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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 15
Northwest IA Crop Update, July 5, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Corn development nearing tassel
bullet Cupped soybean leaves: what’s at fault?
bullet 1st generation BLB emerging
bullet WBC moths flying
bullet Soybean rust news

A warm and dry week is forecasted.  Current crop condition in west central and northwest IA is very good to excellent with stored soil moisture in good condition in most areas.  Yet this weather pattern will need to be watched as the corn crop is entering the critical reproductive phase.  Environmental stress may also induce some leaf cupping in area soybean fields, be sure to fully investigate the true cause of soybean leaf cupping before blaming it on herbicide drift.  Western bean cutworm monitoring has begun across the state with moths already caught, be sure to monitor the trapping effort and begin scouting your fields if high numbers of moths are caught in your area.  Soybean rust has now been found on soybeans in Florida and Alabama, continue to monitor for new news on this disease, particularly with two tropical storms forecasted in the area.

Weather information
Growing Degree Day  Last week northwest IA accumulated slightly greater than the long term degree-day average (158 vs. 149).  The week started out warm but cooled off by the weekend.  The week ahead looks to be a bit warmer with 180 degree-days forecasted through July 10, compared to the normal for the week at 154.  Note last year’s accumulations for the same two weeks at 116 and 122 degree days.  All corn planting dates continue to expand the difference between current and “average” accumulations with last week’s average of 104% of normal and this week forecasted to move to 106% of normal.  Last year we were at 94% of normal.  Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through July 10.  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 27-03 158 116 138 149
Forcasted July 4-10 180 122 170 154


  Table 2.  2005 Degree-Day % of Average (by planting date)
  10-Apr 15-Apr 20-Apr 25-Apr 30-Apr 05-May 10-May Ave
last week 106% 104% 101% 101% 104% 106% 104% 104%
this week 108% 107% 103% 104% 106% 109% 106% 106%

Crop Management
Corn development ranges from V12 to tassels emerging in early planted fields.  Tassel emergence and pollination typically occurs at 1100 – 1400 degree-days.  As mentioned previously, the number of rows of kernels per ear has already been established, but the determination of the number of kernels per row will not be complete until about one week from silking. Nutrient uptake through the 14th leaf stage of corn is N = 38%, P = 25%, K = 40% of the year’s total.  K demand has been large compared to the other primary nutrients, and will be at 80% of the year’s demand in the next 7 to 10 days.

Soybean Development All soybean fields are in the reproductive phase of R1 (beginning flower) and R2 (full Flower).  The plant canopy is closing in most fields, providing shade that helps to prevent weed growth and soil moisture loss.  On the other hand, closed plant canopies increase humidity and potentially increase disease incidence in the lower canopy.

Cupped soybean leaves With warm and dry conditions forecasted through the next 10 days, cupped soybean leaves may be common again this year.   In previous hot and dry years, area soybean fields have experienced “cupped” leaves similar to symptomology observed from 2,4-D/Dicamba herbicide drift.  Many soybean fields with cupped leaves that I have looked at in previous years have not been attributed to a growth regulator herbicide, but rather were due to environmental (moisture/heat) stress and/or a soybean herbicide application during stress conditions.  When faced with a field with the “cupped” leaf symptomology, first look for patterns in the field.  Herbicide drift patterns should be evident from the source (corn field) while environmentally induced leaf cupping should be consistent field-wide or more prevalent in areas with more environmental stress (sandy, compacted, pest problems, double-planted areas).   See more comments here:

Pest Management
Western Bean Cutworm  Western bean cutworm pheromone traps have been placed throughout Iowa and are catching moths.  According to University of Nebraska research, we anticipate 25, 50 and 75 percent moth flight beginning at 1319, 1422, and 1536 degree-days (base 50) respectively (starting from May 1).  As of July 4, N.W. IA degree-day accumulation is 1051 and should be around 1200 by July 10.  State-wide trap count data is available at this web site:  More detailed information on trap counts, degree-day accumulation (for N.W. Iowa) and control strategies can be found at this web page:

Bean Leaf Beetle  Peak emergence of 1st generation bean leaf beetles will have occurred by the end of this week in soybean fields that emerged before May 20th.  Therefore, plan to begin scouting for first generation Bean Leaf Beetles by the end of this week in soybean fields that emerged in early May.  Research at ISU now allows us to get an estimate of the population size of the 2nd generation by scouting the 1st generation one week after the peak emergence, which generally occurs after an accumulation of 1212 degree-days following emergence of the soybean field.   More information on when and how to scout and thresholds at 

Soybean Aphids STILL remain scarce around the region.  Yet surrounding states, particularly MN, appear to be finding increasing populations (even in S.W. Minnesota).  See MN’s numbers from the MN department of Ag pest survey found at this web address:  Click on the July 1 or most recent pest report found at the top of the list.  Continue to be vigilant and scout for aphids whenever you are in a soybean field. Read about aphids in NE Iowa in the most recent ICM newsletter: Also look to the new ISU website all about soybean aphids  Other soybean aphid information sources can be found on my own aphid page at this web address:

Soybean Rust  The big news late last week in the rust arena was a positive find of soybean rust in two soybean sentinel plots, one in Florida and another in Alabama (which was Alabama’s first rust find of the year).  Prior to these finds, soybean rust had only been found on kudzu in Florida and volunteer soybeans in Georgia (which were later destroyed).  It appears that two tropical storms will impact the southeastern U.S. this week, increasing the potential for inoculum spread and creating favorable conditions in the southeast U.S for infection.  Yet the most recent computer model run by X.B Yang’s lab indicates very low risk for movement of soybean rust spores into the northern soybean producing region through the month of July.


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 07/05/05

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