August 4, 2005
temperatures will provide some relief for the crops, but unfortunately those
who needed the rain the most got little rainfall. Unless a major weather system
moves through the area soon to break the drought pattern, itís likely that the
driest areas will continue to be dry. Although corn pollinated well in many
fields, some kernel abortion has occurred causing tips of ears to be barren.
Northern corn rootworm beetle populations have also been very high in some
fields and silk clipping occurred prior to pollination further reducing yield
potential. Continued dry weather will take its toll on the soybean crop as
well. For those wanting to estimate corn yields, its just a matter of counting
Pull several ears of corn at random and count the number of rows of kernels and
the number of kernels per row. Ignore kernels less than half size at the tip of
the ear. Also measure off 1/1000 acre along one row in several places in the
field and count the number of ears:
1/1000 acre =††††††† 17 ft 5" in
14 ft 6" in 36"
13 ft 9" in 38" rows
(For 20" rows, count 2 rows 13' 1" long)
Number of kernels per 1/1000
acre = kernels per row X rows per ear X ears per 1/1000 acre.
Divide this number by about 90 to get bu/A (since
there are about 90,000 kernels per bushel). †If you think kernel size and weight will be
smaller than normal, divide by a larger number (100+).
Two Spotted Spidermites
Spider Mites can be
found in many fields in the area now, especially in the eastern part of the
area I cover. Spraying has been common in Jones
County and some spraying has occurred
in Linn and Johnson
Counties. The cooler
temperatures should help somewhat to reduce the problems with the pest. An
insecticide is recommended if spider mite damage is occurring in the field,
mites are present, and hot, dry conditions are expected to continue. Spider
mites are more common in areas where the soybeans are under stress. If damage
is localized along the edge of the field, and mites can not be found in the
rest of the field, it may not pay to spray the entire field. If mite damage is
localized but mites can be found in low numbers in the rest of the field,
spraying the entire field may pay off, especially if dry conditions persist.
Most have been spraying Lorsban for spider mites,
because of its better effectiveness on soybean aphids which are also present in
most fields. For more information see the July 22, 2002 ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2002/7-22-2002/spidermites.html
and the July 18, 2005 article at http://www.ipm.iastate.edu/ipm/icm/2005/7-18/spidermite.html.
Soybean aphid numbers
have increased above the economic threshold in some fields and many fields have
been sprayed in the past several days in Jones County.
Some spraying has also occurred in Linn and Benton Counties.
Aphid numbers usually peak in mid-late July and then have a second peak in
August. Numbers have declined in some fields in the past week even when they
are increasing in nearby fields. Every field is different, so fields need to be
monitored closely for at least a couple of more weeks. Usually aphid numbers
drop off rapidly sometime after mid-August. Treatment is recommended if aphid
numbers are increasing and surpass 250 aphids/plant.
The cooler temperatures will decrease moisture stress to the soybeans, but
unfortunately may result in greater soybean aphid reproduction. Under the right
conditions, populations can double in 2-3 days, so 250/plant can increase to
1000/plant in less than a week. For more information on soybean aphids see http://www.ent.iastate.edu/soybeanaphid/
and the July 11, 2005 ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2005/7-11/aphids.html.
Asian Soybean Rust
There have been more
finds in the far southern U.S. Almost all of the finds have been on sentinel
plots, research plots, or on kudzu. For the latest see http://sbrusa.net/.
Other Soybean Foliar Diseases
For some reason, downy
mildew, a disease favored by cool, damp conditions, has been common this year
in many soybean fields. Frogeye leafspot is also
showing up in some fields. Photos of common soybean diseases can be found at:
Soybean Cyst Nematode
With all of the
attention on aphids and rust, don't forget the soybean pest in Iowa that still causes
the most yield lost state-wide. Damage from the
pest is more evident in dry years. The July 25 ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2005/7-25/scn.html
provides photos and scouting information to help deal with this pest.
Spider mites are also
being found in some corn fields. An article in the July 22, 2002 ICM Newsletter
shows a table to calculate economic thresholds for spider mites in corn based
on work done in Nebraska and Texas. If a market value of $300 per acres
is assumed for the corn and a treatment cost of $10/A is assumed, a treatment
may pay if more than 20% of the leaves have 10% or more of their area covered
with mites. Insecticides labeled for mites in corn include dimethoate
and Capture. See the article at http://www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/nospidermites.html.
numbers continue to be high in many fields, so scouting of hay fields continues
to be an important activity. For 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
. A couple of sources for sweep nets are http://www.gemplers.com/a/shop/product.asp?T1=R13101&src=21SM001