Disease Update: Brown Stem Rot, Sudden Death Syndrome, and Stem Canker Compared – Hillary Mehl, Extension Plant Pathologist

Posted on September 3, 2013 by David Holshouser

We are receiving reports and samples of soybean with symptoms of Brown Stem Rot (BSR) on an almost daily basis at the Tidewater AREC. The most obvious symptom of the disease is interveinal chlorosis and necrosis of the leaves though this can be indicative of other pathogens and should not be considered diagnostic. “Look-alike” diseases we have seen this year and how to distinguish them from BSR are indicated in the Table below. A few of the soybean samples submitted to the Tidewater AREC have been diagnosed with Stem Canker and one sample had signs/symptoms of Sudden Death Syndrome (SDS, we are in the process of confirming this with a root/stem biopsy).

Depending on the region, Brown Stem Rot is reported as being either insignificant or economically important in soybean. In south-eastern Virginia, Dr. Pat Phipps has not previously observed the severity of BSR that is being reported this year. In previous years, it was common to see diseased and healthy plants side-by-side with diseased plants scattered throughout the field. This year we have seen some large areas up to an acre with all the plants exhibiting symptoms of BSR. The increased incidence and severity of the disease is likely due to a combination of factors including the wet weather we have had this year as well as increased soybean production and shorter rotations out of soybean. The fungus causing BSR (Phialophora gregata) overwinters on soybean residues and builds up in the soil with soybean cropping. Severity of BSR and yield-reducing potential has been associated with fungal population levels in the soil, so reducing inoculum by rotating out of soybean for 3-5 years is one of the best management strategies for BSR.

Plant Part	BSR	SDS	Stem Canker
Roots	healthy	necrosis	healthy
Outer stem	healthy	healthy	dark, reddish-brown sunken canker starting at node
Interior stem (pith)	brown pith (center)	tan to light brown cortex; white/green healthy pith	slight browning at nodes to completely deteriorated stems
Leaves	interveinal chlorosis and necrosis	similar to BSR, leaflets may detach from petioles	general yellowing to necrosis

Brown Stem Rot Showing Up in Virginia Soybeans

Posted on August 30, 2013 by David Holshouser

I seem to be getting several calls every day regarding soybean plants dying in spots within the field. Although not always the case, most fields are exhibiting symptoms of interveinal chlorosis and necrosis. In the worst areas, the leaves are either burning up and dying. In some cases, the leaves are falling off of the petiole. The symptoms look similar to, but usually worse than, various nutrient deficiencies. Actually, what is occurring is very similar to a nutrient deficiency in that something is restricting the roots and/or vascular system from moving water and nutrients through the plant.

A common concern is that this is sudden death syndrome (SDS). We tend to hear a lot about this disease in the media, in a seed catalog ratings, and throughout many extension bulletins in the Midwest. And the name makes the disease sound like the entire field is getting ready to die. SDS is neither devastating in most cases nor widespread in Virginia. The disease rarely comes on suddenly but is building up slowly as the season progresses. The infection actually took place much earlier in the year. Furthermore, it’ll rarely result in widespread death of the soybean crop. Instead, you’ll see in in spots and patches in the field. Worth noting however is that SDS is commonly associated with soybean cyst nematode infestations. So, if your field has been diagnosed with SDS, then you may want to sample for nematodes.

In general, we’ve seen very little SDS in Virginia soybean over the years. More likely, the above symptoms are association with brown stem rot (BSR), which has been and continued to be the most common root/stem disease in Virginia. Most of the plant samples that I’ve brought back to our plant pathology lab for Drs. Mehl and/or Phipps to examine have been diagnosed as BSR. Like SDS, it usually doesn’t cause widespread death and it usually limited to small spots or patches in the field. In general, it will not continue to spread over the rest of the field. However, the patches will tend to enlarge over time. Yield loss is usually minimum and restricted to the infected areas.

Another symptom usually evident with BSR is a brown pith (center) of the stem and taproot right at the soil level. If you see the above ground symptoms shown above, start splitting the stems. If you see a white pith, then the disease could be SDS. Another diagnostic tool is to look at the leaves. If the leaflets fall off but leave the petiole attached to the stem, it is likely SDS. if the leaves don’t fall, it’s likely BSR. Finally, you’ll seed more rotted roots with SDS. But, to find the rotted roots, you’ll need to dig them up and wash the soil from the roots. Jerking the plant out of the ground will likely strip off most of the roots.

The last disease that could be a problem and exhibit the same symptoms is red crown rot (in peanut, this is called cylindrocladium black rot or CBR). This disease can however be recognized by red fruiting bodies found at the base of the stem. Red crown rot was more common when Virginia’s peanut acreage was greater and soybean and peanut were being rotated with each other.

Regardless of the disease, what can you do about it? Unfortunately, there is nothing that can be done this year. Foliar fungicides will not control a disease that is inside the stem and roots. But, in the future, rotate out of soybean for one or more years. Also, you may want to select a variety with resistance to that disease when you plant soybean in the field. Finally, as mentioned earlier, take a nematode sample. Just because you’re seeing SDS or BSR in your field this year doesn’t mean that you have nematodes. But anything that is restricting root growth could aggravate SDS or BSR; therefore the symptoms would be more evident in fields infested with nematodes.

How Late Can You Spray for Frogeye Leaf Spot?

Posted on August 30, 2013 by David Holshouser

Frogeye leaf spot is easily recognized by the circular to angular spots on the upper and lower leaf surfaces. These spots are usually surrounded by dark, reddish-brown margins and can be found throughout the canopy. If these spots are numerous enough to cover 30% of the leaf area, leaves will quickly wither and fall off of the plant prematurely. This is a serious disease and can cause substantial yield loss, so keep a close eye on your fields.

Many, if not most of our soybean varieties have some resistance to the disease. If the variety is labeled as “resistant”, you’ll usually not experience the disease. However, I have seen and continue to see significant disease on varieties will “moderate resistance”, especially in non-rotated fields.

The main question that I’m getting now is “How late is too late to spray for frogeye leaf spot?” First, check the fungicide label that you intend to use. Applications with some fungicides must cease when the soybean reach the R6 stage. Others may have a 14 to 21 day pre-harvest interval. The R6 stage will last nearly 3 weeks and the R7 stage will last another week to 10 days; so the pre-harvest interval is not my major concern. Regardless, make sure that you follow the label.

Assuming that there are no label concerns with fungicides, my inclination is to spray the field, especially if yield potential is good. My concern with not treating is that if weather conditions are favorable, the infected areas will produce more spores and cause additional infection. Keep in mind that at R5, only about 25% of the yield has been made; at the start of R6, only 50% has been made. So, from this standpoint, there’s still lots of yield to lose if the problem were to get worse.

If you decide to spray, I highly recommend a combination product such as Priaxor, Quilt XL, Quadris Pro, or Stratego XL. There are reports of failures to control frogeye with only strobilurin fungicides (Headline, Quadris, Approach). We have not documented this, but using a combination product may help prevent. In addition, a strobilurin + a triazole (or carboxamide in the case of Priaxor) should provide better control.

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.

Brown Marmorated Stinkbug and Kudzu Bug Update

Posted on August 13, 2013 by David Holshouser

Both the brown marmorated stinkbug and kudzu bug continue to expand their territories. Below is the latest update of this expansion.

A few fields have needed treatment. We’ve been successful with edge of field treatments with the brown marmorated stinkbug, as they tend not to move into the middle of the fields. Hopefully, this IPM strategy will continue to remain effective.

For the kudzu bug, the threshold is 1 kudzu bug nymph per sweep. So, to trigger a spray, you’ll need to average 15 nymphs per 15 sweep sample. Also keep in mind that insecticides vary in their effectiveness. See the chart below for the most effective products. Products highlighted in pink are recommended.

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.

Corn Earworm Survey—2013

Posted on August 5, 2013 by David Holshouser

D. Ames Herbert, Jr.
Extension Entomologist
Virginia Tech Tidewater AREC

Annually, we conduct a survey to estimate Helicoverpa zea (corn earworm) infestation levels in field corn in mid- to late July. Corn is considered a nursery crop for earworm, allowing the pest to complete a lifecycle and then move on to other crops such as soybean, cotton, and peanut in August. Over 30 years of data show that there is a linear correlation between the infestation level in corn and the amount of soybean acreage that gets treated with insecticide for this pest.

To conduct the survey this year, the number of corn earworms found in 50 ears of corn was recorded in 5 corn fields in each of 27 counties, totaling 6,750 ears and 135 fields sampled. When fields were known to contain Bt or non-Bt corn, this was noted. Otherwise, samples were considered to be random and assumed to be representative of the actual Bt/non-Bt composition in each county. Age of earworms, or if they had already exited the ears, was also recorded (data not shown). We greatly appreciate the help of Virginia Cooperative Extension Agriculture and Natural Resource (ANR) Agents, Virginia Tech faculty and staff, and volunteers in this effort. These cooperators are acknowledged below. We also would like to thank the many growers who graciously allowed us to inspect their fields for earworm.

Results of the survey can be found at the Virginia AG Pest Advisory (http://www.sripmc.org/Virginia/View.cfm?lngNewsID=1011). Statewide, approximately 18% of ears were infested with earworms. For comparison, 30% of ears were infested in 2012, 33% of ears were infested in 2011; 40% in 2010; and 36% in 2009. Regional averages for 2013 were 9.2% infested ears in the Northern Neck, 15.1% in Mid-Eastern, 15.7% in South-Central, and 23.4% in the Southeast.

This survey is intended to be a representative sample, not a complete picture. We always recommend scouting individual fields to determine exactly what is happening in terms of corn earworm as well as other pests and crop problems. Also, please check the black light trap data on the Virginia Ag Pest Advisory and other reports posted weekly to keep up-to-date on the insect pest situation.

Application Equipment for Effective Insect Pests and Foliar Disease Control

Posted on July 2, 2013 by David Holshouser

Bobby Grisso, Ext. Agricultural Engineer and David Holshouser, Ext. Agronomist

Several years ago, we stressed that proper application equipment, specifically nozzle selection and spray volume, was critical to obtaining good disease control with foliar fungicides. At that time, we were trying to prepare for an invasion of soybean rust. The problem was that soybean rust (and most other diseases) would begin establishing itself in the middle and lower leaves of a full-canopied crop. Getting enough spray droplets deep into the canopy was required for adequate control.

Although we have yet to see soybean rust early enough in the growing season and at great enough levels to cause a yield loss, we think our efforts to educate the agricultural community on proper application paid off. Foliar fungicide sprays, now being used to control other soybean diseases, are reaching their target and penetrating to the middle and lower canopy. Most importantly, it takes spray volumes of 15 to 20 gallons per acre (GPA) and medium-sized droplets to obtain good coverage.

Now we have another pest, the kudzu bug, which will likely require a similar spray strategy in order to get the pesticide down into the canopy where it needs to be. Although this pest can be found on upper leaves and stems, past observations from states to our south indicate that it likes to bury itself well into the canopy on stems and lower leaves. Therefore, revisiting nozzle and sprayer strategies for optimum control of this and other pests are warranted.

The single most important factor affecting prevention of disease and/or control of insect pests is good coverage of the plant with pesticide. Fortunately, the technology is available. You may however incur additional expenses with adaptation to current application equipment.

Remember the five major principles that result in satisfactory and economic control of the problem: 1) Positive identification of the insect pest or disease; 2) Correct pesticide; 3) Selection of the right equipment, particularly the right type and size of nozzle; 4) Timely application; and 5) making sure of the accuracy of equipment to confirm correct application amounts based on label recommendations.

First and probably most important, apply 15 to 20 gallons per acre of spray solution when using ground applicators. Only with enough volume can we penetrate a large soybean canopy and cover the leaves with pesticide. With aerial applications, this can be reduced to 5 gallons per acre (see the pesticide label for specific requirements).

Just as important, select the nozzles and pressures that will result in medium sized droplets (226 to 325 microns). Pressure is not as important. Depending on the nozzle selected, too high of pressure will just create fine and very fine droplets that are prone to drift. Too low of pressure may create large droplets that may penetrate the canopy but not coat the leaves.Flat-fan pattern nozzles are still the best choice as long as most of the droplets from these nozzles are categorized as medium. A flat-fan nozzle will normally be operated between 30 and 60 pounds per square inch (psi), with an ideal range of 30-40 psi. Extended-range flat-fan nozzles are a modification of the flat fan that allows the applicator to spray at lower pressures and still maintain uniform distribution. Regardless of the type of flat-fan nozzle selected, insure that the selected nozzle size will produce a medium-sized droplet at the operating pressure range and speed that the sprayer will be traveling. Most nozzle catalogs will contain this information. An example is shown below.The twin orifice flat-fan nozzle produces two spray patterns: one angled 30 degrees forward and the other directed 30 degrees backward. Such a nozzle may provide better penetration and coverage of plants with fully developed canopies. This is mainly due to being able to double the gallons per acre while still maintaining the desired droplet size. Still, hitting the target from two different angles, with one forward and one backward spray pattern may also provide a more effective coverage than spraying with just one spray pattern shooting down. Twin orifice nozzles can be designed with a twin spray pattern from one tip or special fittings/caps that allow the producers to place two nozzles in the same cap, one pointed forward, and the other one pointed backward. Nozzles producing cone pattern are not recommended for foliar soybean diseases; droplets are usually too fine and will not penetrate the canopy.Here are recommendations to help achieve the best coverage and control when spraying for soybean rust.

Keep spray volume above 15 gallons per acre for best results (20 gallons per acre is better).
Choose the appropriate size and type of nozzles and operate them at a pressure that will allow them to produce medium-size droplets (200-300 microns).
Nozzles producing a flat-fan pattern provide better coverage than the nozzles producing cone pattern when there is full canopy.
Use directed spraying, if applicable, to improve coverage.
Use twin nozzle/pattern technology. Two spray patterns, one angled forward and one angled backward, generally perform better than single nozzles spraying in one direction. This is primarily due to being able to increase the spray volume, while maintaining medium-sized droplets.

For more information on sprayer technology, see the following VCE publications:

Nozzles: Selection and Sizing. Virginia Cooperative Extension publication 442-032, May 2013; http://pubs.ext.vt.edu/442/442-032/442-032.html (accessed: June 2013)

Droplet Chart/Selection Guide. Virginia Cooperative Extension, publication 442-031, May 2009; http://pubs.ext.vt.edu/442/442-031/442-031.html (accessed: June 2013)

Asian Soybean Rust – Frequently Asked Questions VI: Sprayer and Nozzle Technology. Virginia Cooperative Extension publication 450-306, May 2009; http://pubs.ext.vt.edu/450/450-306/450-306.html (accessed June 2013)

Kudzu Bug Update

Posted on July 2, 2013 by David Holshouser

Ames Herbert, Extension Entomologist

The map below lists the progression of kudzu bug in Virginia from 2011 and 2012 (blue and orange counties) though this year (purple counties). As of June 27, 2013, we have documented kudzu bug (KB) infestations in soybean fields in 21 of those counties (Accomack, Amelia, Appomattox, Brunswick, Campbell, Charles City, Culpeper, Dinwiddie, Franklin, Greensville, Goochland, Hanover, Isle of Wight, Middlesex, New Kent, Orange, Prince George, Southampton, Suffolk, Sussex, and VA Beach). The problem is spreading quickly and almost daily I get word of an infestation in another county. If you find KBs in a soybean field in a county that is NOT listed, please contact me with that information. If you are growing soybeans (or crop advising) in a county on the list, you should make the effort to check fields. Although adults are still present, nymphs are hatching from eggs masses and dispersing to stems and petioles. Adult KBs have a strong aggregation pheromone that results in clusters on individual plants with many plants not infested. This will begin to change as nymphs emerge.

Their tendency is to disperse to new feeding sites, new plants or areas of plants which will result in a more widespread and more uniform infestation. As of the last week in June, the nymphs we are seeing are quite small. You can see them with your naked eye, but it takes either really good vision (those days are over for me) or a hand lens to see that those tiny light colored things on stems are indeed KB nymphs. This too will change as they gradually grow and molt into larger nymphal instars. Based on all that we know, we should try to keep the management recommendations as simple as possible, trusting those that have done the research—that using their recommendations will result in the best possible outcome: control at the least cost. As we move forward in the season, the best advice is to treat fields that are flowering or developing pods when an average of one nymph (big enough to see, see image below) is captured per sweep net sweep—or, 15 nymphs in a 15-sweep sample. If this situation is encountered, we are advised to treat that field. Remember, this insect is a slow feeder—gradually drawing down a plant’s vigor. This is good in a way, as this gives us plenty of time to sample fields and react with a treatment if needed. KBs do not eat holes in leaves and do not take bites from pods or seed. You may find nymphs and second generation adults on pods, but the damage is not direct like a corn earworm that eats the seed or a stink bug that punctures the seed. This is a new pest for us and we will all have to learn how best to deal with it. For now, we should abide by the recommendations above. Given the number of infested fields, I fully expect that some will have to be treated, eventually.
What about product choice. We have covered this in an earlier advisory (http://www.sripmc.org/Virginia). There are many good choices. Below is an insecticide efficacy chart that was developed by researchers at Clemson University and University of Georgia. They (and I) do not recommend using any product that falls below 80% control and the higher the better. I have been asked about a lot of products, some on this chart, some not. I go with the chart.