Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information

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August 25, 2005

August 25, 2005

SOYBEAN

Two Spotted Spidermites

 

Spider mite damage is evident in many field across the area, but in most cases the mite population is gone or diminished with the wetter, cooler conditions. It is getting too late to spray most products anyway, as harvest could begin within a couple of weeks.

 

Soybean Aphids

 

Insecticides seemed to have been very effective in fields that I have looked at that were sprayed for aphids. Populations are also dropping in most unsprayed fields.

Asian Soybean Rust

 

On August 22, soybean rust was confirmed in a commercial field in Appling Co., GA, the tenth Co. in Georgia with a positive find.  The first report from South Carolina in 2005 was in a commercial soybean field in Hampton Co. on the SC/GA border and is adjacent to Effingham Co., GA that had a previous positive find. Soybean fields in SE and EC Iowa are now or will be soon in the R6 stage, so beyond the window for treating for soybean rust, so looks like we made it through the season. For soybean rust updates see http://sbrusa.net/.

 

Frogeye Leaf Spot

 

Frogeye leafspot is more common this year than what I have seen before. Photos of the disease, and management tips are provided at the following site:
http://www.ipm.iastate.edu/ipm/icm/2001/12-24-2001/frogeye.html.

 

Sudden Death Syndrome

 

Sudden death syndrome is showing up again this year, but appears to be very patchy and not killing very large areas in most fields where it is being found.

 

CORN

Western Bean Cutworm Damage

 

It is becoming apparent that western bean cutworm damage is more widespread this year than what we have seen in the past. Along with the drought conditions, the increased insect damage could lead to more ear rots this fall, and increase the importance of timely harvest and proper drying. The northern part of the area (Benton, Linn, and Jones counties) had the large moth flights in July, and that is where a lot of the damage is being found. It is not uncommon to find several larva per ear. To manage the pest in the future, hybrids resistant to the pest need to be selected or fields need to be scouted in July and treated if necessary. For information on the western bean cutworm see the July 19, 2004 issue of the ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2004/7-19-2004/wbc.html.

 

Ear Rots and Mycotoxins

 

It is likely that we will see more ear rots this fall with the drought conditiond. Alfatoxin is created by a yellow-green mold called Aspergillus flavus, which might already be observable in a field or which may become observable any time before harvest.  However, the presence of the mold does not necessarily mean that aflatoxin is present.  In addition, Aspergillus flavus may be working on the inside of the kernels with no outward signs. Iowa State University publication Pm-1800 Aflatoxins in Corn will answer many of your questions.  In addition, be sure to watch for other ear molds as described on pages 158 - 160 of the October 6, 2003 (Fall 2003 Special Edition Drought Issue) of the ISU Integrated Crop Management Newsletter. Early harvest and proper drying can help reduce the impact of the rots.

 

Harvesting Corn Silage

 

Harvesting corn for silage has been occurring for the past week or more in the area. The milk line is often used to estimate whole-plant harvest moisture for corn silage production. The whole plant reaches 100% accumulation of digestible nutrients at 1/4 milk line. Ensiling at this stage through 3/4 milk line maximizes yield of digestible forage. Various storage structures suggest a different harvest moisture to optimize the fermentation process of ensiling. The following table provides some common ranges of harvest moisture for different structures.


Structure

Harvest moisture

approx. milk line

Bunker or Bag

67 - 70%

1/4

Oxygen limiting silo

50 - 60%                

1/4 - 1/2

Upright silo

63 - 67%

3/4 - initial black layer



However, the milk line is only designed to be an indicator of when to sample some forage and test for its actual moisture content.


Drought-damaged corn silage:


In general, drought-stressed corn silage quality is about 90% of normal silage.  The main problem is reduced yield. The following resources may be useful:

1) Handling Drought-Stressed Corn
http://corn.agronomy.wisc.edu/WCM/2003/W138.htm

2) NCH 58, Utilizing Drought-Damaged Corn
http://www.ces.purdue.edu/extmedia/NCH/NCH-58.html

An additional concern could be high nitrate levels.  The following link discusses nitrate concerns and testing.
http://www.extension.iastate.edu/ag/drought/NitrateToxicity.pdf

ALFALFA

Fall Harvest Management

 

To reduce winterkill it is best to give alfalfa a 5-6 week rest period between the last harvest and the first killing freeze (24F for stopping alfalfa growth). These temperatures generally occur during late October in much of the area. Cutting alfalfa from mid-September to mid-October increases the risk of winter-injury. After mid-October, or a hard freeze, taking a final cutting can be done without much increased risk of winterkill.

 

 

 FOR YOUR CALENDAR

Crawfordsville Fall Field Day - September 14 1:30 p.m.

 

The annual fall field day will be held at the SE Iowa Research Farm near Crawfordsville on September 14 at 1:30 p.m. Alison Robertson and X.B. Yang will be featured at the tour. They will present information from this year's fungicide trials and also will discuss alfatoxin and other mycotoxin concerns in corn. Mike Duffy, ISU Economist, will compare the economics of continuous versus rotated corn. Also featured will be Jim Jensen, on Low Sat And Low Lin Soybean Opportunities, and Kevin VanDee on Strip Intercropping.

 

Nashua Fall Field Day - September 8 1:30 p.m.

 

The afternoon program will feature topics on both crop production and grain marketing, and will give you an opportunity to meet the new ISU corn specialist, Roger Elmore.

 

If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: August 25, 2005
Contact: Jim Fawcett fawcett@iastate.edu


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