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East-Central and Southeast Iowa Crop Information

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July 14, 2005

July 14, 2005


SOYBEAN

Two Spotted Spidermites

Spider mites are now causing damage in some soybean fields in the area. If the hot, dry weather persists, it is likely that we will be seeing more problems with this pest. With all of the focus on soybean diseases, some are confusing the yellow "stippling" that the spider mites cause with plant diseases. If you shake soybean leaves over a white piece of paper and then look closely, you will be able to see little "dots" that move on the paper. A 15-20X lens is needed to actually see that the dots are two-spotted spider mites. For details on scouting for and managing this pest go to  http://www.extension.iastate.edu/Pages/eccrops/spidermite.html or pages 148 - 149 of the July 22, 2002 Integrated Crop Management Newsletter.  Organophosphates (Dimethoate 400 or Lorsban 4E) are the most effective products for this pest.  Dimethoate 400 will have less residual activity and is less effective against soybean aphids, so using this product may kill the predators and allow any soybean aphid population to increase more rapidly.  Likewise, other products labeled for soybean aphid have little activity on spider mites but may again kill predators, allowing the spider mite population to increase more rapidly.  Bottom line:  Before making an insecticide treatment, be sure to take a complete inventory of insect pests and spider mites before making a final product selection.

Soybean Aphids

Soybean aphids are still being found in the area in low numbers. Aphid reproduction is thought to be favored by cool and dry conditions, so hopefully the hot weather will not favor the aphids. Numbers can increase very rapidly, so fields need to be monitored closely. The economic threshold is considered to be 250 aphids per plant. For more information on soybean aphids see  http://www.ent.iastate.edu/soybeanaphid/ and the latest ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2005/7-11/aphids.html. Potato leafhoppers are also common in soybean fields; be careful to not misidentify them as soybean aphids.  Potato leafhoppers do not usually cause economic injury to soybean.  Before making an insecticidal application for soybean aphid, be sure to also check for two spotted spider mites; see the item above.

Soybean Cyst Nematode

While scouting any soybean fields where the presence of soybean cyst nematode is not known, be sure to carefully dig up a few plants, gently shake off any soil, and examine the roots for the presence of soybean cyst nematode.  For more on soybean cyst nematode, see pages 115 - 116 of the June 30, 1997 ICM Newsletter or http://www.ipm.iastate.edu/ipm/icm/1997/6-30-1997/scoutSCN.html, IPM-47s "Scouting for Soybean Cyst Nematode  http://www.extension.iastate.edu/Publications/IPM47S.pdf, and Pm-879 "Soybean Cyst Nematode"  http://www.extension.iastate.edu/Publications/PM879.pdf.

Asian Soybean Rust

The first case of Asian soybean rust in a commercial soybean field in the USA was reported on July 12 in Baldwin County, Alabama. This is a 60 acre field about a mile from where rust was reported on June 28 in sentinel plots. All previous observations of rust have been in kudzu, volunteer soybeans or sentinel plots. Hurricane Dennis may have transported some spores to the eastern part of the midwest. Bacterial blight and brown spot are common in many soybean fields in Iowa now. Bacterial blight is in the upper canopy and brown spot on the lower leaves. Even if rust spores do arrive in Iowa, the current weather is certainly not favorable for rust infection. To see the latest visit http://sbrusa.net/.

Malformed Soybean Leaves

I've received many reports of soybean fields showing malformed leaves similar to dicamba or 2,4-D injury. Whenever these symptoms are seen, any possibility of sprayer contamination or spray drift needs to be investigated. If there is not a spray pattern or drift pattern and the symptoms show up over the entire field, it may not be an herbicide causing the symptoms. Sometimes when soybeans are under stress or when they go through a growth spurt they can exhibit leaf symptoms similar to growth regulator herbicide injury. It is not well understood why these symptoms sometime occur. Usually the soybeans recover with little to no affect on the yield. For more information see http://www.ipm.iastate.edu/ipm/icm/1999/7-19-1999/malsoy.html.

 

ALFALFA

Potato Leafhopper

Potato Leafhopper numbers continue to be high in many fields, so scouting of hay fields continues to be an important activity.  For information on managing potato leafhopper, see pages 107 - 110 of the June 21, 1999 Iowa State University Integrated Crop Management Newsletter or  http://www.extension.iastate.edu/Pages/eccrops/potatoleafhopper.html. A couple of sources for sweep nets are   http://www.gemplers.com/a/shop/product.asp?T1=R13101&src=21SM001  and  http://www.forestry-suppliers.com/product_pages/view_catalog_page.asp?id=5280.

CORN

Silk Clipping

High levels of corn rootworm beetles, occasionally combined with grasshoppers and Japanese Beetles, are resulting in considerable clipping of corn silks.  If silks are clipped to within less than 1/2 inch of the husk, the injury may prevent successful pollination.  Once pollination has occurred, silk clipping will have no consequence.  If an insecticidal treatment is needed, labelled products for corn rootworm beetles in corn include Ambush 25W, Asana XL, Lorsban 4E, Mustang, Mustang MAX, Penncap-M, Pounce 3.2EC, Sevin XLR+, and Warrior.  Purdue has a good reference on silking and silk clipping at  http://www.agry.purdue.edu/ext/corn/news/articles.03/Silks-0716.html.

Extended Diapause

Some are seeing evidence of rootworm damage in corn on soybean ground. We do have pockets of the northern corn rootworm (green to yellow beetles) that do exhibit extended diapause in eastern Iowa. Fields need to be identified so decisions regarding future management can be made.

 

If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: July 14, 2005
Contact: Jim Fawcett fawcett@iastate.edu


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