June 28, 2010
Hail has danced across the area, varying from none to light to severe.
To assess injury to corn and options to consider, see http://www.extension.iastate.edu/Pages/eccrops/hailcorn.html.
To assess Injury to soybean and options to consider, see http://www.extension.iastate.edu/Pages/eccrops/hailsoy.html.
As corn fields begin to recover, many are noticing whorls tied up in themselves. The vast majority of those plants will recover and become normal plants.
Roger Elmore has two more detailed articles in the ICM News at http://www.extension.iastate.edu/CropNews/2008/0607LoriAbendrothRogerElmore.htm and http://www.extension.iastate.edu/CropNews/2009/061901.htm.
Lodging and Green Snap / Brittle Snap
We have also had lodging and some green snap / brittle snap of corn during the high winds of some of the storms of the last several days. Fast growing corn is most likely to green snap at growth stages V10 tasseling. The wind speed coupled with the growth stage of the corn is the most important factor contributing to green snap. There are some genetic differences in vulnerability to green snap. And an application of a dicamba product, such as Banvel or Clarity, can make the corn more brittle. An oldie-but-goodie article about research on green snap is at the ICM News site http://www.ipm.iastate.edu/ipm/icm/1998/7-13-1998/greensnap.html.
Yield loss from lodging is less than 5% for corn that is V10-V12, 5-15% for corn that is V12-V15, and up to 30% if the corn is V17 or larger; see page 77 of http://www.agronext.iastate.edu/corn/docs/corn-field-guide.pdf.
Nitrogen Deficiency Symptoms
There has likely been a lot of nitrogen lost again this spring with the excess rainfall. One thing to keep in mind that not all of the nitrogen has been lost even where the corn is very stunted and yellow in areas that have had saturated soils. Roots cannot grow into water, so plants do not do well with continuous saturation of the soil. Some of the yellowing is just from wet feet. A lot of soybean fields are showing this yellowing as well. However, much of the nitrate nitrogen will have been lost by de-nitrification in soils that have been saturated for an extended period of time. Additional nitrogen will likely pay off as long as the corn stand is still there and there is still potential for getting a good yield. An additional 50-60 lb/A of N should be considered on fields that are showing N deficiency symptoms and in fields that have had an extended period of soil saturation. The sooner it is applied the greater the yield response will likely be, but it can pay even up through tasseling. See John Sawyers article for more information at http://www.extension.iastate.edu/CropNews/2010/0614sawyer02.htm.
Potassium Deficiency Symptoms
are again seeing corn fields with areas in the fields that are stunted with the lower
leaves yellowing and browning along the margins. This is a symptom of K deficiency, but is
often due to poor root function rather than a shortage of K in the soil. The end rows are
often better than the rest of the field, probably due to the different soil structure
where the traffic and/or tillage pattern has been different. Usually the fields look fine
until the corn gets to be about calf high, then corn in areas of the field stop growing
and the lower leaves turn yellow. Good corn can be right next to extremely stunted corn
with no apparent reason for it. The corn in these areas remain stunted and the lower
leaves remain yellow, but they usually yield better than expected.
Anything that restricts root growth during the initiation of the nodal root system can lead to the problem. The nodal roots emerge within 0.75 1.0 inch of the soil surface. If there is something in the surface inch or so that the roots don't "like" they don't function properly. There can be large differences among hybrids in showing this phenomenon. Shallow planting and/or soil settling or eroding after planting aggravates the problem. This year, often there is evidence the soil was tilled and/or planted when the soil was just a little too wet. In many fields it is difficult to come up with an explanation why the problem is appearing. It is most common in no-till fields, but shows up in tilled fields as well. In tilled fields, it can show up where the soil is fairly "fluffy", especially under dry conditions. Since the end rows usually look better, it could be that a little surface compaction actually helps to alleviate the problem.
If the soil hasn't been tested recently, soil samples should be taken to make sure it is not a true K deficiency problem. Soils that are low or marginal in K are more likely to show the problem. There is nothing that can be done when the problem appears. An excellent discussion of this problem in both corn and soybean is on pages 123 124 of the June 20, 2005 Integrated Crop Management (ICM) Newsletter or at http://www.ipm.iastate.edu/ipm/icm/2005/6-20/potassium.html.
Year-in and year-out, the best time to apply a fungicide to corn, if needed, is from tasseling through silking (Vt-R1). However, it has been our experience that in years when tasseling occurs unusually early, as is likely to be the case this year, it is often most profitable to delay the application until the blister (R2) stage of development in mid-to-late July. To make the decision about the need to apply a fungicide, Alison Robertson, ISU Extension Plant Pathologist, suggests:
1. If the corn has good genetic resistance, dont worry, be happy.
2. If genetic resistance is moderate or poor, then just prior to spraying time,
a. go to several locations in each hybrid in each field and
b. inspect several plants at each location.
i. Check the ear leaf and the next three leaves down.
c. If the hybrid has poor resistance and if you find any lesions on the leaves in question, have a fungicide applied.
d. If the hybrid has moderate resistance, also consider
i. Did you find a few lesions or many?
ii. Is the weather forecast conducive to further development of the disease?
We have now had three wet springs in a row. Hopefully summer will bring some drier conditions (but not too dry). The persistent wet weather has kept field work to a minimum which means fields are getting pretty weedy. Be sure to check labels regarding maximum corn height when applying herbicides. Drop nozzles will likely be needed in many fields to get good coverage and reduce the injury risk to corn. It is getting hard to see the soybean rows in many fields, which illustrates the value of using a soil applied herbicide in a Roundup Ready system. See Bob Hartzlers article if youd like to know how much yield we are losing with the weedy fields at http://www.extension.iastate.edu/CropNews/2010/0615hartzler.htm.
White Mold Prevention
I have received many questions regarding the chance of a major white mold outbreak and the wisdom of applying Cobra or Phoenix to soybeans. While mold flourishes in fields with a closed canopy with a history of white mold during cool, wet springs and with highly susceptible soybeans. So if the field is planted to a variety with lower susceptibility, that lowers the likelihood of problems in 2010. The recent high temperatures lower the risk for 2010. And if the canopy is still fairly open, that lowers the 2010 risk. Finally, if there in not much of a history of white mold in the field, then there will not be much inoculum present, lowering the risk for 2010.
Work in 2009 by Carl Bradley of the University of Illinois suggests that Cobra is a very viable option in suppressing white mold, and Phoenix has the same active ingredient. Work by X.B. Yang, ISU Extension Plant Pathologist, in the mid-90s suggests that 2 4 ounces per acre of Cobra will provide most of the potential benefit. Most likely the same is true of Phoenix applied at the lower end of the range of rates listed on the label.
The best time to apply a soybean fungicide is at beginning pod set (R3), so we still have about a month before we should consider making a fungicide application to soybean. Some fields now have a fairly high level of brown spot on the leaves; this will provide inoculum for brown spot to recur later in the season, so fields with brown spot now should be watched especially closely as those fields approach R3 to determine the need / lack of need for a fungicide application at that time.
As producers are making the last (hopefully) herbicide application, some are again considering including an insecticide with the herbicide just in case So far bean leaf beetle activity has been miniscule and there is no evidence of soybean aphids moving from field to field. If you look long enough, you will probably find a colony or two of aphids, but predators should take them out. So the application of an insecticide at this time will most likely only kill the predators, leaving the soybeans unprotected a few days later so that even a small infestation later in the season can quickly explode. In addition, unless a contact herbicide is being used, the coverage will probably not be good enough that an included insecticide would kill soybean aphids anyhow.
High numbers of potato leafhoppers can be found in some hay fields. Be sure to also use a sweep net to monitor potato leafhopper numbers and treat if numbers exceed the threshold. For more 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. Remember, waiting to see hopperburn is waiting too long as substantial losses have already occurred by that time.
Lepto Leafspot and Common Leafspot
Lepto Leafspot and Common Leafspot are showing up in many alfalfa fields. In general, there is nothing that can be done except to harvest early to salvage as much leaf material as possible.
For more information on Lepto Leafspot, see the following sites at The Ohio State University:
For more information on Common Leafspot, see:
FOR YOUR CALENDAR
Muscatine Island Research Farm Field Day and 75th Anniversary, Fruitland
The Muscatine Island Research and Demonstration Farm was founded 75 years ago. A special field day will include many special events in addition to the traditional field day. If you have an interest in horticulture, be sure to attend. Information is at http://www.extension.iastate.edu/Pages/eccrops/meetmusc.html.
Northeast ISU Research Farm Field Day, Nashua
June 30, 1:00-4:00
Field day speakers include: Ken Pecinovsky, Farm Superintendent, Robert Hartzler, ISU Extension Weed Scientist, Alison Robertson, ISU Extension Plant Pathologist, John Sawyer, ISU Extension Soil Fertility Specialist, Chad Ingels, ISU Extension Program Specialist, and Brian Lang, ISU Extension Agronomist. CCA Credits available for a fee. More details are at http://www.extension.iastate.edu/Pages/eccrops/meetnerf.html.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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