Author Archives: David Holshouser

About David Holshouser

David serves as Associate Professor & Extension Agronomist at Virginia Tech’s Tidewater Agricultural Research and Extension Center. He provides leadership for agronomic extension and research programs that lead to profitable and environmentally-responsible agriculture.

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.

Lower Seeding Rates & Maintain Yields with Narrow Rows & Uniform Stands

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Before the cost of soybean seed jumped with the introduction of the Roundup-Ready trait, soybean was planted at much greater rates than was needed for maximum yields. Back then, seed was cheap. It only cost a few dollars to insure adequate stands – even if only 50% of those seed emerged. Today, planting extra seed is a luxury that we can no longer afford, especially if the seed includes one or more of the many seed treatments now available. Furthermore, with better planters and drills that easily handle residue and place the seed at the proper depth, we tend to get better stands. But, just how low can we go? How many seed per acre does it take to maximize yield?
Over the past 8-10 years, I’ve collected a lot of seeding rate data on different soils throughout Virginia. The main conclusion from that research was that seeding rate recommendations were too high for full-season (May-planted) soybean and barely adequate for double-cropped (late June- to July-planted) soybean. Over the past three years, I’ve also collected quite a bit of data following a barley crop (early June planted) and I feel comfortable with relatively low seeding rates for this cropping system. My current recommendations are listed below. Note that the table lists the desired final plant population in plants per acre and the seeding rate calculations (in plants per row foot) are based on 80% emergence. Your final stand may vary depending on emergence. In general, for May plantings, 80,000 plants per acre are adequate for all row widths.

Suggested Soybean Seeding Rates for Virginia

Don’t Widen Rows to Save Seed. I’ve heard talk of widening rows to save seed. It’s true that if you move back to 30 or 36” row spacing, you’ll not use as much seed. That’s because you can only crowd so many plants into a row and get a benefit. Basically, too many plants within the row are competing with each other. But, you’re losing the benefit of narrow rows, which is, once again to capture 90% or more of the sunlight by flowering and early pod development. Sure, it’s possible that in a good year on a good soil, you’ll be able to meet these requirements. But over time, it’s not likely. On the other hand, row spacing of 15 to 20 inches should provide adequate canopy if planted in May through mid-June. Keep in mind that the later you plant, the greater the benefit from narrow rows. Also, narrow rows benefit early-maturing varieties more than late maturing varieties. And, as I’ve alluded to earlier, there is less of an advantage to narrow rows on more productive soils. Therefore, some fields will likely respond more to narrow rows than others.

Here’s a good way to check and see if your row spacing is narrow enough for a particular field. This summer, when the soybean crop is in full bloom, walk your fields. As you’re walking, look into the canopy. Can you see any ground? If so, then you’re rows are not narrow enough. If you’re already planting in narrow rows, then consider raising your seeding rate. This will insure that you’re meeting the leaf area requirements.

Uniform Stands Matter. We generally think that soybeans will compensate for poor stands, wide rows, and/or gaps within rows. Soybean will compensate much more than other crops. However, I think that uniformity of stand within the row matters.  The more uniform the spacing between plants within a row, the greater the yield potential. This was shown with double-cropped soybean in research conducted in Virginia that compared a 15-inch planter with a standard drill and a drill that uniformly distributes the seed in the row (see figure below). I’ll refer to this drill as a “precision drill.” Details of the drills used are shown below.

Meteriing wheels for various seed types in the Great Plains 1510P or 1520P Precision Seeding System

Type, make and model, seed singulation method, and row spacing of seeding equipment used in this study.

Stand uniformity with the precision drill was equal to the vacuum-meter planter and better than the standard drill. Yield results are also shown below.  Soybean yield with the standard drill was equal to the vacuum-meter planter in three of four years and less than the vacuum-meter planter in one year. Soybean yields were greater when planted with the precision drill than when planted with the vacuum-meter planter in 2 of 3 years, and averaged 10% over three years of study.

When Should Soybean Be Planted in Virginia?

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We must remember a main goal of many pre-plant and at-planting decisions is obtain adequate canopy and leaf area that captures 90% or more of the sunlight by the R2 (full bloom) to R3 (beginning pod) stages. A secondary goal related to planting date is to position the most critical time of crop growth, the pod (R3-R4) and seed (R5-R6) development stages, during a time of the season where there are sufficient resources (water, light, nutrients, CO2, and O2) to maximize yield.
We might assume that the earlier that we plant, the sooner the crop emerges; therefore, earlier emergence means faster canopy closer and more leaf area development. Furthermore, the earlier we plant, the sooner the crop will reach first flower (R1); therefore, the critical R3-R6 development stages will take place at a time when days are longer and there is more sunlight. While these assumptions are mostly true, the end result will not always contribute to final yield.
Let’s examine the first assumption that earlier planting means more leaf area. This is generally true, but how much leaf area do we really need? Soybeans need enough leaf area to capture 90 to 95% of the sunlight by the R2/R3 stages. This minimum requirement can usually be met by planting in April, May, and even early-June. So, adequate canopy development is not usually an issue until we double-crop the soybean after wheat harvest
Of course canopy development is affected by the environment in which the crop is growing, mainly by soil moisture and temperature. If soil moisture is lacking, growth will be slowed but usually not enough to prevent adequate canopy development, even with late-May and early-June planting dates. Besides, early-season soil moisture is usually not a problem with full-season no-till production unless a cover crop has removed most of it during a dry spring. Even if this were to occur, it can be alleviated by killing the cover crop earlier than planned during dry springs and when continued drought conditions are forecast. Low water-holding capacity soils may also affect canopy development even when there is adequate soil moisture at planting, but other cultural practices can make up for the lower leaf area in these situations (discussed later). I reemphasize that it is rare that leaf area requirements are not met with full-season plantings.
In addition, there is not as much to be gained (from a leaf area perspective) from planting between April and early-June. Why? Growth during April and early-May is slow due to cooler temperatures. Therefore, there is little difference in vegetative soybean growth between an April planting (cooler conditions) versus a May planting versus an early-June planting (warmer conditions). When planted in April, soybean may take up to three weeks to emerge and growth will generally be slow until mid-May. On the other hand, when planted in early June, the crop will emerge within 3 to 5 days and, if soil moisture is adequate, will grow rapidly. Therefore the time between planting dates will be much greater than the difference in the subsequent growth of the crop. In summary, I see few benefits, at least from the standpoint of canopy development, in planting soybean before mid-May to early June in Virginia.
An exception may be if you are planting an earlier-than-recommended maturity group (MG) for your area (MG III or early IV). In Virginia, late MG IV’s and MG V’s are the best adapted varieties, with IV’s doing better in the northern half and V’s doing better in the southern half of the state. If you stick with late IV’s and V’s, there should be no issue with canopy development when planted in April or May. On the other hand, a MG III planted in late-May to early-June may suffer as shown in the below graph. This is because there is not enough time between planting and flowering to allow adequate leaf area development. Still, note that yield suffered only when the MG III variety when planted in early June. The yield response does not always look this way due to rainfall patterns, soil type differences, etc. Sometimes MG III varieties will show a similar response as the later-maturing soybean. But, I think that it is safe to say that if planting early-maturing varieties, you should try to get those planted by mid-May to reduce the risk of low leaf area.
Of course, when planting double-crop soybean, canopy development will suffer. Therefore, other cultural practices such as narrow row spacing and greater seeding rates must be implemented to minimize low leaf area effects. Late-planting and double-cropping will be discussed in next month blog.


So, what about the second assumption that the earlier we plant, the sooner the crop will reach R1; therefore, the critical R3-R6 development stages will take place at a time when days are longer and there is more sunlight. The reasoning is that the longest day of the year is during late-June. With more daily radiation during pod and seed development, there is more energy being put into yield. So, it stands to reason that if we were to plant in April, we could push the pod- and seed-development stages into an environment where we can capture more light. This may work as long as the crop is running full speed ahead and there is no other stress to slow it down. But, is this the case in Virginia? Will there be adequate moisture and temperatures lower than 90O F during July and August? I doubt it.
Let’s examine this a bit further. For April and May plantings, a 3-day delay in planting results in about a 1 day delay in maturity. Therefore, planting in mid-April instead of mid-May will allow the plant to reach flowering (R1/R2) about 10 days earlier. Instead of the first flower being on the plant during the last 10 days of July, it’ll be there during mid-July. Instead of reaching R5 (beginning seed) by the last week in August and R6 (full seed) in early September, the crop can reach theses stages by mid- to late-August. But, is this really a good thing? Do we really want to shift the R4 to R6 stages into typically the hottest and driest time of our season? Even if you irrigate, you’ll have the heat to contend with. And it’s much more expensive to irrigation when the temperatures are higher.
Some may suggest that we should take this a step further and use an earlier maturing variety when planting in April. This is sometimes referred to as the Early Soybean Production System. OK, let’s use a variety with a relative maturity of 3.7 instead of one with a relative maturity of 5.2. This will shift the maturity of the crop 12-15 days earlier. So, we are now putting the critical R4 to R6 stages right into July and two weeks of August. Although the crop is experiencing longer days, it’s also experiencing higher temperatures and, likely, lower soil moisture. The Early Soybean Production System will work if you have a consistent August drought. However, our droughts in Virginia are intermittent. We are just as likely to have a drought in June as we are as in July as we are in August. We can’t predict it. However, it is usually hotter in late-July and August than it is in September. So, in the long run, it’s better to push soybean development in Virginia to a later part of the season, even though we cannot gather as much sunlight during the day. Maybe that’s one of the reasons that soybean following our barley crop usually yields as well or better than soybean planted in May.
To conclude, planting soybean early will have little effect on yield in Virginia. Plus, it could be detrimental, especially on low water-holding capacity soils. My suggestion is that you adjust planting dates to suit and spread out labor and equipment needs, but not to increase yield.

The Good and Bad of a Wet September

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We gained much needed rain in late August with Hurricane Irene.  It also brought substantial lodging, which reduced our yield potential.  But, I have yet to see a poor soybean crop lodge.  That August rain by itself would have pretty much finished out our full-season crop; our full-season soybean may not benefit much from the continued rain that we are still experiencing.  However, the double-crop plantings still had lots of yield to make; therefore, the September rains helped fill out that crop.

Any yield increases with September rains will come mainly in the form of greater seed size.  So, how important are larger seed?  Although number of seeds (mainly through number of pods) affect soybean yield most, seed size is second in importance.  The photo below may help illustrate this point.

In the above example, the weight of the larger seed is more than 50% greater than the small seed.  Of course these differences resulted from two extremely different environments.  The seed to the left of the pencil formed under late-season drought conditions.  The seed to the right of the pencil was produced under excellent seed fill environment, similar to this year.  If we were to assume that there were an equal number of seed and pods in both cases, the crop with the larger seed would have resulted in about 10 bushels more yield in a 20 bushel environment, 15 bushels more in a 30 bushel environment, and so on.  Don’t misunderstand.  Plants with lots of seed and pods will not make very big seed.  We won’t get that much of a yield increase from this year’s late-August and September rains.  A large yield increase would have only resulted if we were to compare to a crop where little to no rain occurred during the seed-fill period.  Still, these late-season rains should definitely add 10-20% to our overall yields.

Now for the bad news – with these September rains comes an increase in disease affecting seed quality.  Furthermore, with heavy October rains, there is the possibility of pod splitting or seed sprouting in the pods before harvest.  This does not happen often, but years like this one will set the crop up for such a scenario.  In the remainder of this article, I’ll focus on the most common seed quality issues.

Phomopsis Seed Decay.  When soybeans mature during warm and wet conditions, we can expect seed quality to deteriorate.  Because this disease develops more rapidly on plants that are maturing under warm and wet conditions, we usually have more problems with early-maturing varieties.  We can however have seed decay on our later- maturing varieties if October is warm.

Infected seed are shriveled, elongated, and cracked.  Severely infected seed may appear white and chalky.  Test weight can be lower.  High occurrence of these seed can lead to discounts or rejection.

There are a few things that can be done to reduce the disease incidence.  It resides in the soil and on infected residue.  So, rotation is very important.  Later-maturing varieties and later planting dates that delay maturity into the cooler parts of the year will reduce the incidence.  Still, timely harvest is the best management strategy.  The longer you leave the soybeans in the field, the worse the disease.  So, only plant as many early varieties as you can harvest in a timely manner.  Foliar fungicides will decrease the incidence of seed decay if applied from mid pod development to early seed filling stages (R3-R5).

Purple Seed Stain.  Purple seed stain is caused by the organism Cercospora kikuchii, the same organism that causes Cercospora blight.  Although little of this disease was present early in the year, its incidence has increased dramatically since late-August.  Right now, fields with Cercospora blight can usually be recognized by reddish leaves and reddish purple blotches on the stem and leaf petioles.  When severe, defoliation of the upper leaves of the plant will take place.  In many cases, the blotching progressed up the stem and to the pods.  In the past, you may have noticed the dark, nearly black pods on some varieties.  Once it progresses to the pods, there is a higher likelihood that the seed will be stained.

Purple seed stain is very noticeable.  The seed will contain pink to pale purple to dark purple splotches, which can cover the entire seed coat.  The purple stain itself does not reduce yield, but seed with nearly 100% discoloration may be lower in oil and higher in protein.  A lot of staining can result in discounts.  Germination of seed with 50% or more staining will likely be delayed.  This year, our insect scout, Mr. Ed Seymore, is already noticing dark pods and some purple seed stain.

Usually, the disease first appears on the plant during early seed development.  If conditions are right (average temperatures over 80o for several days), then the disease will build up rapidly.  We had those conditions this year.  Other weather factors do not generally affect seed infection.  Severity of the infection is largely related to amount of infected leaf debris and residue.  Therefore, rotation with a non-legume crop is critical for control.

Other control measures include variety selection, planting high quality seed free of visual staining, and fungicides.  Varieties differ in their susceptibility of Cercospora kikuchii, but that information is rarely available in seed catalogs.  We routinely evaluate purple seed stain in our variety tests.  Strobilurin fungicides such as Quadris or Headline seem to give some control if applied during pod or seed formation.

Sprouting Seed.  One of the most disturbing late-season issues can be pod splitting or seed sprouting in the pod.  Back in 2005, we saw a lot of this.  That year was very dry up until September and October, when it began raining and didn’t seem to want to quit.  What causes pod splitting?  The verdict is still out on this, but here are my observations.  Generally this happens when the crop is under severe stress, usually drought conditions up until the full-seed stage (R6).  Pods are generally small due to the drought.  Then rains set in between R6, and R7 (Physiological maturity).  The seed grow and grow, and seem to outgrow the pods, causing them to split.  Obviously, this splitting can then lead to seed quality issues.

In 2005, not only were the some pods splitting, but October rains brought on sprouting seed.  The sprouting seed was not always directly related to the pod splitting; many of the pods in which we found sprouting seed were did not split until the seed sprouted.  We saw up to 30% of the pods with sprouted seed in some locations.  This is a very unusual occurrence, but can occur if soybean seed drop below 50% moisture, then increase to 50% or more moisture.  We immediately began looking in other fields and contacting county agents to do the same.  Soon after our discovery, I heard of reports from North Carolina of sprouting and reports from Pennsylvania of seed swelling in the pod.  However, after getting several reports back from county agents and farmers, and after we checked several other fields, we found that the problem was not nearly as severe as in our fungicide trial or as widespread as we may have thought.  Below is a summary of our findings:

  1. The sprouting problem was worse in southeastern Virginia.  Although we had reports as far north as Essex County, the incidence of sprouting seed declined as we moved north and west.
  2. More sprouting was occurring on the later-planted, later-maturing varieties.  This was verified in our variety test and by several county agents.
  3. More sprouting was occurring in pods showing Cercospora blight (very dark pods).  This was verified in a fungicide trial, our variety test, and by several agents.  We did not understand why this happened and could not definitely relate the sprouting to this disease.  But, there seemed to be a relationship.
  4. Sprouting occurred primarily at the top of the plant when dark pods (also occurring primarily at the top of the plant) were common.  Otherwise, most sprouting (especially that occurring on earlier-planted, earlier-maturing varieties) occurred at the bottom of the plant where the relative humidity was higher.
  5. After a week of drying conditions, the sprouted seed had dried up, and in some cases fallen out of the pods.

Why did the seed sprouting occur on the later-planted, later-maturing varieties in 2005?  There are two or three possible explanations.  One is that these varieties may have just dropped below 50% moisture; therefore the seed needed to absorb less total moisture to raise it back above 50%.  A second explanation is that these varieties still had significant amount of leaves, therefore held the moisture within the soybean canopy better.  Finally, in 2005, there was more Cercospora in the later-planted later-maturing varieties (note: Cercospora is not always worse in one maturity group or another,  but depends on the environment during early seed fill).

So, will we see pod splitting or seed sprouting in 2011?  I would expect some pod splitting from fields under drought stress during the summer followed by lots of September rain.  But, usually, the number of split pods is low and will not affect overall yield or quality substantially.  If seed sprouting occurs, the problem will not likely be widespread and should not affect our overall seed quality very much.  At the worst, there could be some lower test weight and seed could contain more foreign material (from the dried up sprouts).  However, the light seed will likely be blown out the back of the combine.  If you do observe the problem and it is severe, I suggest that the air on the combine be adjusted to remove those light, sprouted seed at harvest.  Too many sprouted seed in the bin could lead to rejection by the buyer.

Is it Too Late to Spray Fungicides?

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Over the past week, it seems that we’ve been living in a rainforest.  Today is the first day  in several that it has only rained once.  Pluse, we’ve had several very hard rains each day.  In many areas, we’ve had more rainfall after Hurricane Irene that we received during the that storm.

These wet and humid conditions have set up our double-crop soybean for some late-season foliar disease.  If you haven’t sprayed your soybean and they have not yet reached the R6 (full-seed stage), you may want to consider another fungicide application.  We have seen yield responses, when conditions were right, as late as the R5 stage. The soybean pod below is still in the R5 stage (the seed are not yet meeting), but will be at R6 in just a few days.  R5 is as late as you can legally spray soybeans with fungicides.

If you decide to spray, stick with the strobilurin fungicides such as Headline, Quadris, or Stratego.  Using a triazole fungicide (Folicur, Domark, Tilt, etc.) are not effective on most of our common foliar soybean disease.

Late Season Onslaught of Corn Earworm (Dr. Ames Herbert)

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It may be happening for the first time that I can recall a late season infestation of corn earworms in soybeans. The last couple of nights we have trapped more corn earworm moths (about 1800 and 1500, respectively) in our pheromone traps than we have ever captured. These are alarmingly high numbers and I am getting reports that folks are seeing a lot of moth activity in and around fields of cotton, soybean and peanut. I got the first report today that some growers in southern Southampton County are having to retreat some soybean fields. I have been telling folks that as best I can remember, I have never seen a new infestation of earworms develop in September. Well, as one of my earlier mentors said to me one day. Dont make predictions about insects. Theyll make a liar out of you every time. Seems he was right. So, what crops are at risk? The cotton and peanut crops are safe as we are close to defoliation time with cotton and digging time with peanuts. Only late planted soybean fields that still have susceptible pods (earlier than R7 growth stage) are at risk. The good news is that our corn earworm pyrethroid vial test results have been showing a gradual decrease in the percent of moths surviving (see the attached graph) to levels below 10%, which means that pyrethroid insecticides should provide good control, relatively inexpensively. If I was someones mentor, I would want to go down in history as saying, Never let your guard down. Never stop checking fields for insect pests until the crop is mature

Brown marmorated stink bug in soybean, summary to date (Dr. Ames Herbert)

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The brown marmorated stink bug (BMSB) which was first identified in 2001 in Allentown, PA, is now infesting soybean fields in parts of Virginia. This stink bug, although similar in appearance to our native brown-colored species, can be easily distinguished by the white bands on their antennae, and the white bands on the legs of nymphs. BMSB is a known pest of many crops, wreaking havoc on fruit crops, wine grapes and many vegetable crops, especially sweet corn. Although the epicenter for this pest is still the mid-Atlantic region (PA, DE, MD, VA and WV), a few have been found in states as distant as California. BMSB, like our native stink bug species, feeds directly on developing soybean pods and seed. If the damage occurs very early in seed development, pods will be flat and brown, but still be attached to the plant and easy to see. If damage occurs later in seed development, pods will appear yellow and speckled, and opening the pod will reveal damaged, crinkled, stained seed. Last summer (2010) we began a monitoring program for BMSB in soybean and found them in soybean fields in 15 Virginia counties, but always in low numbers. In Maryland where they had seen these same kinds of low numbers the previous year (2009), last summer (2010) they found large infestations on field edges. The same pattern has occurred for us. This summer (2011) we have found several fields with very high numbers. So far, the heavily infested fields are confined to one geographical areathe north-central piedmont counties of Orange, Culpeper, Madison, Fauquier and Clarke. Very low numbers have been reported in other counties. A pattern seems to be emerging that is playing well for us in terms of managing BMSB in soybean. To date, yield threatening infestations seem to be confined to field edges, not going beyond 30 to 50 feet into the field. Heavy infestations also seem to be associated with fields with wooded borders, especially if there are concentrations of the invasive weed, Tree-of-Heaven (Ailanthus altissima). Both BMSB and Tree-of Heaven are native to China and other parts of Asia. BMSB seems to be strongly attracted to that host, especially when the trees are putting out their seed clusters. Not coincidently, the north-central piedmont area where we are finding the highest densities of BMSB in soybean is the area with the highest concentration of Tree-of-Heaven. I encourage you to Google Tree-of-Heaven and become familiar with what it looks like. This strong field edge effect has made it possible for our local soybean growers to make edge treatments applying insecticides in one spray-boom width around a field, without having to treat the entire field. We are revisiting as many of these edge treated fields as often as we can, and so far, the edge treatments are holding. Another bit of good news is that many of the insecticides commonly used in soybean are effective against BMSB. This summer, we were able to put out three insecticide trials in growers fields in Orange County and most of the products we applied worked very well (including Baythroid XL, Belay, Brigade, Cobalt Advanced, Endigo ZC, Lannate LV, Orthene 97, and Vydate L). The problem for fruit growers is not that they cannot kill these critters, it is that they continue to reinvade their orchard which necessitates repeated sprays. Will this also occur in soybean fields? We are not certain. How should growers react to this new pest? We are recommending that growers stay vigilant until the latest planted fields reach the R7 growth stage when beans would no longer be susceptible to stink bug feeding. Scout field edges, especially fields with wooded edges with clusters of Tree-of-Heaven. Use a sweep net to sample the plants by making successive 15-sweep samples. We have no exact threshold, but suggest that greater than an average of 4 adults or nymphs per 15-sweeps would constitute a risk to the pods and seed. We have encountered fields with 8 to 10 per 15 sweeps, and in some extreme cases, more than 20 to 30 per 15 sweeps. So, the bad news is we have another established insect pest of soybean in Virginia. The good news is we have already made some progress in terms of how best to manage it. As a final note, we are also in the process of doing field cage studies to determine 1) how damage by BMSB may differ from damage by our native stink bug species, and 2) what a damage threshold might be more on this later.