ISSUED: 4-88
James H. Herbek and Morris J. Bitzer
Department of Agronomy

Weed Control
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 are:
good quality seed,
weed free seed,
uniform stands,
avoiding weed seed distribution,
crop rotation.

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 stand.
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 severe.
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 when:
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 practices.
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 before planting.
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 being used.
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 uniform incorporation.
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.

Reduced Tillage
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 the planter.
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).

Residue Carryover
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.

Harvest Aid
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 and restrictions.
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.

Disease Control
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 plant parts.
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 in Princeton.
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 exist.

Table 1. Soybean Variety Response on SCS Infested Fields
Yield (Bu/Acre)
19841 1984-862 Resistance3 Group
Fayette 32 -- R(3,4) III
Sparks 19 15 S IV
Douglas 24 19 S IV
Franklin 40 25 R(3) IV
Egyptian 35 28 R(3,4) IV
Pyramid -- 31 R(3,4) IV
Avery -- 31 R(3,4) IV
Essex 17 -- S V
Forrest 32 -- R(3) V
Nathan 28 -- R(3,4) V
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.

Insect Control
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 populations.
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
Plant Stage
Planting  *
Emergence * * *
Cotyledon * * *
True Leaf * * *
Trifoliate * * * *
Vegetative Stages + * +
Early Bloom + + + +
Full Bloom + * *
Pod Set + * *
Pod Fill * *
Maturity + *
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 prebloom;
- 20% defoliation from blooming to podfill;
- 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 and Planning
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)

Other Publications
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 sale only)
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.