Tag Archives: Disease

Soybean Rust Update: August 28, 2013 – Hillary L. Mehl, Extension Plant Pathologist

Posted on August 28, 2013 by David Holshouser

So far in 2013, Asian soybean rust (SBR) has been confirmed on soybeans in 99 counties/parishes in seven states in the U.S. (AL, GA, FL, MS, LA, SC, and AR) including seven counties in South Carolina. Fields in both North Carolina and Virginia are being scouted, but SBR has not been detected.

Soybean sentinel plots and commercial fields are monitored annually for early detection of SBR and tracking of disease spread. Data are used to make recommendations for timely applications of fungicide sprays for control of SBR. Currently soybean leaflets from sentinel plots are being evaluated on a weekly basis and no SBR has been detected thus far. Diseases in soybeans at this time include Cercospora blight, brown spot, and frogeye leaf spot. Anthracnose and target spot are likely to appear on soybean in the near future.

In 2012, SBR was first detected on samples collected from sentinel plots on October 12. Since soybean was past the full seed (R6) growth stage prior to the appearance of SBR in Virginia, fungicide sprays were not recommended. The risk of yield loss decreases if soybean rust is detected after the R6 development stage. When deciding if and when to spray, it is important to consider disease pressure, weather conditions, and crop maturity. If current weather patterns continue, SBR may appear in Virginia by mid-September. Earlier maturing soybean are less likely to be exposed to high inoculum levels of SBR than later maturity groups. Soybeans at earlier developmental stages (before R6) when SBR arrives in Virginia are more likely to require one or more fungicide applications.

Some sources for more detailed information on SBR are listed below:

The USDA soybean rust website (up-to-date reports of SBR incidence)

http://sbr.ipmpipe.org

Virginia Asian Soybean Rust website (Virginia Cooperative Extension)

http://www.ppws.vt.edu/ipm/soybeanrust/index.htm

For additional information, contact:

Hillary L. Mehl, Ph.D.
Assistant Professor of Plant Pathology
Virginia Tech Tidewater AREC
6321 Holland Road
Suffolk, VA 23437
Telephone: (757) 657-6450
Cell: (530) 906-0807
email: hlmehl@vt.edu

Frogeye Leaf Spot Increasing in Virginia Soybean

Posted on August 22, 2013 by David Holshouser

Ed Seymore, TAREC Ag Technician who is scouting fields for brown marmorated stinkbug (BMSB), aphids, kudzu bug, etc., reported to me today that every soybean field that he is checking west of I95 in Virginia (He’s in the Shenandoah Valley today) has frogeye leaf spot. Some fields are heavily infested (all leaves; up to 20-25 spots per leaflet).

I called this to your attention earlier this year, as I was seeing frogeye leaf spot symptoms in several variety tests. Many of our varieties have resistance to the disease, but some do not. In addition, the level of resistance varies with variety. The disease will also be worse in non-rotated fields (continuous soybean). In the past, I’ve found that varieties with good resistance truly resist the disease. Varieties with moderate resistance tend to hold up pretty well if soybean are in rotation. In rotated fields containing a susceptible variety, the disease can be severe but not devastating. But, the disease can devastate soybean varieties with no resistance when these soybean are following soybean (see photo below from non-rotated field planted to susceptible variety).

So, be sure to scout your soybean fields for this disease. Symptoms are round spots with tan/grey centers and reddish halos around the spot. I have no good threshold for treatment, but if you have a susceptible variety and/or are growing soybean after soybean, a fungicide application is in order. If you have not already applied a fungicide, I’d also suggest using a fungicide that combines a strobilurin with a “curative” fungicide. Combination products that have performed well on other disease, but not necessarily on frogeye leaf spot (frogeye has not been a big problem in the recent past) in Dr. Pat Phipps tests include: Priaxor, Quilt XL, Stratego YLD, and Quadris Top.

Foliar Fungicides May Pay in 2013

Posted on August 13, 2013 by David Holshouser

It seems that it’s been raining constantly in many places this year. In addition, soybean growth is generally very good, creating a canopy that will maintain high relative humidity through much of the day. Although the long-term forecast has temperatures getting into the 90’s on some days, it looks as if the 80’s will be the norm for the next 10 days or so. These high relative humidity, rainfall events, and favorable temperatures will favor foliar disease in soybean. Therefore, fungicide applications to R3 (beginning pod) to R5 (beginning seed fill) may pay off in 2013.

This past week, I’ve looked at several variety trials, both on-farm and the Official Variety Tests (OVT). I’m seeing a significant amount of frogeye leaf spot. This photo was taken in our OVT in Painter. Although most of our varieties have resistance to this disease, some do not. You should check the seed catalog or with your seedsman to determine whether the varieties that you’re using has resistance to this disease. If not, a fungicide will be in order if you see leaf spots forming.
Although frogeye leaf spot can be quite devastating, I’m not overly concerned since most varieties are not showing symptoms. However, I consider this disease an indication that conditions are right for disease formation, sort of a “canary in the coalmine”. If frogeye is prevalent, then other diseases such as Cercospora leaf blight will likely be raising its head as well.

Dr. Pat Phipps is developing a model to help us predict whether or not a fungicide will be needed. He has much experience with and has developed effective prediction models in peanut; therefore, we think that such a model may be effective in soybean. He will present his research at next week’s Virginia Soybean Field Day, so be sure to attend to hear his latest update. In the meantime, listed below are the conditions that will favor disease development. We seem to be meeting these criteria.

Many of you have likely already applied a fungicide to your full-season soybean. The most consistent yield response has been when the fungicide has been applied at the R3 development stage. However, we’ve seen responses as late as R5 (see article in this newsletter for an example of this). So, with the weather conditions that we’re experiencing, I think that we could see a response even with late applications.

Still, keep in mind that fungicides are preventative; therefore, the weather conditions after the fungicides have been applied are most relevant. Unfortunately, predicting temperatures and rainfall events is not always accurate.

Finally, it is very important to select the proper fungicide. Our research has proven that strobilurin fungicides or pre-mixes that contain a strobilurin fungicide are most effective again the most common soybean diseases that we experience in Virginia. The triazole fungicides are not as effective. However, strobilurin/triazole tank mixes or pre-mixes have tended to give us the best control. Products that have proven effective in our tests and that we would recommend using in soybean include:

Headline (pyraclostrobin)

Quadris (azoxystrobin)

Stratego YLD (trifloxystrobin + prothioconazole)

Priaxor (pyraclostrobin + fluxapyroxad)

Quilt Xcel (azoxystrobin + propiconazole)

Quadris Top (azoxystrobin + difenconazole)

Note that all contain a strobilurin, which we think is necessary.

Keep in mind that if soybean rust were to come into Virginia, we will likely need a triazole as these fungicides are most effective against that disease. Although soybean rust is still far from Virginia (see map below), it is on the move. I suspect that we’ll see the disease in Virginia this year. But, I hope that it comes late as it has done in past years.

After the Flood: How much damage? What to Do?

Posted on June 13, 2013 by David Holshouser

Although we are currently drying out after the heavy rains over the weekend, I thought it would be a good idea to review the effects of flooding on soybean. Usually, widespread flooding is not a major issue for us due to the sandy nature of our soils and/or well-drained fields. Still, it can and has happened.

During the week ending on June 12, some parts of Virginia received 6 to 10 inches of rainfall (see map below). Plus, more may be on the way.

This heavy rainfall has resulted in saturated soils and in some cases, flooding. This field is a Soybean Research Verification Field in Dinwiddie County that I’ve been scouting. The photo was taken after a hard rain on Friday, June 7.

It’s difficult to know the long-term effect of flooding on soybean fields. Research is limited, but we do know that the fate of flooded fields will largely depend on 1) the development stage during which the flood took place; 2) the duration of the flood; 3) the temperature during and right after the flood; and 4) the drying rate after the flood.

Basically a flooded field depletes the roots of oxygen (O₂), causing photosynthesis to slow. After several days without O₂, the plant may turn yellow, grown very slowly, and possibly die. Other indirect effects of flooding can include reduced nitrogen-fixing bacteria (but they will recover), nutrient imbalances, and increased disease pressure.

Effects on Germination & Emergence. The most damaging effects of flooding on un-emerged soybean occur when the duration of the flood is greater than 24 hours and/or when the soil temperature is low. In controlled research conducted during the late 1990’s, researchers examined the effect of flood duration, soil temperature, and time after seed imbibition that the flood occurred. They found that saturated conditions decreased germination by 15 to 43 percentage points when averaged over temperatures of 59^O or 77^O F (germination was 62% in the non-flooded control, averaged over temperature treatments). When flooding lasted only 1 to 12 hours, germination was only decreased by an average of 15 percentage points, regardless of when the seed began imbibing water. Even after 48 hours of saturated conditions, germination was only reduced by 20 percentage points if the flooding occurred one day after the seed imbibed water. But, if the flooding occurred 2 or 3 days after the seed imbibed water, then the germination was lowered 33 or 43 percentage points, respectively. Therefore, the farther along the seed was in the germination process, the more susceptible they were to flood damage. Lower temperatures during the germination process increased the damage. At 59^OF, germination was lowered by 21 to 25 percentage points, regardless of the duration of the flood. But at 77^OF, germination was only lowered by this amount if the flood lasted 48 hours. The researchers suggested that damage to the seed under brief flooding was primarily physical (e.g., seed membrane damage), but damage with longer flood duration was physiological (e.g., ethanol toxicity, O₂ deprivation, CO₂ build up).

Flooding can also result in soil crusting. This will be worse in tilled fields that are low in organic matter and surface residue. Unless the crust is broken with a rotary hoe or similar implement, the emerging seedling, already stressed by the flood, will have an even more difficult time emerging and growing.

There is also an increased chance of seedling disease, but this is less of a problem at this time of the year with warm soil temperatures. Poor stands will be the biggest issue. To determine whether or not re-planting will be beneficial, refer to Virginia Soybean Update Vol. 13, No. 3 (June 2012).

Flooding After Emergence. Soybean can generally tolerate up to 2 days of flooding. But if the saturated soil conditions persist for more than 2 days, significant yield reductions may occur. The amount of yield reduction will vary with development stage, duration of the flood, the type of flood (stream overflow vs. low land depressional), temperature during the flood, the drying rate after the flood, and the overall environmental conditions after the flood.

In general, the following comments can be made. Flooding in the reproductive stages causes more damage than in the vegetative stages. Yield is reduced more with longer flooding duration and with a slower drying rate of the field (e.g., a well-drained sandy soil will recover faster than a poorly-drained or heavier textured soil). Higher temperatures after the flood will lead to greater yield losses. This is because the recovering plant will deplete its stored energy at a faster rate. In addition, high soil temperatures will result in greater microbial respiration, which lead to greater depletion of oxygen. The best conditions for a recovering soybean crop are cool, cloudy days and cool, clear nights.

In a Arkansas study conducted on two poorly drained soils with slow permeability (Sharkey clay and Crowley silt loam), researchers found that yield decreased by 1.8 and 2.3 bushels per acre per day of flooding on the Sharkey clay when soybean were flooded from 2 to 14 days at the V4 stage (4 trifoliate leaves on the main stem) and R2 (full flower) stages, respectively. On the Crowley silt loam soil, yield was reduced 0.8 and 1.5 bushels per day of flooding at V4 and R2, respectively. It’s worth noting that this research did not have an un-flooded control treatment; therefore, the 2-day period of flooding was considered the control. This research clearly showed that duration of the flood greatly impacts yield. In addition, flooding during reproductive stages caused more yield loss that if the flooding occurred at vegetative stage. Eight varieties were tested, but variety did not affect the flooding response. Other researchers have however indicated that certain varieties exhibit greater flooding tolerance.

Another study in Louisiana evaluated 7 days of waterlogging on V2, V3, V7, R1, R3, R5, R6, and R6.3 stage soybean under greenhouse conditions. The V stages represent the number of trifoliate leaves on the main stem and the R stages represent beginning flower, beginning pod, beginning seed, full seed, and the temporal midpoint of seed filing. In summary, the early vegetative stage (V2) and the early reproductive stages (R1, R3, and R5) were most sensitive to waterlogging.

Although some research has been conducted, estimating the yield loss due to flooding is nearly impossible because we don’t know what the conditions will be like afterwards. The best gauge may be ones past experience with fields that have flooded.

Fortunately, some of these experiences have been documented. An example is research conducted in Ohio in 1998, where researchers monitored several fields that flooded during that year. They reported a 20% reduction in soybean yield in one field and a complete loss in another field that was flooded for 3 days during the V2-V3 stage. The field with the complete loss was due to thick sediment coating the plants and not allowing any recovery. However, flooding for 3 days in two other fields caused no yield loss and flooding actually increased yield in a fifth field. In the field where yield was increased, sediments covered the plants but a light rain soon after the flooding washed the sediment off. In addition, greater residual soil moisture associated with the flooded area when a late-season drought occurred was probably responsible for the greater yield. After 6 days of flooding, yield losses ranged from 0% at two sites to about 60% at a third site and over 90% at another site. At one of the sites with no yield loss, the subsoil had a high sand content, which enable the root zone to quickly return to aerated conditions.

In another study, researchers found that flood irrigation for greater than 2 days reduced soybean yield by 20% as compared to a 1-day flood treatment.

What to Do After the Flood. While it may seem that there is little to do to help a flooded crop, it is very important that we minimize any other stress on the crop that can prevent recovery. First, try to stay off of the field. Wet fields compact easily. Compaction will further stress the crop and slow its recovery. Make sure that the field is dry enough before taking equipment back on it.

Evaluate the stand. If a stand reduction occurred, determine if it’s worthwhile to replant. Remember that after mid-June, every day delay in planting will cost you about ½ of a bushel in yield. The plants that remain are still higher yielding than seed that can now be planted, even if the stand has been substantially reduced.

Stress such as herbicide injury can slow the crop down further. Still, weeds need controlling. But you may want to select herbicides (usually as tank-mix partners to glyphosate) that don’t cause a significant amount of burning.

Cultivation is an option to conventionally tilled fields to help aerate the soil. However, cultivating a wet soil can do more damage than good by causing additional compaction, which in turn would further stress the crop.

You may also want to dig a few roots and inspect for adequate nodulation. Make sure the nodules are pink. I see little that can be done, but knowing that the nodules are still working may give you peace of mind. And while you’re doing that, split some stems and roots to check for any disease. If suspected, send in to one of our Plant Disease Clinics to be further evaluated.

Finally, some will want to apply some type of foliar fertilizer to the crop to “kick-start” it back to health. But, I see little advantage of this. Remember that the real problem is lack of O₂ to the roots and CO₂ buildup in the soil; only after the roots begin to receive O₂ will the recovery process start.

Hopefully you haven’t experienced severe flooding (> 24 hours). But if so, be patient and evaluate the field. Then make good decisions on how to handle it.

Cool Soils Should Alter Your Planting Plans

Posted on May 1, 2013 by David Holshouser

It goes without saying that every year is different and this winter/spring has been wet and cold. I think that most of us have assumed that the soil temperatures are much below normal; therefore, holding off a few days with planting may be a good idea. According to 2013 and five-year historical soil temperature records at Orange and Suffolk (from the USDA-NRCS Soil Climate Analysis Network weather stations), soil temperatures have been fluctuating quite a bit.

After early-April soil temperatures proved to be much colder than normal, a week or two of warm weather put us back on track with average. Then, temperatures dropped off again. Fortunately, the last two weeks of cool weather have not lowered the soil temperatures all that much.

Although our soils have warmed substantially since early April, the temperatures are still less than optimum for soybean germination and emergence. Ideally, I like to see temperatures hold steady at 65 to 70^O or above. The ideal temperature for soybean germination is 77^O and the optimum range is 68 to 86^O. The maximum is 94^O, where germination can be inhibited. However, we can’t always wait for perfect temperatures if we are to get all of our soybeans planted on time.

Still, planting soybean in cool (<65^O) will lead to delayed emergence and increased chance of seedling disease that can reduce stands, weaken emerged plants, and inhibit early-season growth. For a more detailed description of fungal seedling disease in soybean, refer to an article I wrote last May on the subject and can be found in my Virginia Soybean Update blog.

I stress that the greater time required for emergence, the greater probability that the seed will become infected with soil-borne disease. If you are planting into cool soils, I strongly suggest using fungicide-treated seed as an insurance against seedling disease. These treatments will protect the seed and seedling if emergence is delayed.

But, seed treatments should not be a substitute for other practices that encourage rapid seedling emergence. Here is my checklist for insuring a good stand free of seedling disease:

Know the germination and vigor of your seed; adjust the seeding rate accordingly.
Insure good soil-to-seed contact by properly setting your planter to cut through the residue and penetrate to the proper depth.
Plant soybean seed ¾ to 1 inch deep into good soil moisture.
Consider fungicide seed treatments if planting into cool soils.

Now is a Good Time to Evaluate Your Varieties for Foliar Diseases

Posted on September 20, 2012 by David Holshouser

September is a great time to evaluate your crop and the performance of varieties that you chose. In addition to general growth and health of the crop, take some time to determine if you have any of the below diseases. If so, you could be losing some yield. If you sprayed with a fungicide and still have disease, reconsider the product and rate used and the time that the fungicide was applied. Keep in mind the weather conditions when the application was made and the conditions 2 to 3 weeks after or before the product was applied. Cool temperatures (70’s) and high relative humidity (>95% for 12 hours or more) will usually increase disease incidence.

Another caution is to never diagnose a specific disease on the plant without verifying it with a person trained to identify plant pathogens. Only when the reproductive structures are found on the leaf can a disease be confirmed. Many things will cause look-alike symptoms. Be sure before you cast the blame. There are more diseases than just the ones shown below, but these are the most common. Brown spot is normally found in the lower part of the crop canopy (the lower leaves), Cercospora blight and leaf spot will be found throughout the canopy, and the frogeye leaf spot and downy mildew tend to be found in the upper part of the canopy.

Soybean Crop is Still at Risk to Drought & Pests

Posted on September 4, 2012 by David Holshouser

Our soybean crop is moving right along. Much of our May-planted crop has hit or it approaching the R6 stage. Our double-cropped soybeans are in the R4-R5 stage depending on their maturity, planting date, and whether or not they experienced early-season drought. There’s a tendency for us to believe that the crop is made after the seed have met in the pod. However, as shown below, only 50% of our yield has been made at the R6 stage.

Dry weight will continue to accumulate in the seed for the next 3 weeks until the crop reaches the R7 stage (physiological maturity), which is defined as one pod on the plant that has reached its final mature color. Much of this seed weight is due to translocation from other plant parts, especially the leaves and petioles (remobilization). Sometimes soybean is called a self-destructing plant. This is largely true. As photosynthate is moved from the leaves to the seed, the leaves will turn yellow and drop from the plant. If the leaves are yellowing and dropping due to this natural process, then we shouldn’t be concerned. However, if the leaves are dropping due to another reason such as dry weather or disease, we should take notice. It is important to keep the leaves green as long as possible to maximize yield.

It takes about 2 weeks to move from the R5 to R6 stage and another 3 weeks for to move from R6 to R7. This is a long time and much can happen during this period. First, we must protect the developing seed from insects such as corn earworm and stink bugs. After R6, these two pests are not as much of a concern, but defoliators such as soybean looper can still remove green leaf area, which I’ve already stressed, is vital for maximum yield. We also need to be concerned about foliar disease, which can also rob us of green leaf area

Foliar Fungicides for Soybeans

Posted on August 15, 2012 by David Holshouser

Ten years ago, few farmers considered applying foliar fungicides for soybeans. These crop protection products had their home with higher-value crops such as vegetables and peanuts, where a return on investment was more likely. But with soybean futures topping $16, more growers are using fungicides to protect their soybeans from common diseases and hopefully to increase yields.

The question that is still plaguing nearly everyone is will the fungicides pay for themselves this year? The answer is really quite simple – We Don’t Know! Why don’t we know? First, the fungicides that effectively control our most common soybean diseases, the strobilurin fungicides (e.g., Headline, Quadris, etc.) are preventative. In other words, they must be applied before the disease develops. So, we’re applying a chemical to prevent a disease that may or may not progress to yield-reducing levels. Unfortunately, our ability to predict soybean disease development is not very good. Therefore, we rely on results from applied research, experience, and the probability of getting a response.

How likely are we to get a response to fungicides in Virginia? We regularly test fungicides on soybean and have built up a fairly large database. While disease incidence is reduced with fungicide application, a significant yield response occurs only about 1/3 of the time. The average yield response is 3 to 4 bushels per acre, regardless of whether we average over only experiments with significant responses or over all experiments. We’ve seen yield increases up to 12-14 bushels, but that type of response is rare. So, if you can average 3 to 4 bushels per acre over all acres, then a fungicide will likely pay for itself – unless we experience one of those years where disease is nearly absent (remember how dry it was last year?). If there’s no disease, we’ve wasted our money.

Although we do not yet have an effective weather model (such as is being used in peanut) that will guarantee success, we can make an informed, although not perfect, decision. We must remember that three conditions must be present in order for a disease to develop: 1) a susceptible host; 2) the pathogen; and 3) a conducive environment. This concept is commonly referred to as the plant disease triangle. If any of these three is missing, then the disease will not develop. We have the host – soybean. And we usually have some yield-robbing pathogens present – especially the Cercospora species. But the variety used must be susceptible to the pathogen. This includes two things: genetics and stage of soybean development. From the genetic standpoint, we have many varieties with resistance to frogeye leaf spot (Cercospora sojina), probably the most damaging disease that is annually present in Virginia. So, if you’re using a frogeye leaf spot resistant variety, response to fungicides is less likely. Certain varieties are more susceptible to cercospora blight (Cercospora kikuchii); some are less susceptible. But few have true resistance. Cercospora blight is likely most responsible for yield loss to to foliar disease and the reason that fungicides prevent soybean yield losses in Virginia. Regarding stage of development, soybean disease usually appears after full flower (R2) and the soybean are just beginning to pod (R3). This can be attributed to a larger canopy at this stage (which is related to the environment side of the triangle – see below discussion), but may also be related to physiological changes taking place in soybeans as plants transition from vegetative to reproductive growth. So if the pathogen is present on a susceptible host, there is a greater likelihood that a disease will develop. Still, the environment has to be right.

What is the right environment? Temperatures ranging from 60 to 85 F in periods with moisture provided by rainfall, dew, or high relative humidity is usually required. The greater number of days that these conditions are met, the more disease there will be. The weather service is pretty good at predicting temperatures, but not so great with rainfall. Additionally, even if rainfall is predictable, the environment within the soybean canopy (micro-environment) is usually what matters most. A tall soybean crop that has completely closed its canopy and is growing in good soil moisture will result in a micro-environment more conducive to disease development than a short crop that has not closed the canopy and is growing in a relatively dry soil.

In summary, all three conditions shown on the disease triangle must be present for a disease to develop to yield-robbing levels. We do not yet have a good model to predict common soybean diseases – but we are working on it. Until such a model is developed, Dr. Pat Phipps and I suggest the following guidelines on whether or not to spray a foliar fungicide on soybeans:

Soybeans are growing well, have a full canopy, and are in the R3 (beginning pod) to R5 (beginning seed) stage. Research indicates that applying fungicides at R3 is better than later stages. Note that most fungicides cannot be legally applied after they have reached the R6 (full-seed) stage.
Daily air temperatures averaging between 60 and 77 F and accumulations of rainfall were ≥ 0.5 inches in the previous 5 days or ≥ 1.0 inches in the previous 10 days, or periods of relative humidity were ≥ 95% for ≥ 12 hours per day. The greater number of days that these conditions are met, the greater likelihood for a response to fungicide application.

When soybeans were $5 to 6 per acre, very little foliar fungicide was being applied to soybean. However with today’s prices, yield is king; therefore, a small yield increases resulting from an external input such as foliar fungicides can be cost effective.

Fungal Seedling Disease in Soybean

Posted on May 11, 2012 by David Holshouser

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 (<60^O 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 (<60^oF).

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.

The Good and Bad of a Wet September

Posted on September 23, 2011 by David Holshouser

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 80^o 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:

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.
More sprouting was occurring on the later-planted, later-maturing varieties. This was verified in our variety test and by several county agents.
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.
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.
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.