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 9

Northwest IA Crop Update, May 20, 2002
(Word Document)

In this issue 
Soil Temperature Warming (It’s about time!).
DD50 accumulation and Corn development
Alfalfa Aphids and Spring Black Stem.
Use Caution when POST applying herbicides in cool conditions.
Over-winter Bean Leaf beetles and feeding injury.
Just how cold has it really been?

Introduction
Finally, a good week of planting with few interruptions from rain.  It appears that even though soil temperatures have been very cool, most corn is emerging.  Some areas with more marginal conditions need to be monitored.  Soybean planting is well underway and I would expect most of the remaining acres in NW Iowa to be planted by the end of the week.  Alfalfa problems have been more common this last week.  Aphids and Spring Black Stem have been common in some fields causing defoliation and poor growth.  Be on the lookout for these problems in your fields.  If you think it has been cool this spring, you are thinking correctly.  Since the 20^th of April, we have collected half as many corn degree days compared to the same time period last year.  Bean leaf beetles appear to be very hungry again this year.  Feeding damage from over-wintered beetles is normally a cosmetic problem, but if you have the first field up in the county, you may be inundated with the hungry beetles. 

Weather update
Northwest Iowa was relatively rain-free last week except for light showers on Wednesday night.  Temperatures remained cool over much of the upper Midwest, yet the sunshine here in NW Iowa helped to boost soil temperatures and aided corn emergence.  Soil temperature was finally sustained over 55 F for the entire week (Figure 1).  The 7-day extended forecast calls for more swings in temperatures with highs mostly in the low 70’s and lows in the upper 40’s to low 50’s

Growing Degree Day Accumulation
The area accumulated 59 DD50’s over the last 7 days which is a few more than the previous week but (again) less than was anticipated.  According to the 7-day forecast, we should expect 72 DD50’s over then next 7 days.  Emergence of corn should have occurred on fields planted from April 15^th through May 5 ⁽Figure 2).  According to DD50 accumulation, the earliest planted corn should be at the 3rd full leaf stage (count the first leaf with the rounded tip).   

In Figure 2, the line with squares equates to Monona, Crawford, Carroll and southern Ida, Sac and Calhoun counties.  The line with Diamonds equates to Pocahontas, Buena Vista and northern Ida, Sac, and Calhoun counties.  The dotted line is predicted GDD and crop development in 7 days.

Pest Management
Alfalfa Aphids and Alfalfa Spring Black Stem.  Several fields in NW Iowa have been stricken with high aphid populations, Spring Black Stem or a combination of both.  Fields appear to be healthy then quickly digress, sometimes with complete death of the foliage.  If your alfalfa field doesn’t quite look like it should, a quick walk-though may well be warranted.  Look for small green aphids under the leaves and on the stems.  Also look for black spots under the leaves and elongated lesions on the stems (Spring Black Stem).  Pea aphids and Spring Black Stem typically do not cause extensive damage in NW Iowa alfalfa fields, but this year appears to be an exception. Treatment thresholds for Pea aphid populations range from 75 to 100 aphids/stem.  Early harvest may be the best option to stop the spread of Spring Black Stem. 

For more information on Alfalfa aphids, refer to the following web address
www.iastate.edu/carroll/crops/newsletters/2002/Alfalfa_aphids.doc
For more information on Alfalfa Black Stem, refer to the following web address
http://www.ipm.iastate.edu/ipm/icm/2002/5-13-2002/springdis.html

Black Cutworm Trap Catch; Go to the following web page to see local BCW trap catch numbers http://extension.iastate.edu/carroll/crops/bcw_2002_trap_catch.htm  We should be scouting for cutworm activity now in area cornfields.  Corn is typically safe from BCW activity once it is past the V5 stage.  For more information on cutworm scouting refer to http://www.ipm.iastate.edu/ipm/icm/2001/5-7-2001/earlyscout.html.

Uses caution when applying POST herbicides under cool conditions.
Dr. Bob Hartzler (ISU Extension Weed Science) has the following advise when applying herbicides in our current (cool) conditions:  Herbicides perform best under ideal conditions when both the crop and weeds are actively growing. When the environment reduces the vigor of the crop or the weed, undesirable consequences can occur - either crop injury or poor weed control. The most noticeable effect of cool temperatures on herbicide performance is a slower kill of weeds. An herbicide kills a plant by disrupting some physiological process essential for growth. Under cool temperatures, physiological processes slow down; thus the herbicide is slower acting. In some situations the desired effect (dead weeds) will result - it just takes longer to get the job done. In other situations, the slower activity of the herbicide will allow some weeds to survive, or the crop may be injured.

Undesirable crop responses are more likely to occur when using herbicides with lower margins of crop safety. Certain herbicide labels have warnings on the label concerning increased risk of injury under cool conditions. If plans were to use one of these products, the best decision would be to delay application until more favorable conditions occur or switch to a product with a greater margin of crop safety. A partial list of products with cool temperature warnings includes Basis Gold, Accent Gold, Reflex, Lightning, Cobra, and Buctril. Read all labels to determine if restrictions or warnings are present concerning use in cool temperatures.  Crop and weed size also should be considered when determining whether or not to spray.  More detailed information can be obtained by going to the following website:  http://www.weeds.iastate.edu/mgmt/qtr98-2/cooltemps.htm

Bean Leaf Beetle:  We are beginning to find bean leaf beetles (BLB) in fields.  The majority of BLB over-winter in or near wooded areas where they can find good cover.  Yet up to twenty percent may over-winter in the soybean stubble.  As the beetles emerge, you may find these beetles on corn plants planted into last year’s soybean stubble.  BLB typically will not feed on and cause injury to corn plants.   Late planted soybeans may be our best method of reducing the first generation population by not providing a food source when the over-wintering population emerges.   Economic injury to soybeans typically does not occur until the second generation in August, yet with higher than normal populations the last couple of years, extensive feeding injury on soybean has been observed from the over-wintered and first generation beetles. 

Table 1 can be used to help evaluate if treatment for over-wintering bean leaf beetles is warranted.  The threshold is based on economic feeding injury only; it does not account for the potential spread and injury from the bean pod mottle virus. 

Table 1. Early-season bean leaf beetle economic thresholds in soybeans (beetles per plant).^a

^a For beetles per row-foot, multiply number by 7.6.

 Table 2. Insecticides labeled for bean leaf beetle in soybeans.

* Restricted use insecticide.
by Marlin Rice, extension entomologist and Larry Pedigo, professor, Department of Entomology.  Integrated Crop Management Newsletter, May 15, 2000.  http://www.ipm.iastate.edu/ipm/icm/2000/5-15-2000/lotsobeetles.html

Just how cold has it been?
From April 20^th to May 19, northwest Iowa has accumulated only half as many heat units when compared to the same time period last year.  Figure 3 shows accumulated degree-days for 2001 (top line with diamonds) and for 2002 (bottom line with squares).  The lack of heat units can be seen in corn field by the slow emergence and general poor color of the emerged corn.
 

(Word Document)