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When To Stop Spraying For Soybean Aphid

When To Stop Spraying For Soybean Aphid

ISU specialists say it doesn't pay to spray with foliar insecticide beyond R5.5 growth stage of soybean plants.

During the past 2 weeks or so, there have been multiple reports of finding soybean aphids in fields in several areas of Iowa, but to date only a few fields in central Iowa had reached the economic threshold. Since then a few more areas have been treated with a foliar-applied insecticide, but not many, says Mark Johnson, Iowa State University Extension field agronomist in central Iowa.

SDS SYMPTOMS: Soybean leaf on the left is on a plant that has resistance to Sudden Death Syndrome, and it has no symptoms of SDS disease. Leaf on the right shows typical SDS symptoms.

"You are apt to see two recommendations for when to stop treating, either at the R5.5 or R6 growth stage of the soybean plant," says Johnson. "The reason for this is because some states have seen a payoff for treating at R6 at least some of the time. ISU Extension entomologist Erin Hodgson advises to use R5.5 here in Iowa because there is a much higher chance of treating at R5.5 paying for itself than treating at R6."

Johnson has been asked to look at some soybean fields recently to help farmers figure out the cause of yellowing at the tops of plants. There have been some small areas of fields where Sudden Death Syndrome or SDS is showing up, especially since the August rains occurred. Keep in mind that SDS disease shows itself as interveinal yellowing or chlorosis. That is, the leaf area between the veins turns yellow, while the veins themselves stay green.  


SDS SYMPTOMS: Soybean leaf on the left is on a plant that has resistance to Sudden Death Syndrome, and has no symptoms of SDS disease. Leaf on the right shows typical SDS symptoms.

Yellow leaves don't necessarily mean the problem is SDS. It can be something else that's causing the yellowing of the leaves. "Some of the fields I've been asked to look at have yellowing of the leaf margins on some of the leaves, much like the lower leaves that show potassium (K) deficiency earlier in the season," says Johnson. "In these fields we're seeing now some of the leaves are about half-yellow, and are located on the top of the plants and at the tips of branches. In some cases the whole leaf is chlorotic."


TOP DIEBACK: It's a condition that's showing up in areas within some soybean fields this August in central Iowa.

This is a condition called Top Dieback. "We don't know for certain the exact cause of this, but we believe it is K deficiency and shows up in years where the soil has been dry for a while and then we get a good rain," he explains. "There also seems to be at least some relationship with soybean cyst nematode or SCN. If you have a field with these symptoms, consider soil testing this area or areas for K and also for SCN." For more information, read this ICM article from 2008.

Iowa's 2014 corn crop is continuing to mature
Most of the Iowa corn crop is now at least in the dough stage or R4 stage of growth. That's where the kernels are approximately 70% moisture and have about half their mature dry weight. Some fields are in the dent stage (R5).

"At the beginning of the R5 growth stage of a corn plant, kernels are about 55% moisture," says Johnson. "This is when you can observe the milk line moving down the kernel by looking at the side of the kernel opposite of the germ. Take the ear in two hands and break the ear at about the midpoint, then look at the half that is in your hand that is holding the tip of the ear."

A hard early frost, a killing frost, that occurs when the temperature is 28° for several hours before the corn reaches R6 stage of growth, will halt dry matter accumulation and cause a premature black layer to form. Frost-damaged corn is slow to dry. However, hot weather for the past week or so has helped crop maturity get back on track in August 2014. In the accompanying table showing growing degree days, Johnson notes "We are now looking a little better for degree day accumulation than we were earlier this summer."


Iowa is a waterhemp paradise this year
Waterhemp is a weed that's showing up in many fields across central Iowa this summer and in other areas too, especially in soybean fields.


"Looking at these waterhemp infestations as we drive down the road, it reminds us that weeds can develop resistance to other herbicides besides glyphosate," says Johnson. "The repeated reliance by farmers on the use of one herbicide group can lead to resistance developing in the field."

The following picture shows a field that has relied solely on Herbicide Group 14, which are the PPO inhibitor herbicides, for control of broadleaf weeds. The preemergence herbicide applied was a product called Sharpen, and the postemergence applications have been Cobra and Cadet—the field was sprayed twice with postemergence treatments. The previous year had only the herbicide group 14 applied for control of broadleaves weeds—the same as in 2014. This field is planted to soybeans in a 15-inch row width.


WEEDY BEANS: Waterhemp is a 'weed gone wild' in 2014, in many Iowa fields. "In this photo, you can see some soybean leaves, but mostly what you see is waterhemp," notes ISU's Mark Johnson.
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