August 23, 2011
The recent cooler weather has helped to reduce losses in potential yields, but the hot weather during and after pollination has resulted in a lot of tipping back of the grain on corn ears and also has caused some pollination problems. I've also seen zipper ears where several rows of kernels aborted along one side of the ear due to heat stress and/or N deficiency stress. Yields will likely be extremely variable across the area, with some fields yielding 200 bu. / A. or better and others yielding half of that. Some corn fields are already dead or near death in the SE, which is now officially in a moderate drought. Some areas have received less than and inch of rain since late June, and this followed an extremely wet spring, which would have resulted in poor root growth.
The heat did not slow down gray leaf spot, which is very common in most fields, especially on the lower leaves. It does well with warm and humid conditions. Fungicide applications did help to prevent the disease from progressing into the upper canopy, which should help to protect the yield. Now that most fields are dented, it is a good time to evaluate the effectiveness of the fungicides, or to determine whether a fungicide would have likely paid for itself if it had been sprayed. Research across the Midwest has shown that if you have less than 5% of the ear leaf covered by disease lesions at the dent stage, the fungicide has worked well in preventing most yield losses, or if a fungicide was not applied it would not likely have been economical to have been applied. If a fungicide was not applied and the ear leaf has much greater than a 5% incidence of disease lesions at the dent stage, it would have likely paid to apply the fungicide. The image below can help in estimating what 5% looks like with gray leaf spot lesions.
I believe that the heat did reduce the incidence of eyespot and northern corn leaf blight, which are not very widespread this year.
Goss's Wilt Found in More Eastern Iowa Fields
Fungicide applications do 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.
The best defense against Goss's Wilt is to use resistance varieties. Some varieties are very sensitive and others very resistant. Portions of fields are now dead where there is severe Goss's Wilt. See Alison Robertson's article about the survival of the bacterium in soil, residue, silage, and bedding at http://www.extension.iastate.edu/CropNews/2011/0822robertson.htm.
Corn aphid populations have exploded in NW Iowa, but I have not seen any major problems in this area. I have seen fairly low aphid numbers on the edges of some corn fields, especially on the ear husks. See Erin Hodges article on some thoughts about scouting for and managing this pest at http://www.extension.iastate.edu/CropNews/2011/0820hodgson.htm.
There have been a few reports of root feeding and root lodging in fields with the Bt rootworm trait, especially in NE Iowa. Aaron Gassman recently published an article confirming the development of rootworms in the field in Iowa that are partially resistant to the Cry 3Bb rootworm trait.
August is usually a critical month for soybeans, and unfortunately most of the area did not receive the rains in August needed to get top soybean yields. At least the cooler weather should have reduced stresses and some did receive a good shower today which hopefully came in time to improve yield prospects. I think we should still get decent yields because we had so little problems with insects and diseases this year in the soybean crop. Although sudden death syndrome is showing up now in some soybean fields, it is much less widespread than last year, and also should have less impact on soybean yields since it is showing up later in the season.
FOR YOUR CALENDAR
Northeast ISU Research Farm Field Day, Nashua
August 31, 2011, 1:00 p.m.
Program runs from 1:00 to 4:30 p.m. and features 5 speakers. Ken Pecinovsky, ISU Research Farm Superintendant will review crop progress. Charles Hurburgh, Agricultural and Biosystems Engineer, will talk about grain storage and grain quality management. Aaron Gassmann, Entomologist, will cover corn rootworm management and Bt corn rootworm traits. Antonio Mallarino, Professor of Agronomy in soil fertility and nutrient management, will give insight to soil testing interpretations and plant analysis, micronutrients and accuracy of micronutrient soil tests. Bob Hartzler, Extension Weed Management Specialist, will talk about fall and early spring weed control. The field day is free and open to the public. It starts at the Borlaug Learning Center on the Northeast ISU Research Farm near Nashua. Directions: From Nashua at the Jct of Hwy 218 (Exit 220) and Co. Rd. B60, go west on B60 1.1 miles to Windfall Ave., then south 1 mile to 290th St., then east 0.2 miles to the farm. For more information about the event, call Terry Basol at 641-435-4864.
ISU SE Iowa Research Farm Fall Field Day
September 8, 2011, 1:30 p.m.-3:30 p.m.
Featuring Elwynn Taylor on How Heat & Drought Will Impact Corn Yields
Other Stops Will Include:
Crop Season Review by Kevin Van Dee, Farm Superintendent
Advances in Breeding Drought Tolerant Corn (Featuring Pioneer's New Aqua Max Hybrids) By Kendall Lampkey, Agronomy Dept. Head
Soil Sampling and Making Sense of Soil Test Reports By Antonio Mallarino, ISU Agronomist Soil Fertility
CCA Credits Available for a Fee
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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