Tag Archives: Seed Treatment

Early-Season Insect Pests

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

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

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.