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Northwest Iowa Crop Update Newsletter
by Todd Vagts
ISU Extension Crops Specialist
Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.

[Home][Special Topics][Weather Data][Subsoil H20][PDF Info] [ISU Extension][IA State University]

Volume 5, Number 13
Northwest IA Crop Update, June 20, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Warm week ahead
bullet Iron Chlorosis in soybean
bullet Where are the soybean aphids?
bullet Soybean Cyst Nematode
bullet Soybean Rust Sentinel Plot Field day
bullet No new rust finds

All area crops have taken advantage of the warm and sunny weather and are growing well.  First cutting of hay is nearly complete with the opportunity for mowing and drying.  The soybean crop is now rapidly putting on nodes and will enter the reproductive phase by late June or early July.  Iron Chlorosis has begun showing up in the usual areas and now is also the time to begin looking for soybean cyst nematode.  No new finds of soybean rust have been reported, but intense scouting continues in the southeastern U.S. following tropical storm Arlene.  A unique opportunity will be available to visit with western IA’s soybean rust sentinel plot coordinator and scout on June 30th at the Western IA research farm near Castana, call for more details.

Weather information
Growing Degree Day  Last week northwest IA accumulated 125 degree-days, 90% of the normal for the week.  If the weather forecast holds true, we should see a much above normal degree-day week ahead.  The current forecast calls for 192 degree-days, compared to the average of 149.  Note that during the same week last year, the area collected only 80 degree-days.  That week marked the beginning of the cool period experienced through August of 2004.   With the forecasted degree-days, all corn planting dates will once again move above normal in degree-day accumulations.   Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through June 26.  More detailed degree-day accumulations by planting date can be obtained at this URL:

Table 1.  Degree-Day Weekly Accumulation
  2005 2004 2003 14-Yr Ave
June 13-19 125 122 159 139
Forcasted June 20-26 192 80 149 149


  Table 2.  2004 Degree-Day % of Average (by planting date)
  10-Apr 15-Apr 20-Apr 25-Apr 30-Apr 05-May 10-May Ave
last week 104% 101% 96% 97% 100% 104% 99% 100%
this week 108% 106% 102% 103% 106% 109% 106% 106%

Crop Management
Corn development  Corn across the area took advantage of the dry sunny days last week and grew rapidly, creating a full canopy on 30 inch rows in most fields.  Corn now ranges from the V8 to V11 leaf stage.  Corn planted during the cool April 20 – 30th period should be at or approaching the V10 stage and V12 by next weekend.     

By V10, the time between the appearances of new leaf stages will shorten, generally occurring every two to three days.  Nutrient uptake through the 10th leaf stage of corn is N = 16%, P = 15%, K = 25% of the year’s total.  K demand has been large compared to the other primary nutrients, hence the reason K-deficiency often shows up at this time of the season.  Use the corn development chart (figure 2) to help predict timing of corn V stages.  If you want to figure you own crop development schedule, figure that corn requires about 75 degree-days (base 50) for each new leaf.


Soybean Development ranges from V3 (3 node) to V4 (4 nodes).  Expect flowering to begin in the earliest planted fields by June 25 but most fields will not begin flowering until the first week of July.  Warm temperatures in June can accelerate the time to flowering, sometimes by as much as two weeks.  R1 is determined when one open flower is found at any node on the main stem.  Flowering begins on the third to sixth node of the main stem. 

Iron Chlorosis has begun to show up in soybean fields.  Iron chlorosis is often a common occurrence, particularly in cool wet years and in fields with high pH (greater than 7.0). Symptoms of iron chlorosis include yellowing of interveinal areas of young leaves. Brown and necrotic spots may occur in leaf margins and plants can be severely stressed or killed if the disease is severe. Varieties differ in iron chlorosis responses.  For more information, access the 07-02-2001 ICM article on-line at

Pest Management
Soybean Aphid  Last week I was in 5 soybean fields in each county of Carroll, Calhoun, Sac, Ida, Monona and Crawford and did not find a single Soybean Aphid.  Soybean aphids have been showing up in S.E. MN, but still at relatively low numbers.  You can keep track of aphid numbers to our north from the MN Pest Report found at this URL:  The current economic threshold is 250 aphids per plant with an increasing aphid population and few beneficials.   ISU does not recommend applying insecticide (with the early application of herbicide) to control low numbers of aphids or as an aphid preventative treatment.

Soybean Cyst Nematode can be found in soybean fields now.  To determine if your field is infested with SCN, dig the roots (don't pull them out), carefully shake off the soil, and examine for small cysts attached to the roots.  They are smaller than a nodule, and creamy white in color.   The ISU Extension publication on SCN does a good job discussing the pest, and shows pictures of what to look for. Find it at this web address:

Soybean Rust  No new finds of soybean rust have been reported following the movement of tropical storm Arlene into the southeastern U.S.  Researchers expect that it will take up to two weeks for rust to develop to detectable levels.  Keep monitoring the USDA web page for details:

Soybean Rust Sentinel Plot Field Day  An update on soybean rust monitoring in Iowa soybean sentinel plots will be held at the ISU research and demonstration farm near Castana in Monona County on June 30.   Dr. Ralph Von Qualen, western IA’s sentinel plot coordinator, will lead the discussion on soybean rust monitoring in IA.  I will also be on hand to discuss recent developments on the movement of soybean rust in the U.S. and its implications to western IA soybean farmers.   

Participants will be able to observe the sentinel plot located at the research farm and ask questions related to disease and pest management in soybeans.  The program will begin at 10:00 a.m. and should conclude by 11:00.  Contact the Monona County Extension Office at 712-423-2175 for more information.  The farm is located four miles east of Castana on county highway E34.

ISU Soybean Rust Forecast  See the June 13 SBR forecast from XB Yang’s lab at this URL:  XB comments that we are not out of the woods yet for soybean rust entering IA, but if we enter July with no new finds in the S.E., the odds of a major epidemic in this state is diminished.


Print or view this newsletter in  PDF or Microsoft Word format.

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue 
Carroll, IA 51401 
Office: 712-792-2364; Cell: 712-249-6025;  Fax: 712-792-2366

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This page last updated on 06/21/05

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