July 2, 2007
To Spray a Fungicide or Not to Spray a Fungicide
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.”
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
2. The disease pathogen must be
3. The weather must be
conducive for the pathogen to successfully infect the host.
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.
fewer “yeses” you have to the above questions, the lower is your likelihood of
getting your money back.
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.
of Illinois, wrote an
excellent article on pages 117 – 118 of the June 29, 2007 Illinois Pest
Management and Crop Development Bulletin, http://www.ipm.uiuc.edu/bulletin/article.php?id=793.
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 http://www.ipm.iastate.edu/ipm/icm/2007/6-25/fungicides.html. The second article should be in the July 2,
2007 Integrated Crop Management Newsletter and posted at http://www.ipm.iastate.edu/ipm/icm/
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: http://www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/soydefoliation.html.
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,
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 http://www.ipm.iastate.edu/ipm/icm/2002/8-19-2002/jbeetles.html.
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 (http://www.ipm.iastate.edu/ipm/icm/1999/7-19-1999/malsoy.html).
Phytopthora Root Rot
root rot is being observed in some fields.
has a publication on phytopthora root rot at http://agbiopubs.sdstate.edu/articles/FS902B.pdf.
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.
for the latest info.
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
FOR YOUR CALENDAR
Midwest Strip Till Conference – July
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: http://wrc.umn.edu/outreach/striptillageexpo/midwest/index.html