Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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June 29, 2007

July 2, 2007




To Spray a Fungicide or Not to Spray a Fungicide


There is a great deal of interest in spraying fungicides on corn this year.  There are several issues to remember and consider before “pulling the trigger.”


First, remember the disease triangle, which states that in order for a disease to develop, three things must occur simultaneously:


1.     There must be a susceptible host.

2.     The disease pathogen must be present.

3.     The weather must be conducive for the pathogen to successfully infect the host.


Therefore, ask yourself the following questions:


1.     Does the hybrid have a poor or mediocre foliar disease resistance package?

2.     Is there reason to believe the pathogen is present, such as corn following corn with a great deal of residue on the surface or, in the case of the rusts, that it has blown in from the south?

3.     Is the weather and weather forecast / outlook conducive for the infection?  Most diseases like wet weather, with gray leaf spot and southern corn rust liking hot and humid weather while most of the other diseases like cool and wet weather.


The fewer “yeses” you have to the above questions, the lower is your likelihood of getting your money back.


Second, remember that most fungicides are effective for only 14 – 21 days.  There are 55 – 60 days between pollination and crop maturity.  Applying the fungicide too early or too late is a waste of money.  In 2005, generally the early fungicide applications were most beneficial (eastern Iowa excluded because of the drought) while in 2006, the later applications were most beneficial.  This difference was because of the different weather patterns in the two years.  So what is the best timing?  Remember that the goal is to protect the ear leaf and all leaves above the ear leaf.  Applying before the tassels are fully emerged means that the top leaves have not fully emerged and they will not be protected.  Once tassels are fully emerged, scout the field closely for evidence of leaf diseases, examining leaves from the top of the plants to the bottom of the plants.  If lesions are found on or above the ear leaf, or if lesions are found on several (five?) leaves below the ear leaf, the timing is probably about right for an application.


Emerson Nafziger, University of Illinois, wrote an excellent article on pages 117 – 118 of the June 29, 2007 Illinois Pest Management and Crop Development Bulletin, Alison Robertson, Daren Mueller, Carol Pilcher, and Kristine Schaefer wrote the first of two article on the subject on page 197 of the June 25, 2007 Iowa State University Integrated Crop Management Newsletter  The second article should be in the July 2, 2007 Integrated Crop Management Newsletter and posted at soon.



Japanese Beetles


Huge numbers of Japanese beetles have been reported in Illinois this year. It was reported that a total of 71,984 Japanese beetles were caught in a single trap in a 24-hour period in Massac county southern Illinois on June 20-21. The beetles have mainly been a problem near urban areas. The beetles will feed on soybeans and corn (as well as hundreds of other species of plants), but the damage to soybeans usually isn’t sufficient to pay for an insecticide treatment. A general threshold for soybeans is to consider an insecticide if there is 20% leaf defoliation during the reproductive stages. Most people tend to greatly over estimate percent defoliation. The pictures in the following ICM article can help in estimating leaf defoliation:


The beetles can cause a substantial yield loss in corn, since they often feed on the silks, so corn fields need to be watched closely in the next few weeks in the areas where Japanese beetles have been observed, such as eastern Clinton, Jackson, Muscatine, and Scott Counties.  An insecticide should be considered if the beetles are keeping the silks clipped to within 0.5” of the ear. In past years, much of the silk clipping was done by the beetles after pollination was complete. It seems like the beetles are emerging earlier than in the past, which means any silk clipping is more likely to affect pollination. The beetles will continue to emerge for several weeks and can live for 30-45 days, so they can continue to cause problems into August. For more information including pictures of the beetles, see the August 19, 2002 ICM Newsletter at





Cupped Soybean Leaves


I have been getting a number of calls related to soybeans that have cupped up or have malformed leaves that look like dicamba or growth regulator herbicide injury. When this problem occurs, the possibility of spray drift needs to be investigated. If an herbicide has drifted from a nearby corn field or roadside, the symptoms should be greatest near the source of the drift and gradually diminish with a greater distance from the source. Also the symptoms should be less near anything that would have obstructed the drift, such as tall grass or trees. If a drift pattern is not evident, then it is unlikely to be the reason for the symptoms.

 If the symptoms appear after a field is sprayed and there is a sprayer pattern to the symptoms, the possibility of sprayer contamination with dicamba (Banvel, Clarity, Northstar, Distinct) or another growth regulator herbicide should be investigated. However, we do occasionally see these symptoms in the absence of a growth regulator herbicide. Occasionally additives, such as ammonium sulfate (AMS), 28% Nitrogen solution, or surfactants can cause these symptoms. I have also seen symptoms not show up until 2 or more weeks after the field is sprayed. If dicamba was the source of the problem, symptoms should show up within a day or two.

In some cases the entire field showed the problem before anything was sprayed on the field or in neighboring fields. Sometimes the symptoms are uniform across the entire field and sometimes certain parts of the field are worse than others, but there is no drift pattern. I see this most often when the soybeans go through a growth spurt when temperatures are high after some cool weather or after a heavy rainfall. Apparently under these conditions, the balance of naturally occurring hormones in the plant is disrupted, resulting in symptoms characteristic of growth regulator herbicide damage. Usually the soybeans recover from this condition with little to no effect on the final yield. See the July 19, 1999 ICM Newsletter for more information (


Phytopthora Root Rot


Some phytopthora root rot is being observed in some fields.  South Dakota State University has a publication on phytopthora root rot at 


Asian Soybean Rust


Soybean rust has been detected on sentinel plots in central Louisiana, which is the furthest north so far this year.  This is near the area which was the source of the inoculum for the northward movement of the disease last September, so we need to keep an eye on the south. See for the latest info.


Soybean Aphids


Soybean aphids are starting to appear in some soybean fields north of I80 in very low numbers. At these low levels the beneficial insects can keep the problem in check. Unnecessary insecticide applications can contribute to aphid outbreaks by killing off the beneficial insects. The economic threshold for soybean aphids is 250 aphids per plant.





Midwest Strip Till Conference – July 31, 2007

9 a.m. to 3:00 p.m.



Organized by Research and Extension of Iowa State University, the University of Minnesota, the University of Wisconsin, and Hawkeye Community College. Manufacturers will demonstrate equipment for strip-tillage and associated operations, including auto-guidance systems and fertilizer injectors.  Researchers, farmers, and industry representatives will present the latest information on strip-tillage related topics, including equipment selection, fertility management, and guidance technology. Participants will review information booths all day, and lunch is available on site.  This program is free and open to the public. Five Certified Crop Advisor CEUs (4.5 SW & 0.5 NM) will be available for a nominal fee.  Expo details are at:


If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: July 2, 2007
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