SOYBEAN PRODUCTION IN KENTUCKY PART IV:
WEED, DISEASE AND INSECT CONTROL
James H. Herbek and Morris J. Bitzer
Department of Agronomy
Weed control is one of the most important
management practices for optimum soybean production. Weeds cause more yield
reductions in soybean fields than any other factor by competing with the
soybeans for nutrients, moisture and light. Soybean yield reductions due
to weed competition can be quite severe, as much as 75% in some research
studies. Weeds in soybeans are also costly in other ways: energy expanded
in additional tillage operations; delayed harvesting; reduced harvesting
efficiency; increased weed seeds and foreign matter in the harvested crop
contribute to storage problems and dockage at elevators; and they can also
harbor insects and diseases.
Weed control in soybeans involves sound
production practices as well as chemical and mechanical methods of control.
Good cultural practices are very important and often contribute to the
effectiveness of the weed control methods being used. Some of the most
important soybean production practices that can be used to avoid weed problems
•good quality seed,
•weed free seed,
•avoiding weed seed distribution,
Good quality seed is important
to obtain uniform, rapid soybean emergence that will provide early shading
to reduce weed problems.
Weed-free seed prevents the
introduction of problem weeds that may be hard to control and also reduces
the spread of weed seed into relatively clean fields.
A uniform stand provides competition
and suppresses weed growth. Thus, it is important to plant soybeans under
optimum conditions to provide the best situation for obtaining a uniform
Reducing the transportation of weed
seed by farm machinery (tillage, planting, and harvesting equipment)
avoids the widespread distribution of problem weeds from localized infested
fields or areas.
Crop rotation can be a great
aid in reducing weed problems in soybeans. Certain troublesome weeds, particularly
large-seeded, deep-germinating broadleaf weeds (such as cocklebur, giant
ragweed, morning glory and velvetleaf) can be more easily controlled in
other crops than in soybeans. If these are not effectively controlled in
rotation crops, weed problems encountered in soybeans will become more
The major weed problems in soybeans
are annual broadleaved weeds, annual grasses and perennials. Identifying
and knowing the growth cycle of the problem weeds in your field will help
you control them. This information lets you choose herbicides and methods
that provide the best control.
Since many weeds are strong competitors
with soybeans, early season weed control is important. If weeds are allowed
to compete from 4 to 6 weeks after soybean emergence, substantial yield
losses start to occur and become increasingly greater as the length of
time increases before they are effectively controlled. Early season weed
control is also more effective since smaller weeds are easier to control
and more options for control are available. In addition, usually after
4 to 6 weeks, soybean plants provide canopy coverage which reduces later
weed seed germination and also helps suppress weed growth.
A total weed control program can involve
mechanical as well as chemical control methods. Often a combination of
these two methods plus good cultural practices results in the best and
most economical control.
Mechanical Weed Control
Mechanical methods of control include
tillage operations before planting to prepare a weed-free seedbed. Frequent,
shallow cultivations with a disc, harrow or other seedbed preparation tillage
equipment can be effective in overall weed control.
Even though a majority of growers now
depend on chemical control methods, using some type of cultivation (rotary
hoe, harrow or cultivator) can supplement chemical control and be advantageous
•adequate weed control is not obtained,
•the soil has formed a hard crust,
•rainfall is not timely or adequate
for herbicide activation.
The rotary hoe is very useful early
in the season for controlling weed seedlings. This tool can be used anytime
from preplanting until soybeans are 3 to 4 inches tall except when the
soybeans are in the crook stage. During this stage, soybeans are very susceptible
to breakage. Best results are obtained when weeds are just germinating.
When soybeans are too large for rotary
hosing, timely cultivation effectively controls weeds. Small emerging weeds
are more easily controlled with cultivation than are large established
weeds. Cultivations later in the season should be relatively shallow to
avoid excessive soybean root pruning while still permitting weed destruction.
Excessive ridging which may interfere with efficient harvesting should
be avoided. Shallow cultivation also avoids bringing new weed seed to the
surface to germinate. Growers relying on mechanical weed control should
complete cultivations by the time soybeans are 6 to 8 weeks old to minimize
damage to the plants. One of the disadvantages of growing soybeans in very
narrow rows is that using cultivation to control weeds is impossible.
Chemical Weed Control
Making the right herbicide selection
requires some planning. The following factors should be considered: 1)
weed species present in a particular field; 2) method of herbicide
application to be used; 3) the correct time to apply the herbicides;
and 4) crops to be planted following soybeans in a rotation.
You need to know the weed problems
in each field because no single herbicide will consistently control the
entire weed spectrum that may occur. Some herbicides control both annual
grasses and some broadleaf weeds, while others control primarily grassy
weeds or broadleaf weeds. Certain weeds are also difficult to control and
may need a combination of cultural practices and/or herbicides for successful
control. You can take the guesswork out of herbicide selection and do a
better job of planning a soybean weed management program if you know what
weeds to expect. Making a map of weeds present in each field toward the
end of the growing season is the best way to predict what weeds will be
the predominant problems for the coming year. Scouting soybean fields during
the early part of the growing season is also important: 1) to evaluate
your weed management program, 2) to determine if additional control
measures are needed and 3) as part of a planned postemergence program.
Chemical weed control can be effective
if used properly. That means you should know what to apply, how much to
apply, and when and how to apply the herbicide. Consult the product label
for instructions and any use restrictions regarding crop application, crop
uses, and safety and handling. Follow all recommended instructions and
Once a herbicide is selected, its performance
depends on both environmental (moisture, temperature, soils, etc.) and
human (proper timing, correct rate and accurate application) factors. Too
low a rate generally results in poor weed control, while too high a rate,
besides costing more, may injure the soybeans or rotational crops. Some
herbicides have a narrow margin of crop safety. Thus, rates of application
must be adjusted for different soil types, soil textures and levels of
organic matter. Sprayer calibration is extremely important for correct
rates and accurate application and should be re-checked periodically. Several
methods of calibration can be used successfully.
Because complex weed problems are often
encountered, using more than one herbicide and/or application is becoming
common to provide a more effective and broader spectrum of weed control.
If tank mixtures are used, note that some combinations of herbicides and
carriers may not be compatible. Therefore, follow label directions for
approved products and carriers and their mixing sequence. For a simple
compatibility check mix small amounts of the herbicides (and the carrier)
in the same ratio that will be used. If a separation or sludge develops
which does not re-suspend after mixing, the herbicides are not compatible
and the combination should not be used. Do not tank mix products when label
restrictions prohibit their use together.
The method of application will largely
be based on the herbicide selected. It will also be related to the stage
of crop development, the stage of weed growth, tillage methods and available
equipment. The 3 most common herbicide application methods are preplant
incorporated, preemergence, and postemergence.
Preplant Incorporation - Some
herbicides must be applied before planting soybeans and incorporated into
the soil to increase their effectiveness. Incorporation moves the herbicide
into moist soil for activation and is also necessary to prevent losses
from volatilization and
photodecomposition of certain herbicides. It also ensures good activity
in the absence of rainfall. When you use preplant incorporated herbicides,
distribute the chemical uniformly at the desired depth within the soil.
Herbicide injury and lack of effective weed control usually can be explained
by poor application and/or incorporation. Application and incorporation
of these herbicides may vary from a few weeks before planting to immediately
Give special attention to seedbed preparation
when applying preplant incorporated herbicides to the soil surface. For
optimum incorporation, the seedbed should be smooth and fairly level and
the soil moisture suitable for normal tillage operations. The presence
of large clods, if not easily broken up with tillage, may result in poor
weed control and an unlevel seedbed can result in nonuniform horizontal
and vertical distribution of herbicides in the soil. Soil moisture content
is also very critical to the uniformity and degree of incorporation obtained.
The soil should be sufficiently dry so it can readily flow and mix with
the herbicides being incorporated into it. If the soil is too wet, it will
not mix properly and can result in the herbicide occurring in streaks.
For effective control of annual grasses
and small seeded broadleaf weeds, herbicides should be uniformly distributed
within the upper few inches of soil where most of the small seeded weeds
germinate. Incorporating the herbicide too deep may dilute it and reduce
its effectiveness. Deeper incorporation may also increase injury to the
crop since the roots will be developing in the herbicide zone. In most
cases, the herbicide is incorporated into the top 2 to 3 inches of soil.
However, the ideal depth to incorporate depends on the herbicide. Therefore,
follow the specific incorporation instructions on the label of the herbicide
The time permissible for delay between
the herbicide's application and incorporation to prevent loss of activity
by volatilization and/or sunlight decomposition varies with the particular
herbicide. The interval between application and incorporation is critical
for some herbicides and they require immediate incorporation. Always refer
to the product label for this information. For optimum results, incorporate
as soon as possible. One method to eliminate any delay is to have the herbicide
applicator mounted on the incorporating implement and perform both operations
at the same time.
To achieve optimum mixing of the herbicide
with the soil, use the right equipment and be sure it is operated properly.
Information about what equipment to use and how to operate them is on the
label of most herbicides that are applied preplant incorporated. Several
implements can incorporate herbicides successfully. The most commonly used
are disks, field cultivators and power tillers.
The disk is the most widely used implement
for incorporating herbicides. Uniform distribution of the herbicide depends
on the kind of disk used and how it is operated.
•Finishing disks (having individual
disk blades spaced less than 8 inches apart, 20 inches or less in diameter,
and spherical rather than conical in shape) consistently provide the most
•Cutting disks (characterized as having
blade spacings more than 8 inches apart, blade diameters greater than 20
inches, and either spherical or conical blades) designed for primary tillage
do not provide suitable incorporation.
Herbicides will be incorporated about
1/2 to 2/3 the depth at which a finishing disk is set to cut. Therefore,
as a rule of thumb, set the disk to cut about twice as deep as you want
the herbicide to be incorporated. Finishing disks should be set to cut
4 to 6 inches deep so that the herbicide is placed in the top 2 to 3 inches
of soil. Since slow ground speeds usually result in streaking or banding
of the herbicide, use higher operating speeds (5 to 7 mph) for good lateral
movement and mixing of the soil. Make two passes with a disk for thorough
mixing. Make the second pass at a right angle to the first (cross-disking).
One disking does not provide adequate horizontal distribution of the soil
and leaves streaks of soil not moved or mixed.
Field cultivators can also be used
to effectively incorporate herbicides. They have 2 or more rows of sweeps
or chisel points with an effective spacing of 6 to 9 inches (for example,
sweeps spaced 18 inches apart on 3 rows would have an effective spacing
of 6 inches). Sweeps are usually preferred since they slice and lift the
soil more than do chisel points, often resulting in better incorporation.
Set the field cultivator about 1 inch deeper than the desired incorporation
depth. Two passes with the field cultivator, with the second pass perpendicular
to the first, gives more uniform incorporation. Use operating speeds no
lower than 5 mph nor greater than 7 mph. Adding a drag harrow will improve
the soil's horizontal distribution. Two perpendicular passes with a field
cultivator at 5 to 6 mph operated at 3 to 4 inches deep should uniformly
incorporate the herbicide within the upper 2 to 3 inches of soil.
Power tillers have tines which are
forced into and through the soil by a tractor's power takeoff (PTO) assembly.
The various types of power tillers include vertical-action tillers (tines
rotate around a transverse axis), horizontal-action tillers (tines rotate
in circles or oscillate) and combination-action tillers (components have
both vertical and horizontal motion). The amount of interaction with the
soil depends on the relationship of PTO speed to ground speed. Too fast
a PTO speed in relation to forward speed overworks the soil and does not
improve incorporation. Too slow a PTO speed in relation to forward speed
leaves areas not worked sufficiently resulting in poor incorporation. Consult
your owner's manual to determine the correct PTO:ground speed relationship
for a particular machine. Incorporation depth for most power tillers is
the same as the depth at which they are set to operate. However, some of
the horizontal-action tillers may not be able to incorporate as deep as
the vertical-action tillers. Set power tillers for an operating depth of
2 to 3 inches so that the herbicide is incorporated to that depth. If power
till equipment is properly operated under good soil conditions, only one
pass is needed for excellent incorporation.
Preemergence - Herbicides are
placed on the surface before the soybeans or weeds emerge, generally at
planting or shortly afterward. It is best to apply all non-incorporated
surface applied herbicides after planting and not preplant since the planting
operation may bring untreated soil to the surface. Also, better weed control
is usually achieved if the treatment is applied immediately after planting
so that weeds do not emerge before herbicide activation.
Favorable rainfall after application
is essential for a preemergence herbicide to be activated to perform at
its best potential. Usually between 1/2 to 1 inch of rain within 7-10 days
after application is adequate. If weeds emerge before a rainfall, using
a rotary hoe, rolling cultivator or shallow cultivation will kill the weeds
and not decrease the herbicides' effectiveness. Excessive rainfall, however,
can reduce the effectiveness of the more soluble herbicides by leaching
them below the zone of weed seed germination. Also, soybean seedling injury
can occur from some herbicides if the herbicide is moved to the zone of
soybean germination. With certain herbicides, a soybean planting depth
of at least 11/2 inches is recommended by the manufacturer to reduce the
potential of crop injury.
Herbicides applied preemergence may
be broadcast or applied in a narrow band over the soybean row. When herbicides
are applied in a band over the row, weeds between the rows will need to
be controlled by cultivations and/or a postemergence program. The major
advantage for banding is reduced herbicide costs per acre.
Postemergence - These herbicides
are applied after the weeds and soybeans have emerged. Once considered
only as an emergency or salvation operation for weed control in soybeans,
the recent development of effective herbicides has resulted in many producers
now including postemergence applications as an essential part of their
weed control program. These herbicide treatments may be applied broadcast
over the top of the soybean-weed canopy, directed to the weeds below the
soybean canopy or applied selectively to weeds above the soybean canopy.
Most postemergence treatments in Kentucky
are broadcast over the top of the soybeans. Several reasons for this exist:
recent herbicides are somewhat safer on soybeans than previous products;
there is a trend toward more narrow rows; directed application equipment
is not common on farms; and selective application equipment is only used
where weeds are taller than the soybean canopy.
Directed postemergence applications
are favored in certain situations and with certain herbicides that do not
have the safety margin on soybeans when applied over the top. For directed
sprays to be effective, the soybeans must be taller than the weeds to let
the spray cover the weeds with little contact of the soybean foliage. Precision
application is necessary for minimal soybean injury. Direct the spray toward
the soil and lower half of the soybean plant at relatively low pressures.
Read the product label for specific recommendations on the size of both
the soybeans and weeds for proper use of each postemergence directed herbicide.
Selective application equipment, which
generally includes wipers (sponge, wick and roller applicators) and recirculating
sprayers, operate above the soybean canopy. This application technique
applies small amounts of translocated herbicides to the top foliage of
susceptible weeds. The weeds must be taller than the soybeans to avoid
getting herbicide on the soybeans and causing injury. This type of application
is commonly used in johnsongrass control programs.
Many of the postemergence herbicides
can cause some soybean injury. The degree of injury will depend on the
herbicide and rate used, environmental conditions (temperature, humidity
and soil moisture) under which it is used, soybean growth stage and use
of additives. Obtaining good weed control with minimal injury to soybeans
is often a delicate balance. Therefore, follow label recommendations for
proper use of each herbicide.
Postemergence herbicide applications
must be made at the proper stage of soybean and weed growth. Since postemergence
treatments usually work best on small weeds, timing is extremely critical.
Actively growing weeds are also more susceptible to herbicides than weeds
under stress. Since the stage of weed growth for most effective control
can vary according to the weed species and the herbicide used, read the
product label for proper use.
Scouting is important for making decisions
in a postemergence program. Without periodically surveying soybean fields,
you cannot determine the best method of control if control is needed. Scouting
information should include identification of weed species, their size and
numbers, and stage of crop growth to help you select appropriate herbicide
treatments (product(s), rates and time of application).
Although it is known that weeds certainly
reduce crop yields, knowing if a weed control treatment will be economically
justifiable is especially important. The weed species present and their
density (plants/area) are important factors to consider as you determine
whether control is necessary. As a general rule, scattered weeds will probably
not cause measurable yield losses; therefore, complete weed control is
often not economically feasible. Remember, however, that weeds differ in
their ability to compete with soybeans. Thus, the weed density required
to justify control will depend on the weed species. For example, cocklebur
and giant ragweed are more competitive and require fewer plants per area
to reduce soybean yields than do foxtail and crabgrass. Use weed threshold
levels, if available, to justify weed control management decisions.
Depending on the amount of tillage
done, some degree of surface residue will be present. Having large amounts
of plant residue may prohibit the use of herbicides that require incorporation.
In no-till only preemergence and/or postemergence herbicides can be used.
Failure to control weeds is a major
reason for no-till failures. A better level of management is required for
success in weed control. An effective program depends largely on herbicides
since cultivation is usually impossible. With no-till, and in some minimum
tillage systems, a foliar applied herbicide will be needed to control existing
vegetation at planting in addition to soil residual herbicides for preemergence
control. Spray volumes used in no-tillage herbicide applications may range
from 20 to 60 gal of liquid per acre depending on vegetation present and
herbicides used. As a general rule, contact herbicides require more spray
coverage of vegetation for effective weed control and therefore should
be applied in greater volumes than translocated foliar herbicides. Consult
the herbicide labels for specific directions. Post-emergence herbicides
may be needed in no-tillage systems to supplement residual herbicides or
to control escaped weeds later in the season. A wide range of postemergence
herbicides are available for control of broadleaf and grassy weeds.
Several potential problems can be encountered
in no-tillage weed control production systems. These involve difficult
to control weed species, spray coverage and application, crop residues
and seed coverage. Becoming aware of these problems and avoiding them increases
the chance that no-tillage production will succeed.
The use of tillage and cultivation
to control vegetation in no-till soybean production is not practical. Therefore,
control of vegetation present at planting depends heavily on chemical control
before, at or immediately after planting. The control of all vegetation
can be difficult if problem weed species, such as perennials, are present.
Thus, know what weeds are present when considering no-tillage. Generally,
perennial weed species tend to be more prevalent under no-tillage than
under conventional tillage. Some perennial weeds can be controlled in a
no-tillage system, but success will depend on the particular weed species
and herbicide(s) used. Treatments applied before, at or immediately after
planting allow more flexibility in herbicide selection and application.
After soybeans have emerged, the control of certain weed species and the
use of certain herbicides becomes limited.
Associated with herbicide applications
at planting is the need for adequate and uniform spray coverage. Apply
foliar applied herbicides used in no-till in enough carrier to give adequate
spray coverage. Determine the spray volume by the amount of vegetation
present and the herbicide treatment. Also, to get uniform spray coverage
in no-till, adjust the nozzle height relative to the vegetation or stubble
height and not the soil surface. Spraying ahead of the planting operation
can lead to poor weed control in the row if the no-till coulters bring
up untreated soil during the planting operation. Avoid this problem by
spraying after planting or by spraying with the nozzles mounted behind
Large amounts of plant residue from
previous crops can reduce the effectiveness of herbicides by intercepting
the spray solution. Uniform distribution of the plant residue will help
to avoid this problem.
Another problem is that soybeans can
be injured by certain preemergence herbicides because the seed has not
been properly covered by soil. In no-tillage plantings it is sometimes
difficult to obtain uniform planting depths and subsequent covering of
the seed. This problem can be overcome by adjusting the planter (coulters,
seed openers and press wheels), planting depth, planting speed and planting
under suitable soil conditions (not too wet or dry).
Rate of application, rainfall, temperature,
soil Ph and tillage practices affect herbicide residues in the soil. Not
only can soybeans be affected by herbicides applied to preceding crops,
but some soybean herbicides used may injure succeeding rotational crops.
Thus, herbicides need to be carefully selected in regard to all crops involved
in a crop rotation program. The limitations for use in cropping sequences
can be found on the herbicide label.
A few products can be used to defoliate
soybeans and desiccate weeds to aid in harvesting efficiency. These products
are not intended to hasten maturity of soybeans but are intended to facilitate
harvesting by hastening the drying of weeds. Timing of application is critical
since these products will injure immature soybeans if applied too early.
Also, these products may have restrictions regarding soybean use for feed
or food and also utilization of the plant residues. If you are considering
using any desiccant or harvest aid, consult the label for recommendations
Since herbicide recommendations change
from year to year, consult the most recent publication of AGR-6, Chemical
Control of Weeds in Kentucky Farm Crops, for information on currently
recommended herbicides and more detailed information about weed response,
rates and specific comments concerning their use.
Soybean disease have become an important
part of Kentucky soybean production, although considered insignificant
at one time. The rapid increase in acreage and intensified, successive
plantings of soybeans have led to more widespread and frequent disease
problems. The longer a crop is grown in a locality, or grown in succession,
the more prevalent diseases become. Soybean diseases are responsible for
annual production losses of an estimated 10-15%. The severity of yield
losses will vary from year to year and depend on weather conditions, stage
of crop growth, cultural practices, type of disease and extent of infection.
Methods of Control
No one method is best to control all
diseases in soybeans. Those that control one disease may have little effect
on another. However, several practices can help keep soybean diseases to
a minimum and should be part of a good production program.
1) Use resistant varieties.
This is one of the best methods of control. Once a disease problem is determined,
check a list of adapted varieties for their disease reaction and select
a resistant variety for future plantings. Specific diseases in Kentucky
for which resistant varieties are available include soybean cyst nematode
and phytophthora root rot.
2) Use good quality, disease-free
seed. This practice is very effective in reducing seed rot and seedling
diseases which can result in a poor stand and stunted growth. Since certain
diseases can be carried on or in seed, planting high quality seed, while
not guaranteeing absolute freedom from disease, should result in more disease-free
plants than planting poor quality or non-certified seed. Refrain from saving
seed from fields infected with pod and stem blight, brown stem rot, bacterial
blight, purple seed stain, bud blight and soybean mosaic.
Low seed quality, even if not due directly
to seed-borne diseases, adversely affects germination and seedling vigor.
When slow germination and emergence occur, the seedling has greater exposure
to infection by soil microorganisms. If low seed quality is not due to
mechanical damage, fungicide seed treatment can increase germination and
vigor of young seedlings. These treatments give temporary protection and
have not given consistent increases in yield. However, if seed quality
is low and if planting conditions are poor for germination and seedling
establishment, treatment will increase stands. Seed treatments are more
thoroughly discussed in Part 2 of this series on Soybean Production
in Kentucky. For current seed treatment recommendations, get Cooperative
Extension publication, PPA-6, Fungicide Seed Treatment for Control of
Grain Diseases, from your county Extension office.
3) Soybean rotation.
Continuous soybeans can result in disease buildup. A cropping system where
soybeans are rotated with non-host crops is an effective control method
for most diseases. Diseases such as pod and stem blight, brown spot, bacterial
blight and brown stem rot increase in crop residue. Since the microorganisms
that cause these diseases do not persist long in the soil, rotation of
soybeans with other crops is a good practice. Rotation is also very helpful
in preventing substantial yield losses from those disease organisms that
do persist in the soil, such as soybean cyst nematode.
4) Plowing under residue.
For diseases that carry over on old soybean residue, turning the residue
completely under can prevent many organisms from surviving and infecting
new plants the following season. However, this method may not be practical
where soil erosion exists and reduced tillage or no-tillage are used. In
such situations, crop rotation would be more effective.
5) Good weed control.
Since some weeds and other crops can harbor or act as hosts for certain
diseases, preventing weed growth in and around the edges of the soybean
field is important. This practice is important for soybean cyst nematode,
bud blight and soybean mosaic diseases.
6) Foliar fungicides.
Foliar fungicides may be used on soybeans during the pod development stages
to increase yield and/or improve seed quality. Whether the use of fungicides
is economical depends not only on the price of soybeans and cost of fungicide
use but also on the level of disease infection. Results of foliar fungicide
use in Kentucky have been inconsistent and can be related to the level
of disease present which is determined by several factors including cropping
history, maturity of variety, planting date and rainfall.
Diseases for which foliar fungicides
offer control are brown spot, anthracnose, pod and stem blight, stem canker
and purple seed stain. They are more prevalent in early maturing varieties,
early plantings, fields cropped continuously with soybeans, and fields
with above average rainfall and temperature during the pod and seed development
stages of growth. Several foliar fungicides are available and labeled for
use. For yield protection, two spray applications are recommended (at early
pod set and again 2-3 weeks later at the beginning seed stage). Read the
product label for proper rates and usage.
Research studies at the University
of Kentucky have shown variable results using foliar fungicides over the
past few years with an average yield increase between 2 and 3 bu/acre.
Where disease pressure was severe, yield responses of 3 to 7 bu/acre were
common. With light disease pressure, yield responses of 0 to 3 bu/acre
were more common.
Using foliar fungicides to improve
seed quality may be more justifiable since diseases, particularly pod and
stem blight, have been shown to be responsible for lowering seed quality
in soybeans. As a result of research at the University of Kentucky, a state
label for seed beans using a single high rate of benomyl is recommended
when severe disease conditions exist. Since several factors determine the
potential for disease severity, a point system was developed to help Kentucky
seed growers decide when fungicide application might be beneficial. This
system takes into account cropping history, variety selection, planting
date and rainfall. Kentucky seed growers are encouraged to use it.
For further information on foliar fungicides
and using the point system, get a copy of Foliar Fungicides Can Improve
Seed Quality from the Kentucky Seed Improvement Association or Cooperative
Extension publication, PPA-10, Kentucky Plant Disease Control Guide
for Field Crops, from your county Extension office. Your county Extension
agent can also help you use the computerized program, "Points."
Diseases Common in Kentucky
Soybean diseases can be caused by pathogenic
bacteria, fungi, viruses or nematodes. Depending on the specific disease,
they can be prevalent during any growth stage and affect virtually all
Diseases that have been commonly recognized
on soybeans in Kentucky are listed below:
1) Seed and Seedling Diseases: Seed Rot and Seedling Blights.
2) Root and Stem Diseases: Rhizoctonia Root Rot, Phytophthora
Root and Stem Rot, Charcoal Rot, Brown Stem Rot, Southern Stem Blight,
Sudden Death Syndrome and Soybean Cyst Nematode.
3) Pod and Stem Diseases: Pod and Stem Blight, Anthracnose,
Purple Seed Stain and Stem Canker.
4) Foliar Diseases: Brown Spot, Downy Mildew, Bacterial Blight,
Frogeye Leaf spot and Alternaria Leaf Spot.
5) Virus Diseases: Soybean Mosaic, Yellow Mosaic, Bean Pod Mottle
and Bud Blight.
Since recognition of diseases is important
for control measures to be applied, submit suspected plant samples through
your county Extension office to the Plant Diagnostic Laboratories at the
University of Kentucky for diagnosis and recommendations. For additional
information on soybean disease control see Cooperative Extension publication,
PPA-10, Kentucky Plant Disease Control Guide for Field Crops.
Soybean Cyst Nematode (SCN)
SCN is one of the most damaging pests
of soybeans in Kentucky with estimated annual losses of nearly $10 million.
This microscopic roundworm is found in the soil, feeds on the root system
of soybean plants and reduces their capacity to absorb water and nutrients.
First identified in Fulton County in 1957, it has continued to spread north
and east in the state where it has been identified in 32 western counties
representing over 90% of Kentucky's soybean acreage.
Symptoms-Above ground symptoms
may be mistaken for other problems such as nutrient deficiency, drought,
herbicide damage, poor nodulation or poor drainage. Therefore, diagnosis
is very important. Soybean plants infected with SCN usually become yellow
and stunted resulting in lower yields. Initial symptoms usually occur in
randomly spaced circular to oblong areas which become larger each year
when soybean cropping is continuous.
Check the root system when you see
the above ground symptoms. Cyst-infected roots will be stunted, darker
in color, and have less nodulation than healthy roots. Careful examination
reveals the SCN females attached to the roots as white to yellow and eventually
brown spherical bodies (cysts). To check roots for cysts, do not pull plants
from the soil because this will cause the cysts to break off of the roots
and remain in the soil. Carefully dig up plants and gently shake off the
soil. Apart from visual observation of the roots, the only sure way to
confirm an infestation is to collect soil samples and submit them to a
laboratory for nematode analysis. The University of Kentucky has a nematode
analysis laboratory at the West Kentucky Research and Education Center
Occurrence and Spread-Several
life cycles of SCN are possible within a growing season. The cysts, which
may contain 200 to 500 eggs each, are also highly resistant to decay and
the eggs can remain alive within the cysts for 10 to 12 years. Thus, SCN's
ability to produce high populations and the cysts' ability to survive adverse
conditions make the nematode very damaging and difficult to control. In
addition, many other legume crops and weeds can act as hosts for SCN.
Presently 5 races of SCN have been
identified. Race 3 is the predominant race in Kentucky with Race 4 occurring
in some of the far western counties.
Although SCN can move through the soil
only a few inches each year on its own, it can be spread rapidly by any
type of soil movement caused by wind, birds, flooding, movement of farm
equipment with soil attached, or seed containing soil particles.
Control - Controlling SCN has
no easy solution. In most cases a combination of practices will be needed
to reduce losses. Because of its great survivability, once a field becomes
infested, SCN can not be totally eliminated and must be continually managed
if soybeans are to be grown. Growers with SCN problems can use several
methods to control the disease and reduce losses: sanitation, fertility,
resistant varieties, crop rotation and nematicides.
- Sanitation - Since SCN can
easily be spread from infested to non-infested fields on equipment, cleaning
soil from equipment before moving from field to field will help prevent
its spread. Also, be sure soybeans purchased for seed were produced in
non-infested soil or were properly cleaned to remove all soil peds.
- Fertility - Research has shown
that SCN damage is more severe in low fertility soils. Although fertilization
will not eliminate SCN problems, addition of P205
and K20 where needed will increase yields.
- Resistant Varieties - Resistant
varieties are one of the most effective and economical means of SCN control
if used properly (Table 1). Several soybean varieties have been developed
that are resistant to various races of SCN. Selection of a variety should
be based on matching its race resistance to that of the SCN race in an
infested field. The Kentucky Soybean Performance Tests publication designates
the race resistance of varieties included in the tests. Because of SCN's
reproductive capacity and variability, some larvae can infect previously
resistant varieties. Thus, do not plant resistant varieties continuously
in SCN infested fields. This practice can, in just a few years, lead to
development of SCN populations or new races for which no resistant varieties
Table 1. Soybean Variety Response on SCS Infested Fields
1McLean Co., KY; 2Regional SCN IV Test;
3R = resistant; S = susceptible Numbers in parenthesis
indicate race resistance.
- Crop Rotation - Rotating soybeans
with non-host crops (corn, grain sorghum, or small grains) offers a simple,
inexpensive control. Research has shown that SCN populations can be reduced
as much as 75% the first year with a non-host crop. The longer a non-host
crop is grown, the more the SCN population is reduced. When a non-host
crop is grown, the hatched larvae from the cysts have no susceptible plant
to feed on and they die. However, because of the survivability of the cyst
eggs, starving all the nematodes is impossible. As a result, the population
of SCN will increase dramatically when susceptible soybeans are planted
after a non-host crop. Therefore, crop rotation should be a planned, continued
part of a soybean production program to control SCN.
A schedule that includes a non-host
crop for one or more years is a preferred rotation to reduce SCN populations.
Resistant varieties will similarly reduce nematode populations if they
are not used continuously or too frequently. One year in a non-host crop
followed by one year in a resistant soybean variety usually reduces the
SCN population so that a susceptible variety can be grown the third year
without a noticeable yield reduction. However, in heavily infested fields,
a non-host crop may need to be included more than one year and/or the rotation
lengthened before a susceptible soybean variety can be successfully grown.
Several rotation programs can be utilized. The choice will depend on how
well non-host crops fit into the overall farming program and the desirability
of available resistant soybean varieties.
- Nematicides - Using a nematicide
is another control practice that can be used along with crop rotation and
resistant varieties. Nematicides are used primarily when planting a susceptible
variety in a cyst infested field. Since they are an added expense to soybean
production, consider them only if suitable cropping rotations and resistant
varieties cannot be used.
Research results with nematicides have
been inconsistent, even with susceptible varieties, and the nematicides
are not always economically profitable. Research further indicates that
nematicide use on resistant varieties seldom provides economic returns.
They do provide short term protection to soybean roots of susceptible varieties
by reducing SCN populations early in the season and giving the plant time
to establish a sound root system. After several weeks, as the nematicide
breaks down, nematodes begin to infect roots and by the end of the season,
the populations have generally returned to a high level. If you use nematicides,
follow all safety precautions and label instructions.
Further information on the soybean
cyst nematode can be found in PPA-3, Soybean Cyst Nematode, and
information about collecting soil samples on the Soybean Cyst Nematode
Analysis Form, available at your county Extension office.
Economically important insect injury
to soybeans in Kentucky is a sporadic event. Outbreaks tend to be localized
and very difficult to predict. However, where infestations do occur, reduced
yield and seed quality will result if proper controls are not utilized.
As soybean acreage has increased in the state and production intensified,
there has been a general increase in soybean insects.
Insects may injure soybeans during
all stages of plant development, both above and below ground parts. Mechanical
damage caused by insect feeding is the most common problem but insects
can also damage soybeans by transmitting certain plant diseases.
Scouting for Insects
The initial step in control of an insect
problem is early detection and identification. Growers should check their
soybean fields at least once each week from planting until maturity. Several
survey methods can be used. These include: 1) visual inspection; 2) sweep
net method; and 3) shake cloth method.
Smaller plants can be visually inspected.
However, as plants become taller, this method becomes inaccurate because
it is difficult to observe insects in a full canopy. A visual survey includes
walking a length of row and counting and/or collecting the insects observed.
The sweep net method consists of collecting
insects by swinging an open net through the foliage. The insects collected
can then be counted and recorded. This method is the most convenient and
fastest procedure but may not always be an accurate assessment of insect
The shake cloth method is the most
accurate and common method of sampling for insects. A white cloth (42"
x 24") is placed in the row middle between two rows and approximately two
row feet of plants from each row are shaken onto the cloth. The insects
are then identified and counted and this process is repeated at 10 different
locations in the field.
Evaluation of Insect Damage
Soybean growers are most concerned
about how much yield or seed quality loss will result from the presence
of insects. Loss of yield will result only when the amount of injury exceeds
the plant's tolerance. It has been shown that soybeans can tolerate significant
insect damage without a reduction in yield.
Defoliation is the most common and
visible form of insect damage to soybeans. When the amount of defoliation
reaches a certain level, yield loss will result. Treatment recommendations
are based on the percent defoliation and presence of an insect population.
However, determining the percent defoliation can become difficult because
the dense foliage can hide damaged leaves. Most growers tend to overestimate
defoliation. A good method is to pull up plants from several locations
in the field, place the leaves against a light background, and then estimate
the percent defoliation.
The defoliation level needed before
a control measure is applied will vary depending on insect numbers, stage
of plant development, cost of treatment, market price of soybeans and anticipated
yield. These factors should all be taken into account to determine defoliation
required for control measures to be justified. Refer to ENT-13, Insecticide
Recommendations for Soybeans, for tables which show the amount of defoliation
required for economic injury to soybeans (damage level needed to justify
treatment). These tables are based on stage of plant development, anticipated
yield, soybean market price and cost of treatment.
Estimating the amount of injury to
pods and seeds is more difficult. The estimate should be based on both
the number of pods and/or seeds damaged as well as the loss of quality.
Treatment decisions for the control of pod and seed feeders are normally
based on the number of insect pests that need to be present for control
measures to be justified. The threshold levels (number needed to justify
control) of most soybean insect pests are listed in ENT-13, Insecticide
Recommendations for Soybeans.
Soybean Insect Problems
A general calendar for common insects
found in Kentucky soybeans is shown in Table 2. The dates of most probable
occurrence listed in the table can occur earlier or later than indicated
because of many environmental factors. However, this calendar will provide
a good estimate of when major insect problems might occur.
Table 2. Soybean Insect Calendar
1SCM = seed corn maggot, BLB = bean leaf beetle, CW = cutworm,
MBB = mexican bean beetle, GH = grasshopper, GCW = green cloverworms, GSB
= green stinkbug. * = Period when insect populations are likely to cause
economic damage. + = Insects present and should be watched.
Soybean insects are commonly placed
in one of 3 primary categories: seedling pests, foliage feeders and pod
feeders. A brief discussion follows for each of these groups and the insect
pests that may be encountered in Kentucky.
* Seedling Pests - These insects
attack soybean plants above or below ground early in the growing season.
Insects in this group include the three-cornered alfalfa hopper, bean leaf
beetle and cutworms. Losses by these insects have been minor to date and
generally non-economic. Soil insecticides may be used; however, since soil
insects or seedling insects are rarely a problem, they are not recommended
as a regular practice unless a history of the problem develops.
* Foliage Feeders - This large
group of insects receives the most attention and concern in soybean fields.
Most insecticide applications are used to control this group of insects.
Since soybeans can withstand a good amount of leaf damage before yield
losses occur, the degree of damage can often be misleading in terms of
justifying control. Foliage feeders remove plant tissue and reduce yield
through loss of photosynthetic surface and increased water loss in the
plant. Soybeans in the vegetative growth stages can tolerate more leaf
area loss without a serious reduction of yield potential than can soybeans
in the reproductive stages where leaf area loss is more critical.
If you discover the following percentages
of defoliation at the various soybean growth stages in your fields, then
controls may be justified:
- 40% defoliation from planting to
- 20% defoliation from blooming to
- 30% defoliation from podfill to maturity.
These are general guidelines, and as previously discussed, the actual
percentage defoliation needed to justify control will vary with economic
factors (treatment cost, soybean price and anticipated yield).
The common insects that may be encountered
in Kentucky in this group include Mexican bean beetle, green cloverworm,
loopers, bean leaf beetles, velvetbean caterpillar, blister beetles and
grasshoppers. Other foliar insects which have been encountered are spider
mites and thrips. These damage plants with piercing-sucking mouthparts
rather than by chewing.
* Pod Feeders - Pod feeders
have the greatest potential for decreasing soybean yield and quality. They
can cause serious damage both by direct seed feeding and damaging pods.
Pod damage can lead to pod abortion or allow entrance for seed diseases.
Insects in this group include corn earworm, fall armyworm, stinkbugs and
bean leaf beetles.
* Other Insects - Other groups
of insects that may be of concern include soil insects (wireworms, grubs,
seed corn maggot, grape colaspis and bean leaf beetle larvae) and storage
insects (Indian meal moth).
For further information on soybean
insects, get Cooperative Extension publication, ENT-25, Soybean Insects,
from your county Extension office.
Methods of Control
Do not rely on one method of soybean
insect control. All possible means of control, including chemical, biological
and cultural practices, should be considered to maintain insect pest levels
below the economic threshold (that number of insect pests or level of damage
capable of reducing yields and/or quality exceeding the cost of control
measures). The most integral part of a soybean insect management program
is to scout and monitor fields for insects on a regular basis.
Cultural practices that can
be used, depending on the insect pest, include optimum planting dates,
destruction of crop residues, good weed control and crop rotations. No
soybean varieties commercially available at present in Kentucky offer a
significant degree of insect resistance.
Biological control can be economically
important. Naturally occurring insect predators and parasites and disease
pathogens aid in keeping insect pests below economic levels. Apply insecticides
only when needed so that beneficial insects (those that feed on insect
pests) are maintained. Untimely application may reduce the beneficial insects
and allow insect pest populations to increase again in their absence. Certain
insect pests may also be controlled by natural diseases. Several "biological
insecticides" are now labeled for use on soybean insect pests. Their action
differs from traditional insecticides by causing a disease or a physiological
dysfunction instead of poisoning the insect pest.
Chemical control should be used
only when necessary. If economic threshold levels have been reached, insecticide
treatments are needed to stop insects from doing any further damage. On
the other hand, indiscriminate use of insecticides is expensive, may result
in poor pest control and increases the potential need for additional applications.
When you select insecticides, make
sure they are appropriate for the particular insect pest. Some insecticides
are more effective in controlling certain insects than others. Because
insecticides vary in their labeled crop uses, rates, application methods,
selectivity and toxicity, read the insecticide label and follow the directions.
For current insecticide recommendations,
threshold levels and economic injury defoliation tables, get Cooperative
Extension publication, ENT-13, Insecticide Recommendations for Soybeans,
from your county Extension office.
Suggested References and Related Publications
University of Kentucky Publications
1) AGR-6, Chemical Control of Weeds in Kentucky Farm Crops
2) AGR-128, Soybean Production in Kentucky-Part I: Status, Uses
3) AGR-129, Soybean Production in Kentucky-Part II: Seed Selection,
Variety Selection and Fertilization
4) AGR-130, Soybean Production in Kentucky-Part III: Planting
Practices and Double-Cropping
5) AGR-132, Soybean Production in Kentucky-Part V.- Harvesting,
Drying, Storage and Marketing
6) ENT-13, Insecticide Recommendations for Soybeans
7) ENT-25, Soybean Insects
8) PPA-3, Soybean Cyst Nematode
9) PPA-6, Fungicide Seed Treatment for Control of Grain Diseases
10) PPA-10, Kentucky Plant Disease Control Guide for Field Crops
11) Foliar Fungicides Can Improve Seed Quality (Available from
Kentucky Seed Improvement Assoc.)
12) Kentucky Soybean Performance Tests (Annual Progress Report)
13) Soybean Cyst Nematode (multi-state publication) (Available
from UK, Dept. of Plant Pathology)
14) Soybean Cyst Nematode Analysis Form (Diagnostic form with
soil sampling procedure)
15) Soybean Disease Atlas (multi-state publication) (Available
from Dept. of Plant Pathology)
1) Compendium of Soybean Diseases (Available from American Phytopathological
Society, St. Paul, Minn.) (For sale only)
2) Identifying Seedling and Mature Weeds Common in the Southeastern
U.S. (Available from North Carolina State University, Raleigh, NC) (For
3) Modern Soybean Production (Available from American Soybean
Assoc., St. Louis, MO) (For sale only)
4) Weed Identification Guide(s) (Available from Southern Weed
Science Society, 309 West Clark St., Champaign, IL 61820) (For sale only)
The authors wish to acknowledge the
following specialists for their contributions to this publication: Jim
Martin, Extension Weed Control Specialist,, Don Hershman, Extension Plant
Pathologist,, and Doug Johnson, Extension Entomologist.