes the disease may appear as blue fungal growth (spore masses) on the main root (tap root) of the soybean plant.

• When the main root or tap root is split lengthwise with a knife, the internal tissue of the root will be gray to reddish brown, not healthy white.

• On soybean leaves, the areas between the leaf veins will turn bright yellow, then eventually brown. The dead, brown tissue between veins may fall out, leaving large ragged holes in leaves.

• The leaf blades will fall off of the petioles (petioles are the thin "stems" that connect the leaf blades to the main stem), but the petioles remain attached to the stem.

McGrath suggests you read an article available from ISU online, as it addresses the SDS issue quite well. However, there is a piece of updated information that isn't included in the article, he notes, which was written a few years ago. That is, now in 2015 there is at least one soybean seed treatment available that is labeled to use for suppression of SDS.

Soybean aphid is infesting some fields in areas of Iowa
Aphids are also bugging some soybean fields in Iowa this summer, mainly northern Iowa. "While soybean aphid numbers vary widely both among and within fields, we have some areas in southwest Iowa where this insect pest is nearing the economic threshold to justify treatment with a foliar insecticide," says McGrath. In a few cases the aphid infestation has already reached that threshold, meaning it will pay to spray. Here are a few of McGrath's thoughts on soybean aphid management and control:

* The current recommended economic threshold for soybeans through the R5 stage is 250 aphids per plant with 80% of the plants infested and populations increasing.

* This should give about a five- to seven-day window schedule for treatment before aphid populations reach economically damaging levels.

* If populations do not increase during this period, you may be able to get by without treating but scout well as aphid populations can explode in cooler conditions like those forecast for the next seven to 10 days.

* Factors favorable for aphid increase are relatively cool temperatures (forecast says this is likely), plant stress (particularly drought, not a problem right now for most), and lack of natural enemies.

* Watch the PHI's (preharvest intervals) on the insecticides if you go ahead and treat.

* If you spray the beans using a ground-rig sprayer, apply 15 or more gallons per acre. McGrath prefers 20 gallons per acre at 40 psi spray pressure, or better, depending on the type of nozzles. Remember, he says, these aphids live down in the soybean canopy. You need to blast the insecticide spray down in there.

* Consider aerial application if you decide to spray for aphids this time of year. That's because, depending on a lot of factors, wheel traffic can cost up to 2.5% of the yield. Read this article at


 Why do you need to blast the plants with more gallons per acre?
"Sounds contradictory doesn't it? Blast 20 or more gallons per acre down in there yet I'm recommending considering aerial application," notes McGrath. "Turns out that propeller wash from the airplane doing the sprayer, the turbulence or some other mechanism, allows for control to be very comparable between ground-driven sprayer and aerial application if both are done well. My experience has shown us that, and some research done at the University of Minnesota has also shown us that.

What about insecticide product residual and performance? Is there a difference in products to provide this? McGrath says this topic is "a can of worms" He adds, "I hear all sorts of talk about what products have more or less residual, for longer control. But the bottom line is that application rate, weather and a lot of other tertiary factors impact length of residual provided by the insecticide. So it is nearly impossible to predict. Don't get too wrapped around the axles trying to pick the perfect insecticide product to apply for aphid. Work with your local dealer and chose a product that offers a workable PHI or preharvest interval. And, apply a strong labeled rate."

Most foliar insecticides very effective at reducing aphid populations
What about the hot vs. cold weather consideration we hear about with insecticides? "Good question," says McGrath.

"There is something we call the temperature co-efficient; inverse for most pyrethroid insecticides and positive for most organophosphates. In other words, pyrethroids tend to work better at temperatures below around 90 degrees F, while organophosphates tend to have more volatile activity in higher temperature situations. For the purposes of soybean aphid management, this is rarely a big deal. This temperature concern tends to be more of an issue when fighting spider mites in hot, droughty conditions."

McGrath quotes ISU entomologists who sum this up quite well: "To date, most foliar insecticides are very effective at reducing soybean aphid populations if the coverage is sufficient." Also, McGrath recommends you click on the following link for more soybean aphid


Late season 2,4-D applications to control weeds in corn
McGrath and fellow ISU Extension agronomist in southwest Iowa, Aaron Saeugling, are both getting some questions about the advisability of knocking back some broadleaf weeds that are poking above the corn canopy. The idea of spraying 2,4-D late in the season on corn is to kill those weeds and try to slow the weed seed production. McGrath recommends you read this article by ISU weed specialist Bob Hartzler who offers practical tips.

Cover crop seeding time is almost here
Citing another timely crop topic, "We are starting to get a lot of questions about cover crops with seeding time not too far off," says McGrath. ISU Extension will have seven to 10 cover crop plot sites across southwest and west-central Iowa this year, trying 10 different species and mixtures. This is the fourth year of the studies done in partnership with Practical Farmers of Iowa, and the third year they've had at least seven of these demonstration sites across the area.

"We've learned a lot… and we still have a lot to learn about cover crops," says McGrath. You can go back and listen to the archived version of the August 19 webinar on cover crops sponsored by Iowa Learning Farms. The August 19 ILF monthly webinar featured "Reaching the Full Potential of Cover Crops In Iowa", by Dr. Tom Kaspar, a USDA agronomist stationed at ISU at Ames. Additional details here. Also, there's a new ISU ICM News article by ISU Extension agronomist Mark Licht that is very helpful.

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