August 9, 2010
Soybean Sudden Death Syndrome
Soybean Sudden Death Syndrome (SDS) is appearing again in 2010. Brown Stem Rot (BSR) can cause leaf symptoms identical to SDS. See pages 70 72 of the March 26, 2007 Integrated Crop Management Newsletter or http://www.ipm.iastate.edu/ipm/icm/2007/3-26/bsr_vs_sds.html for identification and management of SDS and BSR.
If the field has not been tested for Soybean Cyst Nematode (SCN), the presence of SDS in the field should prompt a soil test for SCN as SCN is usually present if SDS is present. The sample submission form and instructions for taking the sample are in PD-32 Plant Nematode Sample Submission Form which is available at Iowa State University Extension offices or can be downloaded from http://www.extension.iastate.edu/Publications/PD32.pdf.
Soybean aphid numbers continue to be very low. The economic threshold is 250 aphids per plant with 80% of the plants being infested and with populations increasing. Once the soybeans reach growth stage R 5.5, an insecticide application is not needed. If there is a seed 1/8 inch long in a pod at one of the top two nodes with a fully expanded trifoliolate leaf on the main stem of the plant, the plant is about right at R 5.5.
An alternative for conventional scouting is to use the speed scouting method developed at the University of Minnesota. You only have to be able to count to 40 to use this method, but need to take a spreadsheet to the field to take notes and make a decision. You will most likely hit threshold with speed scouting before you actually hit the threshold using the conventional method. The data suggests that if you hit the threshold using the speed scouting method, there is an 82 per cent chance you will soon go over the threshold using the conventional scouting method. The following site describes the method and provides a link to download the spreadsheet. http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm
Scouting techniques and management information can be found in SP 247, Soybean Aphids in Iowa 2007, which can downloaded from http://www.extension.iastate.edu/Pages/eccrops/transfer/07SBA.pdf.
I have yet to received a call about or be in a field with White Mold. However, this is the time of year when infections may show up. While the evidence of the infections may show up at this time of year, the infections took place shortly after the beginning of flowering in late June. The infection itself is no longer spreading, but the evidence of the infection gives the appearance of the disease spreading as more plants show the symptoms of the disease.
There most likely will be little positive effect of a fungicide application at this time because of the advanced progress of the disease.
The most important thing for growers to do at this time is to note the presence of white mold in the field and then select for varieties with lower susceptibility or higher tolerance for white mold the next time soybeans are grown in the field. Wider rows may help with white mold, but wide rows have other drawbacks. If the conditions are good for white mold infection (cold and wet) at the beginning of flowering, the application of an appropriate fungicide at that time may help. An application of Cobra at or just before the first bloom has also been shown to lessen the impact of white mold.
FOR YOUR CALENDAR
Muscatine Island Research and Demonstration Farm, Fruitland
Demonstration Garden Field Day on August 9, 2010 at 6:30 p.m.
Details are posted at http://www.extension.iastate.edu/Pages/eccrops/meetmusc.html.
Northeast Iowa Research and Demonstration Farm, Nashua
Fall Field Day on August 26, 2010, 1:00 p.m.
Details will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetnerf.html.
Southeast Iowa Research and Demonstration Farm, Crawfordsville
Fall Field Day on September 15, 2010
Tentatively there will be a manure application field day in the morning followed by a more traditional field day in the afternoon. More details will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetserc.html.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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