August 25, 2005
SOYBEAN
Two Spotted Spidermites
Spider mite damage is
evident in many field across the area, but in most
cases the mite population is gone or diminished with the wetter, cooler
conditions. It is getting too late to spray most products anyway, as harvest
could begin within a couple of weeks.
Soybean Aphids
Insecticides seemed to
have been very effective in fields that I have looked at that were sprayed for
aphids. Populations are also dropping in most unsprayed fields.
Asian Soybean Rust
On August 22, soybean
rust was confirmed in a commercial field in Appling Co., GA, the
tenth Co. in Georgia
with a positive find. The first report from South Carolina in 2005 was in a commercial
soybean field in Hampton Co. on the SC/GA border and is adjacent to Effingham
Co., GA that had a previous positive find. Soybean fields in
SE and EC Iowa are now or will be soon in the R6 stage, so beyond the window
for treating for soybean rust, so looks like we made it through the
season. For soybean rust updates see http://sbrusa.net/.
Frogeye Leaf Spot
Frogeye leafspot is more common this year than what I have seen
before. Photos of the disease, and management tips are provided at the
following site:
http://www.ipm.iastate.edu/ipm/icm/2001/12-24-2001/frogeye.html.
Sudden Death Syndrome
Sudden death syndrome
is showing up again this year, but appears to be very patchy and not killing
very large areas in most fields where it is being found.
CORN
Western Bean Cutworm Damage
It is becoming
apparent that western bean cutworm damage is more widespread this year than
what we have seen in the past. Along with the drought conditions, the increased
insect damage could lead to more ear rots this fall, and increase the
importance of timely harvest and proper drying. The northern part of the area
(Benton, Linn, and Jones counties) had the large moth flights in July, and that
is where a lot of the damage is being found. It is not uncommon to find several
larva per ear. To manage the pest in the future,
hybrids resistant to the pest need to be selected or fields need to be scouted
in July and treated if necessary. For information on the western bean cutworm see
the July 19, 2004 issue of the ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2004/7-19-2004/wbc.html.
Ear Rots and Mycotoxins
It is likely that we will see
more ear rots this fall with the drought conditiond. Alfatoxin is created by a yellow-green mold called Aspergillus flavus,
which might already be observable in a field or which may become observable any
time before harvest. However, the presence of the mold does not
necessarily mean that aflatoxin is present. In
addition, Aspergillus flavus
may be working on the inside of the kernels with no outward signs. Iowa State
University publication Pm-1800 Aflatoxins in Corn will answer many of your
questions. In addition, be sure to watch for other ear molds as described
on pages
158 - 160 of the October 6, 2003 (Fall 2003 Special Edition Drought Issue) of
the ISU Integrated Crop Management Newsletter. Early harvest and proper
drying can help reduce the impact of the rots.
Harvesting Corn Silage
Harvesting corn for silage
has been occurring for the past week or more in the area. The milk line is
often used to estimate whole-plant harvest moisture for corn silage production.
The whole plant reaches 100% accumulation of digestible nutrients at 1/4 milk
line. Ensiling at this stage through 3/4 milk line maximizes yield of
digestible forage. Various storage structures suggest a
different harvest moisture to optimize the fermentation process of
ensiling. The following table provides some common ranges of harvest moisture
for different structures.
|
Structure
|
Harvest moisture
|
approx. milk line
|
|
Bunker or Bag
|
67 - 70%
|
1/4
|
|
Oxygen limiting silo
|
50 - 60%
|
1/4 - 1/2
|
|
Upright silo
|
63 - 67%
|
3/4 - initial black layer
|
However, the milk line is only designed to be an indicator of when to sample
some forage and test for its actual moisture content.
Drought-damaged corn silage:
In general, drought-stressed corn
silage quality is about 90% of normal silage. The main problem is reduced
yield. The following resources may be useful:
1) Handling Drought-Stressed Corn
http://corn.agronomy.wisc.edu/WCM/2003/W138.htm
2) NCH 58, Utilizing Drought-Damaged Corn
http://www.ces.purdue.edu/extmedia/NCH/NCH-58.html
An additional concern could be high nitrate levels. The following
link discusses nitrate concerns and testing.
http://www.extension.iastate.edu/ag/drought/NitrateToxicity.pdf
ALFALFA
Fall Harvest Management
To reduce winterkill
it is best to give alfalfa a 5-6 week rest period between the
last harvest and the first killing freeze (24F for stopping alfalfa growth).
These temperatures generally occur during late October in much of the area.
Cutting alfalfa from mid-September to mid-October increases the risk of
winter-injury. After mid-October, or a hard freeze, taking a final cutting can
be done without much increased risk of winterkill.
FOR YOUR CALENDAR
Crawfordsville Fall Field Day - September 14 1:30 p.m.
The annual fall field
day will be held at the SE Iowa Research Farm near Crawfordsville on September
14 at 1:30 p.m. Alison Robertson and X.B. Yang will be featured at the tour.
They will present information from this year's fungicide trials and also will
discuss alfatoxin and other mycotoxin
concerns in corn. Mike Duffy, ISU Economist, will compare the economics of
continuous versus rotated corn. Also featured will be Jim Jensen, on Low Sat And Low Lin Soybean Opportunities, and Kevin VanDee on Strip Intercropping.
Nashua Fall Field Day - September 8 1:30 p.m.
The afternoon program
will feature topics on both crop production and grain marketing, and will give
you an opportunity to meet the new ISU corn specialist, Roger Elmore.
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