July 29, 2008
Foliar leaf diseases are starting to show up in corn, but are still at fairly low levels. The main disease showing up is common rust. Many are trying to determine if it will pay to spray fungicides on corn. Experience from last year’s trials indicate that the corn hybrid can make a lot of difference. In areas where gray leaf spot was severe, fungicides increased yields of sensitive hybrids by up to 30 bu/A, where little to no yield increase was seen by spraying hybrids that had a good disease package.
Now is the time to be looking at fields to see if disease lesions are common. If you are growing a sensitive hybrid and disease lesions are showing up on most plants on the lower leaves a few leaves below the ear leaf (or higher), a fungicide may pay off. Fungicides will not cure infections that have already occurred, but you don’t have to spray before any lesions are present to prevent yield losses. It makes more sense to do some scouting and make sure a fungicide is needed, rather than spraying just in case. It is important to wait until after tasseling is complete in a field before spraying. In some uneven fields this year that may mean waiting for 2 or more weeks after tasseling has begun. Daren Mueller has a nice article on things to consider in making decisions on spraying fungicides on corn or soybeans (including a photo of gray leaf spot lesions) at http://www.extension.iastate.edu/CropNews/2008/0714Mueller1.htm.
Japanese Beetles will continue to cause problems through August. Most fields being sprayed for the beetles are nowhere near the threshold. To avoid having to spray a field more than once, it makes sense to hold off spraying until about 20% of the leaf area is gone. This is on the entire plant, not just the upper leaves. Most people tend to overestimate defoliation. If it looks like 20% defoliation, most likely its only 10%. The pictures in the following article can help in estimating percent defoliation: http://www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/soydefoliation.html. Corn should also be watched closely now for the pest since the beetles will clip silks. For more information and insecticides labeled for Japanese beetles see the July 28, 2003 ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2003/7-28-2003/japanesebeetle.html.
Asian Soybean Rust
Scouting for soybean rust continues in sentinel plots in Iowa. So far rust is confined to the far southern US and the dry weather there means the risk to the northern states is still very low. For more information see http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi.
FOR YOUR CALENDAR
Soybean Cyst Nematode Management Tour
Monday, August 11 6:00 p.m.
Dave & Karen Peterson Farm – Atkins
Iowa State University Extension & the Linn Co-op will be presenting a field day on the Peterson farm where the Corn Soybean Initiative is conducting soybean cyst nematode research. Soybean varieties with different sources of resistance to the pest were planted at the site. Dr. Greg Tylka, ISU Extension Nematologist, will discuss advances in soybean cyst nematode management at the tour. A free meal, courtesy of Qualisoy, will be available at 6:00 p.m. followed by the tour. Please call Benton Co Extension for a meal reservation and directions to the site at 319-472-4739.
Pest Management Tour
Tuesday, August 12 6:00 p.m.
John Heisdorffer Farm - Keota
Iowa State University Extension and Vision Ag will be presenting a tour on the John Heisdorffer farm south of Keota the evening of Aug. 12. Dr. Aaron Gassman, ISU Entomologist, will be discussing managing corn rootworms in a corn-bean rotation. Gassman is doing research investigating problems with rootworms on soybean ground, including some work on the Heisdorffer farm. Corn and soybean fungicides will also be discussed. A free meal will be available at 6:00, courtesy of BASF. The farm is located 1 mile south of Highway 92 on 330th Ave.
Nitrogen Management Tour
Wednesday, August 20 5:30 p.m.
Bob McArthur Farm – Whittier
Iowa State University Extension and Linn Co-op will be presenting a tour on the Bob McArthur farm just west of Whittier the evening of Aug. 20. Many rates, sources, and timings of nitrogen fertilizer are being investigated on the farm. Dr. John Sawyer, ISU Extension Agronomist – Soil Fertility, will be discussing advances in nitrogen management at the tour. A free meal will be available at 5:30 p.m., followed by the tour.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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