August 5, 2011
With the hot weather the corn is moving through the growth stages in record time, which is not good news. Most of the corn is in the dough stage now and some is starting to dent. I don't think the hot weather had a large impact on pollination, but it is bound to reduce yields by shortening the grain fill period. At least this should help to reduce the need for drying the corn this fall.
It may still pay to spray fungicides on corn that has fungal foliar diseases showing up on or near the ear leaf, but with each passing day, the chances for a yield response becomes less. Gray leaf spot, northern corn leaf blight and physoderma brown spot are fairly common on the lower leaves in many fields. See Alison Robertson's recent article on late corn fungicide applications at http://www.extension.iastate.edu/CropNews/2011/0731robertson.htm for more discussion.
Goss's Wilt Found in Some Eastern Iowa Fields
Fungicide applications will not control the Goss's Wilt that is showing up in a number of fields in eastern Iowa, because it is caused by a bacterium. It causes large lesions, especially on the upper leaves. Last year a lot of Northern Corn Leaf Blight was misdiagnosed as Goss's Wilt in eastern Iowa. Both have large lesions and can occur on the upper leaves, but Goss's Wilt also will show a lot of black freckles along the edge of the lesions, and may also have a greasy or shiny appearance due to the bacterial ooze. It is more common in fields that suffered some hail damage, or in fields where there was hail damage the last time the field was in corn. An article that shows pictures of Goss's Wilt and Northern Corn Leaf Blight can be found at http://www.extension.iastate.edu/CropNews/2009/0731robertson.htm.
Japanese beetles were not a big problem last year, but they have come back with a vengeance this year. Numbers have declined in some fields, but are still in high numbers in other fields. During flowering and pod set, it generally does not pay to spray unless the total defoliation in the fields reaches 20%. For help on estimating defoliation see http://www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/soydefoliation.htm. Most people overestimate percent defoliation. During pod fill you may want to consider an insecticide application at 15% defoliation.
Spider mites are showing up in areas that have received less rain. Where they are most likely to show up first is in fields where an earlier application of a synthetic pyrethroid insecticide was made. The pyrethroids are not effective on spider mites, but do kill the predators that keep the spider mites in check. An organophosphate insecticide, such as Lorsban, can reduce spider mite populations. A good article on spider mites appeared recently in the U of Wisconsin newsletter at http://ipcm.wisc.edu/WCMNews/tabid/53/EntryId/1224/Twospotted-Spider-Mite-Potential-on-Soybean-in-Dry-Areas.aspx.
FOR YOUR CALENDAR
ISU SE Iowa Research & Demonstration Farm Fall Field Day
Details will be posted soon.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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