July 13, 2005

 

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.

SOYBEAN

Potassium Deficiency Symptoms

Tip Blight / Top Dieback / Potassium Deficiency has again begun to show up in some soybean fields.  The new leaves exhibit yellowing along the margins, often followed by necrosis of the tissues.  The debate about the relationship of disease and potassium in this phenomenon continues, but higher levels of potassium often lessen the severity of this phenomenon.  Likewise, genetics and disease levels make a difference; be sure to check plants for signs of root rots and soybean cyst nematode (See the item below.).  Also, poor rooting and/or root function due to soils being compacted, too loose, and/or too dry can promote these symptoms.  For more details, see the pages 172  173 of the September 13, 1999 Integrated Crop Management Newsletter.

Two Spotted Spidermites

As the dry weather continues, the instances of two-spotted spidermites damaging soybean are increasing.  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 labelled for soybean aphid have little activity on spidermites but may again kill predators, allowing the spidermite population to increase more rapidly.  Bottom line:  Before making an insecticide treatment, be sure to take a complete inventory of insect pests and spidermites before making a final product selection.

Soybean Aphid

Soybean aphids are now being found in eastern Iowa in some soybean fields, especially in the area along and north of Highway 92 (Grandview to Columbus Junction). A few fields have been sprayed, but aphid numbers have been very low so far in most fields where they have been found.  The economic threshold is considered to be 250 aphids per plant. For more information on soybean aphids see http://www.ent.iastate.edu/soybeanaphid/.  Potato leafhoppers are also common in soybean fields; be careful to not misidentify them as soybean aphids.  Potato leafhoppers do not cause economic injury to soybean.  Before making an insecticidal application for soybean aphid, be sure to also check for two spotted spidermites; see the item above.

Bean Leaf Beetle

Bean Leaf Beetle numbers in some fields suggest that numbers will become high enough during the second generation (early August) to warrant an insecticide application.  Remember, there is a strategy for using first generation numbers to predict the need for an second-generation spray, allowing that application to be made before the second generation causes significant damage.  For more information on that strategy see http://www.extension.iastate.edu/Pages/eccrops/blb3.html or pages 90 - 91 of the July 12, 2004 ICM Newsletter.  Before making an insecticidal application for soybean aphid, be sure to also check for two spotted spidermites; 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 observation of soybean rust in soybeans planted this year was made recently in southern Alabama and Florida. In both cases the rust was found in sentinel plots. Despite extensive scouting for the disease, there have been no reports further north. Brown spot can now be found in many soybean fields in eastern and southeastern Iowa. Like soybean rust it also first shows up on the lowest leaves. You need to look for pustules with a 15X or 20X magnifier in order to differentiate between brown spot and soybean rust. Pocket magnifiers can be found at the same 2 sources given above for sweep nets. If you think you see a pustule, make sure it is not a piece of dirt. It is not likely that we will see soybean rust in Iowa until it is confirmed further north in the U.S. To see the latest visit http://sbrusa.net/.