Volume 2, Issue 9
August 23, 2019
This newsletter is an update on current topics and events in agriculture in Boone, Dallas, Jasper, Madison, Marshall, Polk, Story, Tama, and Warren counties.

Meaghan Anderson
Extension Field Agronomist

ISU Extension & Outreach
220 H Avenue
Nevada, Iowa 50201
Office:  515.382.6551
Cell:     319.331.0058

Upcoming Events

IDALS Apr 19 - Dec 19 Pesticide Testing Schedule

Aug 28 - Ag Engineering and Agronomy Farm Field Day, Boone

Aug 29 -
Cover Crops for Extended Grazing Field Day, Milo

Aug 30 -
Drainage Workshop for women, Boone

Sept 5 -
Northern Research Farm Field Day, Kanawha

Sept 9 - Ag Coffee - prep for fall harvest, Toledo

Sept 17 - IFGC Pasture Tour & Field Day, Winterset

Handy Links

Central Iowa Crop Update
Updates from an agronomist and weed nerd in central Iowa.

In this issue:

  • Aug. 28 Ag Engineering and Agronomy Farm Field Day
  • 3rd generation thistle caterpillars out but hopefully small
  • Weather conditions and crop development
  • Potassium deficiency and other upper canopy ailments of soybean
  • Soybean gall midge present in Dallas and Madison Co.
  • Scout for corn earworm in corn fields
  • Aphids showing up in soybean fields

Aug. 28 Ag Engineering and Agronomy Farm Field Day

Next Wednesday, August 28, is the Ag Engineering and Agronomy Farm Field Day between Boone and Ames. This field day will start at 9:00 a.m. and include a welcome from the new Dean of the College of Ag and Life Sciences at ISU, a weather update from Dennis Todey, and a demonstration and discussion on soil compaction and equipment from Mark Hanna. Attendees can receive 2.5 Soil and Water Certified Crop Adviser (CCA) CEUs at the event. The field day will conclude at noon with a free lunch! Please join us for this great event!


3rd generation of thistle caterpillars out but hopefully small

The third generation of thistle caterpillars are out and feeding in central Iowa soybean fields. These caterpillars will develop over the next several weeks, pupate, and hopefully fly south.

The threshold for treatment remains the same, 20% defoliation (whole plant, across the field), so it should be more difficult for caterpillars to consume enough tissue to reach this threshold now that plants are more mature, but caterpillar populations may be bigger as well. This article from the University of Nebraska-Lincoln has a nice graphic to demonstrate a quick method to scout for defoliation. As has been the case throughout the season, it seems the later planted fields will probably be at more risk (in general). See photos below of new caterpillars out in soybeans late last week.

Photo of the 3rd generation of thistle caterpillars from August 15 (4 larvae on one trifoliate leaf).


Weather conditions and crop development

Rainfall over the weekend and earlier this week brought a lot of relief to crops in much of central Iowa. I received over 3 inches at home between the two rainstorms. The Drought Monitor has reflected some of this rain and removed the D0 (abnormally dry) rating from much of central Iowa, but Boone/Story/Marshall/Tama County have seen the area of abnormal dryness increase. 

Crop development will slow as we've got cooler than average temperatures in the short-term forecast. While most corn is denting and soybeans are at R5 (seed 1/8th inch long in pod at one of four uppermost nodes on stem), dry weight accumulation will be slow(-er than normal) and steady as long as temperatures are cool. The Useful to Usable Corn GDD Calculator is a very handy tool to estimate when your corn crop might mature given the relative maturity, planting date, and conditions (GDD accumulation) this year in your geographic area.

See the tables below from my colleague Brian Lang to see more about the approximate timing of corn and soybean development in reproductive stages.

Stage Description of stage Comments Time to next stage GDD
R1 Silk Maximum plant height
~ 10 days to R2 220
R2 Blister (clear liquid in developing kernel) Maximum vegetative dry matter.
Minimal grain dry matter.

~ 8 days to R3 170
R3 Milk (white liquid in developing kernel) Outside of kernel is yellow.
Starch accumulation increasing.

~ 6 days to R4 125
R4 Dough Starch accumulation increasing.
Kernel moisture starts decreasing.

~ 7 days to R5 (dent stage) 135
R5 Dent Hardening starch causes a depression (dent) in butt end of kernel. The kernel hardens from butt to tip causing a visual horizontal “milk line” on the kernel face the progressively moves from the butt end to the tip end of the kernel.
  ¼ milk line Often begin silage harvest for bunkers. Whole plant is about 70% moisture. 65% DM in kernel.
~ 10 days 185
  ½ milk line Often a target for silage harvest for upright stave silos. Whole plant is about 65% moisture. 90% DM in kernel.
~ 10 days 175
  ¾ milk line 97% DM in kernel.
Grain is about 37% moisture

~ 14 days 200
R6 Physiological maturity (black layer) 100% DM in kernel.
Grain is about 35% moisture.

Total of 65 days Total of 1210 GDD
Stage Description of stage Comments Time to next stage
R3 A pod at least 3/16-inch long at 1 of the 4 uppermost nodes of the main stem with a fully developed leaf. Most popular stage for foliar fungicide applications other than for White Mold control where R1 stage is recommended, and in some cases R1 + R3 stage applications for White Mold control.
About 9 days to R4 stage.
R4 A pod a pod at least 3/4-inch long at 1 of the 4 uppermost nodes on the main stem with a fully developed leaf. Beginning of the most crucial period of plant development in terms of stress influencing seed yield.  Rapid and steady dry weight accumulation by the pods. Still a timely stage for fungicide application for Frogeye leaf spot & Cercospora leaf blight.
About 9 days to R5 stage.
R5 Seeds are 1/8-inch long in the pod at one of the four uppermost nodes on the main stem with a fully developed leaf. By R5.5 stage, plants obtain max. height, leaf area and node number. Rapid and steady seed dry weight accumulation. Final scouting for soybean aphids. If they do not reach 250/plant by R5.5, we ignore them.
About 15 days to R6 stage.
R6 Pods contain green seeds that fill the pod to capacity at one of the four uppermost nodes on the main stem with a fully developed leaf. Period of rapid, steady seed dry weight accumulation continues up to the R6.5 stage.  Rapid leaf yellowing begins shortly after R6, from the lower canopy spreading upward. About 18 days to R7 stage, physiological maturity.


Potassium deficiency and other upper canopy ailments of soybean

I'm sure I'm not the only one noticing yellowing of some upper canopy leaves in soybean fields across central Iowa. Several things can cause yellowing in the upper canopy, including a number of issues that may not be fully understood at this point. See a few resources below.

Potassium deficiency in upper canopy of plots kept in low-very low soil K range.

While we typically think of potassium deficiency as showing up in the lower crop canopy, it often shows on upper leaves late in the growing season. I believe most of what I've been seeing has been this. Read more about late-season potassium deficiency in this 2018 article, but a new version will be posted to our ICM News page soon.

Some plants may have interveinal chlorosis (yellowing) and necrosis (death) on leaves in the upper canopy. A whole host of potential causes exist, and this article from the Crop Protection Network does a nice job of outlining the causes and how they differ.

If you see yellowing and death in the upper part of the canopy that cannot be explained by the interveinal necrosis or does not appear to be potassium deficiency, please let me know.

Soybean gall midge present in Dallas and Madison Co.

Copying this from my last newsletter with the update that soybean gall midge has now been found in Dallas County and Madison County. It was found in the western part of Dallas County in early August and in Madison County today (Aug. 23). It is likely that larvae may be leaving the stems as we found bright orange larvae early this month (last instar), so you may find damage and no larvae. Please continue to keep an eye out for fields that appear to have had this pest this year.

We recently learned that soybean gall midge is creeping closer to central Iowa and has been identified by an agronomist in Adair and Guthrie counties. Please scout the edges of soybean fields for dying plants or those with galls, scarring, or browning at the base of the stem. Inside the stem, tissue will be brown and we can often find the larvae present. Larvae are easy to spot as the 3rd instar is bright orange. 

Soybean stems with scarred, brown tissue and galls forming on some stems. These stems will be brittle and plants can fall over while still appearing healthy from above or can wilt and appear to be diseased.

Soybean gall midge larvae have three instars. The 3rd instar is bright orange.

These resources will help with scouting and identification of soybean gall midge in soybean fields. Please reach out if you think you've found them.

Scout for corn earworm in corn fields

We've been hearing reports of high levels of corn earworm pressure in some Iowa corn fields. These levels have been, in some cases, above the pressure expected for traits that suppress earworms. Last year, I helped some USDA-ARS researchers collect larvae from a late-planted corn field with significant pressure. They are currently working to identify if populations in Iowa are resistant to some traits and are looking for more fields to sample across the state.

Please let me know if you are seeing this issue. Here is a Bt trait table that shows which traits are marketed for management or suppression of corn earworm (CEW).


Aphids showing up in soybean fields

I've seen some aphids in soybean fields in central Iowa in the last couple weeks, but it seems that their populations may be falling with cooler weather and the rainfall that came through. Levels seem relatively low and many fields are reaching the stage (growth stage R5.5) where we will no longer be concerned about threshold levels, but continue to scout fields that were planted later and are in earlier reproductive stages.

Remember to scout for threshold levels: 250 aphids per plant, 80% infestation, increasing aphid numbers, and before stage R5.5 in soybeans. Speed scouting is a nice method to quickly scout a field.

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