Monthly Archives: May 2012

Early-Season Insect Pests

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Potato Leafhopper. This insect overwinters in gulf-coast states and migrates northward each year, typically arriving in Virginia between late April and early June. Adults and nymphs injure the plant by inserting their piercing-sucking mouthparts into plant tissue and removing liquids. High populations can result in visual injury (cupping of leaves) and under drought conditions, can stunt growth, Injury is more severe on varieties with little leaf pubescence. But, the injury will not necessarily result in yield loss. Very dry conditions will increase injury and likelihood of yield loss. The insect can be controlled with pyrethroid insecticides.

Thrips may be the most abundant insect pest species on soybean.   But, the feeding alone will not usually cause yield reduction.  Under favorable environments, soybean will outgrow thrips damage.  However, if high numbers of thrips coincide with droughty conditions early in the season (seedling plants), then growth can be severely stunted and yield loss might occur.  Thrips feed by rupturing the cell walls of leaf cells and sucking the exudates.  Leaves will take on a silvery appearance from thrips feeding.  The insect can be controlled with insecticides from several chemical classes.  Early-season control can be obtained with insecticide seed treatments.  Ames Herbert is updating thrips counts in cotton and other crops on a regular basis in his Virginia AG Pest Advisory found at http://www.sripmc.org/Virginia/.

Bean leaf beetle is a common pest through all soybean production areas and has become more of a concern in the Midwest in recent years.  These beetles are defoliating insects, whose injury is easily recognized by small round holds between major leaflet veins.  The insect can also feed on the surface of soybean pods, leaving the seed vulnerable to excess moisture and secondary pathogens.  The insect can feed all year, but most concern is during the early vegetative stages.  However, soybean can normally grow out of this injury, without yield loss.  This insect can transmit the virus, bean pod mottle virus.  However, viruses have not traditionally been a problem in soybean.  There is resistance and/or tolerance in many varieties.  However, we suspect that some newer varieties have less tolerance.  The insect can be controlled with insecticides from several chemical classes.

Soybean Aphid.  Soybean aphid is a relatively new pest, first discovered in Virginia just 10 years ago.  It feeds by sucking plant sap, which can cause leaf curling and plant stunting and pod abortion.  At high levels, yield can be seriously reduced.  While an early-season pest in the Great Lake states, it has never occurred in Virginia before July, and rarely before August.  In addition, it only reaches threshold levels on relatively few acres in Virginia each year.  We only mention this pest here because some companies are promoting early-season control of aphid with soil insecticides.  Although soil insecticides may provide some control to seedling soybeans, this is not an issue in Virginia.  Management of this pest depends on regular scouting and applying insecticides when threshold levels are reached (250 aphids/plant before R5).

White Grubs.  With less tillage and more residue buildup on our soils, grubs have become more of a concern.  White grub damages soybean by feeding on soybean roots, killing young plants, and reducing stands.  Insecticide seed treatments have some, but limited effect on grub.

Wireworms.  As the name implies, wireworms are wire-like worms that feed on soybean seed, preventing germination.  This leads to poor and spotty stands when populations are high.  They may also feed on the underground base of the plant.  Later, they may feed on roots.  To determine if wireworms are a problem, bait stations can be employed.  Seed treatments are effective against wireworm.

Lesser cornstalk borer can be a problem on seedling soybean; problems on older soybean are infrequent.  Outbreaks are more likely under hot, dry conditions and in sandy fields with weedy hosts.  Larvae of this insect bore into the main stem at or just below the soil surface.  Numerous seedlings can be injured by a single larva.  Seedlings can be cut off at the soil surface or the tunneling can cause wilting and death.  Surviving plants may lodge and be lower yielding.  Insecticide seed treatments or other applications are not effective.

Insecticide seed treatment to soybean is of limited value in Virginia.  Seed treatments can reduce feeding by some species of insects early on the season, for the first 3 to 4 weeks after plant germination.  However, we do not typically treat for insects early, nor is there data to support the value or need.  Early season insects include thrips (various species) and bean leaf beetle.  Ames Herbert, Extension Entomologist, spent several years doing tests across the state trying to determine the value of treating for thrips and was never able to find a yield advantage.  Bean leaf beetle can feed on seedling plant leaves, but he has never seen a yield reduction from the feeding.  In the north central US, growers use seed treatments to reduce first generation soybean aphid.  In Virginia, we do not see aphids until late July or August long after any seed treatment would be out of the plant system.  Seed treatments may have some utility for wireworms and grubs.

Kudzu Bug.  Although not necessarily an early-season insect, this new pest is showing up early this year just to our south.  Little is known about this insect, but we are learning quickly.  This insect was discovered in Georgia in 2009, moved into South Carolina in 2010 and through North Carolina in 2011.  It was also found in Patrick County, Virginia in 2011.  As of May 2012, it has already been found in at least six N.C. counties and in Greensville County, VA.  It feeds on wide range of legume hosts including kudzu, wisteria, some vetches, and soybean.  It has several generations per year, moving from sheltered areas such as bark or rocks in the winter to kudzu and then on to soybeans.  Like an aphid, it has piercinig and sucking mouthparts, therefore does its damage by sucking juices and nutrients from the plant.  Of the studies conducted in 2010 and 2011, it has reduced soybean yield by an average of 21%.  We need to track this insect, so timely spray recommendations can be implemented.  So if you see this insect, please notify your County Extension office or you can contact Ames Herbert directly at the Tidewater AREC.  You will be hearing more about this insect, so stayed tuned.

Fungal Seedling Disease in Soybean

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David Holshouser, Extension Agronomist

Pat Phipps, Extension Plant Pathologist

Rhizoctonia Damping Off and Root Rot.  Rhizoctonia root rot is probably the most common soilborne disease in Virginia soybeans.  Even if other diseases pre-dominate in a diseased plant, rhizoctonia could easily be a component of the problem.

Preemergence symptoms are typical of common seed rots, but are not usually recognized just because these plants never emerge.  More recognizable is the damping off that occurs in the seedling stage.  This will usually occur before the first trifoliate leaf develops.  Infected plants will have a reddish brown lesion on the emerged shoot at the soil line.  This lesion is most visible after the seedling is removed from the soil.

 Resistance to rhizoctonia is not available; variations in variety tolerance have been reported though.  Stresses such as herbicide injury, poor soils, insect damage, and feeding by soybean cyst nematode will increase damage.  Several fungicide seed treatments are effective for this disease.

Fusarium Root Rot.  Fusarium is another common disease in Virginia.  It is one of the diseases that has been implicated in “Essex Syndrome” that we continue to battle in some parts of Virginia.  There are several species of fusarium and each can cause a different plant reaction and/or disease.

 Two of the species, F. oxysporum and F. solani can cause root rot.  The root rot caused by F. oxysporum usually develops on seedlings and young plants during cool weather (<60O soil temperatures).  Older plants are generally less susceptible than younger ones.  Seedlings will emerge very slow and the resulting seedlings are stunted and generally unhealthy.  Symptoms are usually found confined to the roots and lower stems.

F. solani causes preemergence damping-off and root rot.  Damping off after the seedlings emerge is less of a problem, but can occur.  Lesions are generally on the roots and are dark brown to reddish brown to black.  Lesions can also occur on the young stem.

This disease is common in nematode-infested fields.  Soybean cyst, root knot, and sting nematodes will predispose seedlings.  Soybeans growing in soybean cyst nematode-infested fields will frequently develop fusarium symptoms.  This is less likely in root knot infested fields because the injury to the plant from root knot nematode is limited to the root tip.  In contrast, larvae of soybean cyst nematode migrate within the cells and cause more wounding.  In addition, F. oxysporum often interacts with rhizoctonia.

There is some variety resistance to the disease, but this information is not always published in the company literature.  Warm soils that are well-drained are helpful in managing the disease.  Good soil fertility should be maintained and soil compaction avoided.  Fungicide seed treatments provide some, but limited control.

Pythium Damping-Off and Root Rot.  There are many different species of pythium and the dominant species that is present will vary from geographical region to region, usually depending on temperature.  Pythium will cause pre- and postemergence damping-off during the young seedling stages.  It can also cause a root rot in later vegetative stages.  Seedlings may fail to emerge and will have a short, discolored root.  After emergence, symptoms can resemble those of other seedling diseases, especially fusarium and phytophthora.  The disease begins as water-soaked lesions on the young stem or on the cotyledons (seed leaves), and then followed by brown soft rot.

Variety resistance to pythium is not available, but fungicide seed treatments containing metalaxyl or mefenoxam will control the disease.  The best way to avoid the disease is to avoid planting into cool soils (<60oF).

Phytophthora Root Rot.  Of all the seedling disease that you may have heard about, phytophthora is probably the one that you hear and read about most.  It is a serious problem in the Midwest and affects young seedlings and older plants.  Many of our varieties that we grow in Virginia have varying levels of resistance to multiple races of phytophthora.  Yet, most of you have probably never had the disease.  Why is that?

Phytophthora rot is most severe in poorly drained clay soils that are readily flooded.  Most of our soils are sandy in nature, or if a clay, are well-drained.  This doesn’t mean you can’t have the problem just that it is less likely.  Plant loss can occur in lighter soils or on well-drained soils if they are saturated for an extended period of time when the plants are young.

Symptoms are the typical root rot and pre- and postemergence damping off.  The disease is often not diagnosed because it is confused with flooding damage.  Root and stem rots occurring later in the season will occur under similar, saturated conditions.  Tolerant cultivars may escape damage.  Damage does increase with reduced tillage, especially no-till, mainly because those fields absorb more rainfall and can be more easily saturated if the field is poorly drained.  Like most diseases, continuous soybean will increase likelihood of infection and damage.

Seed Treatments Have Their Place

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Planting soybean into cool and wet soils is a recipe for more seedling disease problems.  Sometimes, you may never notice that there is a problem and, other times, certain areas of the field may be almost wiped out.  More commonly, the field in general is just not growing the way it should.

Some of our most noteworthy seeding diseases include: fusarium root rot, phytophthora rot, pythium damping-off and root rot, and rhizoctonia damping off and root rot.  Of these, fusarium and rhizoctonia are the most common in Virginia.  Some of these diseases can be managed with fungicide seed treatments, but some cannot.

Certain insect pests can also cause problems to seedling soybeans.  Thrips or leafhoppers can stunt growth when in high numbers on drought-stressed plants.  Bean leaf beetle seem to feed on young plants every year.  Both insects can transmit certain viruses.  Some companies are promoting insecticide seed treatments to help manage soybean aphid, but this is not relevant in Virginia.  Other soil insect problems include seed-corn maggot, wireworm, grub, and slugs.

Seed treatments are becoming more and more popular in all crops.  Benefits over soil treatments include lower use rates, less direct contact with toxic chemicals, and ease of use.  Fungicide seed treatments are sold under various brand names, but usually include one or more of the following active ingredients (with their most common trade names):  captan (Captan), thiram (Thiram), fludioxonil (Maxim), thiabendazole(TBZ), carboxin (Vitavax), PCNB (Rival), metalaxyl (Allegiance, Acceleron DX-309), mefenoxam (Apron XL), ipconazole (Racona), azoxystrobin (Dynasty), pyraclostrobin (Acceleron DX-109), or trifloxystrobin (Trilex).    Insecticide seed include the active ingredients:  thiamethoxam (Cruiser) and imidacloprid (Gaucho, Acceleron IX-409), and clothianidin (Poncho).

Finally, there is a new seed treatment (VOTiVO) that employs a biological mode of action with bacteria.  The product is being marketed in combination with clothianidin as Poncho/VOTiVO.  The bacteria lives and grows with young roots and supposedly creates a barrier against nematodes.  The verdict is still out with this product.  We have seen it increase yields in some nematode infested fields but not in others.  We will continue to evaluate this product.

In the near future, I’ll be exploring some of the disease and insect pests that could be causing early-season problems in soybeans.  We’ll start with an overview of individual pests and describe their potential damage.  Then later, we’ll talk about the potential benefits, if there are any, to applying one of the seed treatments currently available.

Insure Rapid Seedling Emergence and Survival

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Once the seed is in the ground with good soil-to-seed contact (insuring a good environment for germination), we must then turn our attention to getting the growing plant out of the ground as quickly as possible and insuring that it survives.  The longer the seed takes to germinate and emerge, the greater likelihood of poor stands and unhealthy plants.

Soil Moisture and Seedbed Condition.  Always try to plant into moist soils.  However, I recognize that this is easier said than done, especially when several hundred acres of soybean needs to be planted and it’s getting closer and closer to wheat harvest or you need to side-dress the corn, or many other things need doing.  Still, seed are expensive and replanting is even more expensive.

If the soil is completely dry, then the seed will usually wait on moisture.  Unfortunately, during dry times, the field is rarely completely dry.  If there is enough moisture to swell the seed but not get the seedling out of the ground, then we have a problem.  Seed need to imbibe over half of their weight in water to germinate.  The critical seed moisture content for germination is 20%.  Germination is not affected if the seed has imbibed water for 6 hours (seed is swollen, but seed coat not broken), then it dehydrates to 10% moisture.  If seed has imbibed water for 12-24 hours (seed coat is broken, no radical), then germination may be reduced 35-40%.  If the radical has emerged and seed drops to 10% moisture, few if any seedlings will survive.

Surface soil compaction will physically hinder emergence and also result in less soil moisture because of less pore space.  Row spacing will affect seedling emergence under crusted or compacted soil conditions because seed are closer together within the row; therefore, will exert more upward force, per foot of row, to emerge.

Soil Temperature.  The ideal temperature for soybean germination is 77OF and the optimum range is 68 to 86OF.  The maximum is 94OF, where temperature will actually inhibit germination.  Planting early (mid-April to mid-May) may delay emergence due to cool soils.  Planting in July increases risk of high soil temperatures, but soil moisture during this time of year is usually the bigger problem.  I did see high temperature inhibition in some of our double-crop plantings in 2010.  Regardless, if the soil temperature is not in the optimum range, emergence will be delayed.  With this delay comes a greater likelihood of the seedling being attacked by disease or insects.

Planting Depth.  Soybean seed depth should be between ¾ to 1¼ inches.  If topsoil moisture is lacking, use the deeper placement, but never plant deeper than 1½ inches.  If topsoil is adequate shallower seed placement may speed up emergence, but probably won’t make much difference under warm soil conditions.   Just make sure that the seed is in contact with moist soil.  Placement depth is controlled by the gauge wheels.  Ideally, these should be adjacent to the disk opener.  If controlled by the seed firming/closing wheel, then uniformity of seed depth could be erratic.  The deeper you plant, the longer it takes for the seedling to emerge.  The seedling physically has further to go to emerge, plus the soil may be cooler at deeper depths.  Germination and emergence rate is controlled by temperature.  So, when soils are warm, deeper planting depth has less detrimental effect on emergence.  If soils are hot, deeper placement may actually be beneficial.  Finally, uniform time of emergence of seedlings should be a goal even though soybean compensate well.

Protecting the Seed and Seedling.  Certain environments such as cold soils, non-rotated land, poor quality seed, or a field history of insect pests or disease favor specific pests.  Again, the greater time required for emergence, the greater probability that the seed will become infected with soil-borne disease.  Therefore, rapid emergence is the best defense against seedling disease.  No-tillage/high residues, early planting, planting into cold soils, and deep seed placement are all factors that may delay emergence; therefore, your management will need to be adjusted when one or more of these conditions are present.  Seed treatments will protect the seed and seedling if emergence is delayed, but should only be used if other methods to decrease emergence time have been exhausted.

To review, here’s a checklist for rapid seedling emergence:

  • Know the germination and vigor of your seed.
  • Insure good soil-to-seed contact
  • Plant soybeans ¾ to 1 ½ inches deep and into sufficient soil moisture.
  • Plant into warm soils (68 to 86OF).
  • Consider fungicide seed treatments if planting early into cool soils.

Three Principles of No-Till Planting

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Nearly all of Virginia’s soybean acres are planted no-till.  Therefore, most of you are experts at getting a good stand even in heavy residue.  Still, a quick review of the principles of no-till planting is always helpful.  This can be boiled down to three simple steps: 1) Cut the residue; 2) Penetrate the soil to the proper depth; and 3) Insure good soil-to-seed contact.  Some may add a fourth step – close the seed-V, but I consider that part of step 3.  For more information, see VCE publication 442-457, Planter Drill Considerations for Conservation Tillage Systems, which can be found at the VCE web site, http://pubs.ext.vt.edu/442/442-457/442-457.html.

1. Cut the Residue.  In order to cut the residue, it first needs to be spread uniformly over the field.  This begins with harvest of the previous crop.  Residue cannot be cut adequately when residue levels are piled in one place.  Furthermore, disk openers don’t really cut through piles of chaff; instead, it is tilled into the soil, which hinders the third step of insuring good soil (not chaff)-to-seed contact.  Chaff in the seed zone will only pull moisture away from the seed.

Standing residue is easier to plant through than mowed/shredded residue.  So, don’t shred stalks and consider stripper headers for small grains.

Allow the residue to dry and become crisp before planting.  Planting too early in the morning is one of the biggest mistakes made.  Regardless if the planter is set right or not, cutting wet or tough residue is a challenge that might not be overcome.  Remember, 75% of soybean yield is established when you put the seed into the ground.  Don’t get in a big hurry.  Allow the residue to dry.

This should go without saying and I rarely see this being done in Virginia.  But, never burn the straw!  This is a valuable resource.  Burning will remove any nitrogen and carbon and send it up into the air.  This adds to pollution and throws away probably the two most important resources for producing a good crop and improving soil quality.

Once the residue is spread evenly, we can then turn our attention to the planter or drill.  First, adjust the disk openers.  Coulters and disk openers should be sharp, have a diameter large enough to create a “scissoring” action between the blade and ground, and not be bent or damaged.

Double-disk openers should be set to work together with little to no gaps between disks.  Maintain approximately 1 to 1½ inch of contact between the two disks.  If this blade-to-blade contact cannot be maintained, if blade diameter is worn below the manufacturer’s recommendations, or if the blade edge is bent, chipped, or jagged, the blade should be replaced.  Watch the leading edge of offset double-disk openers for significant wear and bending.  Essentially, the leading edge of the disk takes the abrasion and wear of cutting straw or stalks and penetration the soil.  Gradual wear leads to a bigger gap between the double-disks.  If the gap becomes too big or the leading disk edge becomes bent, the disks will push residue into the furrow instead of cutting through it.  For offset double-disk openers, a business card-width gap should be maintained to insure proper operation and prevent the trailing disk from cutting into the leading disk blade.

Once insuring that the disk openers can do the job, the next thing to bring your attention to is adequate down pressure.  This is not the same thing as weight.  We can add weight later.  Down pressure is controlled by the springs or hydraulics on the planter.  It’s the amount of downward pressure being applied by each planting unit.  Although adequate down pressure is most related to step 2 – disk openers must penetrate to the proper depth to adequately cut the residue – practices and adjustments within each step may overlap.  The coulters and/or disk openers must act like a pair of scissors.  The drawing to the right illustrates this concept.  The dashed line represents the soil surface and the solid line represents the angle between the coulter and soil.  Note that the coulter is running at the proper depth and the contact angle is about 45O.  At this angle, the cutting is scissor-like and residue will be cut.  Keep in mind that the size of the coulter will affect this angle; bigger is usually better.

Although not necessary, coulters can be added in front of the planter openers to ensure residue cutting.  Like disk openers, the cutting angle must be correct.  All the same principles mentioned thus far and in step 2 apply.  Because coulters are usually mounted several feet in front of the seed opening/placement device (in the case of coulter caddies even further), many use wide-fluted coulters.  A pivoting hitch or a steering mechanism will keep the seed openers tracking in the coulter slots.

2. Penetrate the Soil to the Proper Depth.  The primary differences between conventional planter/drill systems and those designed for conservation tillage systems are down-pressure and weight.  Since openers and soil engaging devices must deliver more down pressure to penetrate firmer no-till soils and cut the residue, conservation planter/drill systems include heavy-duty down-pressure devices, are built heavier, and have the ability to carry much more weight than conventional tillage systems.  Penetrating the soil to the proper depth may require up to 500 pounds of down-pressure per planting unit.  Down-pressure springs are adjustable and multiple springs can be added if insufficient pressure is achieved.  Hydraulic down-pressure controls are also available.  Only after adequate down-pressure is achieved are we ready to add weight to the planter/drill.  Adding weight by itself will not ensure penetration to the proper seeding depth.  Add sufficient weight to the planter to ensure penetration of the coulters and seed furrow openers into untilled soil, and to keep the seed-metering drive wheels on the ground.

Soil type affects planting depth.  The planter/drill will tend to sink and begin dragging up residue if it moves out of heavier soil into a lighter soil, if the planter moves from a no-till to a tilled area, or if the planter moves from compacted land to that which is not compacted.  The result is that the coulters move too deep and begin pushing, instead of cutting, the residue.  Never set the planter on the field edges where the soil is more likely to be compacted or in an unrepresentative soil.  Wide gauge wheels, drill units that do not run side-by-side but are offset, and high clearance will reduce residue dragging when in a field with highly-variable soil types
.

3. Insure Good Soil-to-Seed Contact.  Good soil-to-seed contact cannot be achieved unless the first two steps are performed correctly.  If the first two steps were carried out correctly, the last step will be much easier.  There are two methods for seed-depth control on most no-till planter/drill systems: 1) setting the depth from a gauge wheel adjacent to the seed furrow device or 2) adjusting press wheel pressure behind the seed furrow openers.  The disadvantage of any system using the press wheel for depth control is its distance from the seed opener.  As the distance increases there is a greater possibility that irregular terrain will influence both depth control and the press wheel’s ability to provide good soil-to- seed contact.  Depth control from an adjacent gage wheel is preferred.  In either case, keep adequate pressure on the gauge or press wheel to force the openers into the soil to the proper depth.  For more detailed discussion on the advantages of different types of press wheels (i.e., flat, ribbed, angled, etc.), see VCE publication 442-457 referenced earlier.

Sufficient weight must remain on the press wheels to ensure firming of the seed into the soil.  Wet soil is easily compacted and care must be taken not to over pack the soil, making it difficult for seedling roots to penetrate the soil.  In dry soil conditions, extra closing force may be needed.  The key is to evaluate seed-to-soil contact, not just closing the top of the seed-V.  As long as the contact is maintained, something as simple as a harrow that acts to close the top of the V and pull light residue cover back over the V may be all that is needed.  This is a common practice on drills that use a narrow press wheel.

These three principles will make you successful at no-tilling soybeans, or any crop for that matter.