Effect of Planting Date and Seed
Treatment on Diseases and Insect Pests of Wheat
Dr. Bob Hunger, Extension Wheat
Pathologist (1)
Dr. Jeff Edwards, Small Grains
Extension Specialist (2)
Dr. Tom Royer, Extension
Entomologist (1)
Mr. Terry Pitts, SW Area Extension
Specialist – IPM (1)
Departments of (1) Entomology &
Plant Pathology, and (2) Plant and Soil Sciences
Oklahoma State
University
Approximately 40 to
60% of the winter wheat in Oklahoma is sown with the intent of being used as a
dual-purpose crop. In this system wheat is grazed by cattle from late October to
early March and harvested for grain in early summer. In a grain-only
system, wheat is generally planted in October, but in a dual-purpose system
wheat is planted in early to mid-September to maximize forage production.
Planting wheat this early significantly increases the likelihood that diseases
such as wheat streak mosaic virus, high plains virus, the aphid/barley yellow
dwarf virus complex, and root and foot rots will be more prevalent and more
severe.
Wheat streak mosaic
virus (WSMV), the high plains virus (HPV), and Triticum mosaic virus
(TrMV): WSMV and HPV
are transmitted by the wheat curl mite (WCM). Within the last 2-3 years,
Dr. Dallas Siefers with Kansas State University at Hays, KS identified a third
virus, TrMV that also is transmitted by the WCM. TrMV causes the
expression of symptoms similar to those caused by WSMV and
HPV.
WCMs and these
viruses survive in crops such as wheat and corn, as well as many grassy weeds
and volunteer wheat. In the fall, WCMs spread to emerging seedling wheat,
feed on that seedling wheat, and transmit the virus to the young wheat
plants. Wheat infected with WSMV, HPV, or TrMV in the fall is either
killed by the next spring or will be severely damaged. No seed
treatments are effective in controlling these viruses. However,
planting later in the fall (after October 1 in northern OK and after October 15
in southern OK) and controlling volunteer wheat are two practices that provide
some control. It is critical to completely destroy volunteer wheat at
least two weeks prior to emergence of seedling wheat because WCMs have a
life span of 7-10 days. Thus, destroying volunteer wheat at least two
weeks prior to emergence of seedling wheat should greatly reduce mite numbers in
the fall. In addition to these cultural controls, two winter wheat
varieties (RonL from Kansas and Mace from Nebraska) now have resistance to WSMV;
however, their adaptation to production in Oklahoma is not known. For more
information on WSMV and HPV, see OSU Extension Facts 7636 or go to the Plant
Disease & Insect Diagnostic Laboratory web page at: http://www.ento.okstate.edu/ddd/hosts/wheat.htm.
Aphid/barley yellow dwarf virus
(BYDV) complex: BYDV is transmitted by many
cereal-feeding aphids. Fall infections by BYDV are the most severe because
the virus has a longer time to damage the plant as compared to infections that
occur in the spring.
Several steps can be taken to help
control BYDV. First, a later planting date (after October 1 in
northern Oklahoma and after October 15 in southern Oklahoma) helps to reduce the
opportunity for fall infections. Second, some wheat varieties (e.g.,
2174, Duster, Endurance, Overley, Everest) seem to tolerate BYDV better than
other varieties; however, be aware that no wheat variety has absolute resistance
to the aphid/BYDV complex. Third, control the aphids that transmit
BYDV. This can be done by applying contact insecticides to kill aphids, or
by treating seed before planting with a systemic insecticide.
Unfortunately, by the time contact insecticides are applied, aphids frequently
have already transmitted BYDV. Systemic seed-treatment insecticides
including Gaucho (imidacloprid) and Cruiser (thiamethoxam) can control aphids
during the fall after planting, but in some years aphids are sparse in the fall
and planting insecticide-treated seed in a year with no or sparse aphids in the
fall would not be as beneficial as in years when aphids are numerous. Be
sure to thoroughly read the label before applying any chemical. For more
information on the aphid/barley yellow dwarf virus complex, go to the web page
for the Plant Disease and Insect Diagnostic Laboratory at: http://www.ento.okstate.edu/ddd/hosts/wheat.htm.
Hessian fly: Hessian fly infestations
occur in the fall and spring. Fall infestations arise from over-summering
pupae that emerge when climate conditions become favorable. Delayed
planting (after October
1 in northern Oklahoma, and after October 15 in southern Oklahoma) can help
reduce the threat of Hessian fly, but a specific “fly free date” does not exist
for most of Oklahoma as it does in Kansas and more northern wheat-growing
states. This is because smaller, supplementary broods of adult flies
emerge throughout the fall and winter. Some wheat varieties are either
resistant (e.g. Duster and Centerfield) or partially resistant (e.g. Hatcher,
Shocker, 2145, 2174, Chisholm, Ike, OK 102 and
Okfield) to Hessian fly infestations.
Hessian fly infestations can be reduced somewhat by destroying volunteer
wheat in and around the field at least two weeks prior to emergence of seedling
wheat. Seed treatments that contain imidacloprid or thiamethoxam will also
help reduce fly fall infestations, especially if combined with delayed planting
and volunteer destruction.
Root and foot
rots: These include several
diseases caused by fungi such as dryland (Fusarium) root rot, Rhizoctonia root
rot (sharp eyespot), common root rot, take-all, and eyespot
(strawbreaker). Controlling root and foot rots is difficult. There
are no resistant varieties, and although fungicide seed treatments with activity
toward the root and foot rots are available, their activity usually involves
early-season control or suppression rather than control at a consistently high
level throughout the season. Often, there also are different “levels” of
activity related to different treatment rates, so again, CAREFULLY read the
label of any seed treatment to be sure activity against the diseases and/or
insects of concern are indicated, and be certain that the seed treatment(s) is
being used at the rate indicated on the label for activity against those
diseases and/or insects.
Later planting (after October 1 in
northern Oklahoma and after October 15 in southern Oklahoma) also can help
reduce the incidence and severity of root rots, but planting later will not
entirely eliminate the presence or effects of root rots. If you have a
field with a history of severe root rot, consider planting that field as late as
possible or plan to use it in a “graze-out” fashion if that is consistent with
your overall plan.
For some root rots, there are
specific factors that contribute to disease incidence and severity. For
example, a high soil pH (>6.5) greatly favors disease development of the root
rot called take-all. OSU soil test recommendations factor in this
phenomenon by reducing lime recommendations when continuous wheat is the
intended crop. Another practice that can help limit take-all and some of the
other root rots is the elimination of residue. However, elimination of
residue by tillage or burning does not seem to affect the incidence or severity
of eyespot (strawbreaker). For more information on wheat root rots,
take-all and eyespot (strawbreaker), see OSU Extension Facts F-7622 or go to the
web page for the Plant Disease and Insect Diagnostic Laboratory at: http://www.ento.okstate.edu/ddd/hosts/wheat.htm.
Seed
treatments: There are several reasons to
consider planting treated seed including:
1.
Control of common bunt (also called
stinking smut) and loose smut. The similarity of these names can be
confusing. Both affect the grain of wheat, but whereas common bunt spores
carryover on seed or in the soil, loose smut carries over in the
seed. Seed treatments are highly effective in controlling both
diseases. If common bunt was observed in a field and that field is to be
planted again with wheat, then planting certified wheat seed treated with a
fungicide effective against common bunt is strongly recommended. If either
common bunt or loose smut was observed in a field, grain harvested from that
field should not be used as seed the next year. However, if grain
harvested from such a field is to be used as seed wheat, treatment of that seed
at a high rate of a systemic or a systemic + contact seed treatment effective
against common bunt and loose smut is strongly recommended. For more information on common bunt & loose smut, see:
http://www.entoplp.okstate.edu/ddd/hosts/wheat.htm, consult the “2010
OSU Extension Agents’ Handbook of Insect, Plant Disease, and Weed Control (OCES
publication E-832),” and/or contact your County Extension
Educator.
2.
Enhance seedling emergence, stand
establishment and forage production by suppressing root, crown and foot
rots. This was discussed above under “Root and Foot Rots.” Refer to
Table 1 for a more detailed description.
3.
Early season control of the
aphid/BYDV complex and Hessian fly. This can be achieved by using a seed
treatment containing an insecticide. Refer to Table 1 for a more detailed
description of seed treatments with insecticidal activity.
4.
Control fall foliar diseases
including leaf rust and powdery mildew. Seed treatments are effective in
controlling foliar diseases (especially leaf rust and powdery mildew) in the
fall, which may reduce the inoculum level of these diseases in the spring.
However, this control should be viewed as an added benefit and not necessarily
as a sole reason to use a seed treatment.
5.
Partial control of Hessian
fly. This was also discussed previously, see Table 1.
Often a combination of chemicals is
present in seed treatments, which can include a combination of fungicides for a
broader spectrum of activity, or a combination of fungicides with an insecticide
so activity against diseases and insects is achieved. Examples of this
last type of compound include CruiserMaxx, Gaucho XT, and Rancona Crest, which
contain an insecticide and fungicides so control and/or suppression of aphids
(and hence BYDV), Hessian fly, wireworms, smuts and bunts, and seedling root
rots is available in one treatment (Table 1). Other seed treatments such
as Raxil MD, Dividend Extreme, Charter PB, and Charter F2 contain
only fungicides, but can easily be mixed with an insecticide such as Gaucho 600
or Cruiser to obtain activity against bunts, smuts, seedling root rots and
insects as well. Therefore again, I would emphasize that if a seed treatment is used, be sure to carefully read
the label to ensure that the treatment is intended (and labeled) for your
desired goal, and that it is applied at a rate labeled for the desired
activity. For more information on seed treatments, their intended
uses and rates consult the “2010 OSU Extension Agents’ Handbook of Insect, Plant
Disease, and Weed Control (OCES publication E-832),” and/or contact your County
Extension Educator.
Table 1. Common
Seed Treatments for Use against Wheat Diseases and Insect
Pests This
information is provided only as a guide. It is the
responsibility of the pesticide applicator by law to read and follow
all current label directions. No endorsement is intended for
products listed, nor is criticism meant for products not
listed. NOTE: Many seed treatments have a required
post-planting interval before grazing is allowed; check the
label! Product
&
Active
Rate
Activity against
diseases/pestsA
(company)
ingredients
(oz/cwt)
Ins/BYDV
Sm/Bu
RR
DO
FFol CruiserMaxx........................
thiamethoxam
5.0
AB
A
A
A
A (Syngenta)
difenoconazole
mefenoxam Cruiser
5FS...........................
thiamethoxam
0.75-1.33
A
NAB
NA
NA
NA (Syngenta) Dividend
Extreme...............
difenoconazole
1.0-4.0C
NA
A
A
A
A (Syngenta)
mefenoxam Gaucho 600.........................
imidacloprid
0.8-2.4
A
NA
NA
NA
NA (Bayer CropScience) Gaucho XT............................
imidacloprid
3.4
A
A
A
A
A (Bayer
CropScience)
metalaxyl
tebuconazole Raxil
MD...............................
tebuconazole
5.0-6.5
NA
A
A
A
A (Bayer CropScience)
metalaxyl Rancona
Crest.....................
imidacloprid
5.0-8.33
A
A
A
A
A (Chemtura)
ipconazole
metaxyl
The
following alone or in various combinations (all are BASF
products): Charter.................................
triticonazole
3.1
NA
A
A
NA
A Charter
PB............................
triticonazole
5.5
NA
A
A
A
A
+ thiram Charter
F2.............................
triticonazole
5.4
NA
A
A
A
A
+ metalaxyl Stamina
F3
HL
pyraclostrobin
1.0
NA
A
A
A
A
+ triticonazole
+ metalaxyl Axcess...................................
imidacloprid
0.8-2.4
A
NA
NA
NA
NA Acquire.................................
metalaxyl
0.1-0.375
NA
NA
A
NA Stamina.................................
pyraclostrobin
0.4-0.8
NA
NA
AD
NA
NA A
Ins/BYDV=insects (aphids, Hessian fly, wireworms)/barley yellow
dwarf virus; Sm/Bu=smuts/bunts;
RR=root rots; DO=damping-off; FFol=fall (early season) foliar
diseases. B
A=active (indicates a range of control from partial to complete –
check label for details); NA=not active. CActivities
listed are for the 4 oz rate. DActivity
against root rots caused by Rhizoctonia solani and
Fusarium spp. Oklahoma State
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services. Issued in
furtherance of Cooperative Extension work, acts of May 8 and June
30, 1914, in cooperation with the U.S. Department of Agriculture,
Robert Whitson, Director of Cooperative Extension Service, Oklahoma
State University, Stillwater, Oklahoma. This publication is
printed and issued by Oklahoma State University as authorized by the
Dean of the Division of Agricultural Sciences and Natural
Resources.