Northwest Iowa Crop Update Newsletter by Todd Vagts ISU Extension Crops Specialist Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.
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Volume 2, Number 14

Northwest IA Crop Update, June 24, 2002
(Word Document)

Introduction
After several consecutive hot days, another warm and dry week is in store for west central Iowa.  The crops are growing rapidly and with continued hot and dry conditions, soil moisture may become a concern in the near future.  Evidence of iron chlorosis and cyst nematode can be found in area soybean fields, scout and record these problem areas so that resistant cultivars can be selected for the next soybean cropping season.  Cultivators have been out in force making one last attempt to control weeds, but is the cultivation really necessary?   University research has shown that unnecessary cultivation may cost yield and money.  Insect pests are showing up now, the soybean aphid has been detected in Woodbury County, grasshoppers are munching on crops and gardens and ECB moths continue to be caught in higher than usual numbers.  Look to the most recent ISU ICM newsletter for aid in managing corn leaf diseases in area seed fields.

Weather update
Warm, windy and dry has been the norm the last few days and is forecasted to continue through the next 3 to 5 days.  The best chance of rain (according the weather channel) will be on Wednesday.

Growing Degree Day Accumulation and Crop Development
Northwest IA picked up 193 Degree-days (base 50) last week and is forecasted to pick up 200 this week (Figure 1).  NW Iowa has been accumulating greater than normal DD50’s over the last four weeks and is well above 11-yr average trend line for this time of the year.

Corn Development   The majority of corn is at or shortly past the V-10 leaf stage, adding 2 to 2 ½ leaves last week.   The time between the appearances of new leaves will shorten, generally occurring every two to three days. Nutrient uptake through the 10^th leaf stage of corn is N = 16%, P = 15%, K = 25% of the year’s total.  K demand has been the large compared to the other primary nutrients, hence the reason K-deficiency often shows up at this time of the season.

Soybean Development ranges from V3 (3 node) to V5 (5 nodes).  The soybean plant has taken up 9 and 8 percent of its total Nitrogen and Potassium requirements for the season. 

Iron Chlorosis has been showing up in soybean fields over the last week.  Iron chlorosis can be a common occurrence, particularly in cool wet years and in fields with high pH (greater than 7.0). Symptoms of iron chlorosis are 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. Some varieties are tolerant to the disease and yield much better in fields with higher pH.  For more information, access the 07-02-2001 ICM article on-line at http://www.ipm.iastate.edu/ipm/icm/2001/7-2-2001/soychlor.html

Row Cultivation in corn and soybeans has been common over the last week, but is this cultivation really necessary?  Recent research at several universities suggests that unless a significant weed problem exists that cannot be solved through chemical control, cultivation just for the sake of cultivating can reduce grain yields.  Cultivation should not be used for aesthetic purposes or for the purpose of "opening or airing" the soil out.  Cultivation destroys the root mass close to the soil surface and I believe may create deep fractures in the soil (when the soil is dry) where the tractor tires run.  These fractures may break roots at these deep depths.  In areas that have had excessive precipitation and/or wet spots in fields, shallow root systems will be common.  Cultivation may damage a greater portion of this type of root system, creating water and nutrient stress problems as we move into a dryer period.  Many times cultivation is used to throw soil to the plant base which helps brace root development, which I do agree helps, but I also believe the crop most times develops a more than adequate brace root system on its own.

Pest Management
Soybean Aphid reported in western IA.  Hal Tucker, a Crop Consultant in NW Iowa, recently reported finding soybean aphids in a field in eastern Woodbury County.  Soybean aphids typically are found on the underside of the newest leaves in the upper canopy of the soybean plant.  If you happen to find soybean aphid in your area, please report them to me or your local extension office.

Grasshopper damage along borders of soybean fields has been reported (primarily) in Calhoun county.  Grasshoppers can cause significant crop defoliation to both soybeans and corn as they migrate from maturing grasses in ditches and waterways.  Spot treatments along field edges can be successful at lowering grasshopper feeding pressure, but for best success, the source of the grasshoppers (ditches and waterways) should also be sprayed with an insecticide.  Many insecticides are not labeled for both crop and non-cropland applications, so be sure to read the label before making the application.  Malathion, Sevin and Asana are labeled for non-cropland and many row crops.  More information on grasshopper control can be obtained from the following web pages:

NE CropWatch (06-21-02) http://cropwatch.unl.edu/
NebFacts NF02-328  http://www.ianr.unl.edu/pubs/insects/nf328.htm
ISU ICM Newsletter (06/28/99)  http://www.ipm.iastate.edu/ipm/icm/1999/6-28-1999/scoutsoygh.html

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: http://www.extension.iastate.edu/Publications/PM879.pdf

Corn Leaf Disease in Seed Corn  For help in identifying corn leaf diseases and control strategies, look to ISU’s ICM newsletter http://www.ipm.iastate.edu/ipm/icm/2002/6-24-2002/cornleafdis.html

European Corn Borer moth flight trap catches continue to indicate a somewhat greater moth flight this year when compared to the past few years.  For help in Identifying all the potential caterpillars found in corn, use the following descriptions provided by Dr. Marlin Rice (ISU Extension Entomologist) written in the 6-28-1999 ICM newsletter http://www.ipm.iastate.edu/ipm/icm/1999/6-28-1999/midscorninj.html or visit the ISU Caterpillar key found at http://www.ent.iastate.edu/pest/cornborer/key/

University of NE Black Light Trap Catch Numbers at Concord NE. http://nerec.unl.edu/ipm/lighttrap.htm