Category Archives: Disease

Soybean Rust Update: October 7, 2013

Posted on October 7, 2013 by David Holshouser

Hillary L. Mehl, Extension Plant Pathologist, Virginia Tech Tidewater AREC, Suffolk VA

Last week, soybean rust (SBR) was confirmed in several more counties in Virginia bringing the total to eight (Suffolk, Chesapeake, Virginia Beach, Isle of Wight, Hanover, Prince George, Brunswick, and Sussex). The level of infection on soybean leaflets was low, and though we have confirmed the fungus is present throughout the region, we are not seeing much disease development. This is probably due to dry weather conditions, and fortunately, this has allowed for most of the soybean crop to reach R6 and escape the threat of yield losses from SBR. The fact that the fungus has spread but caused little disease is a good illustration of how important environmental conditions are to the development (or lack of development) of disease epidemics. Even if the weather conditions do become more conducive, there is not enough time left in the season for SBR to reach high enough levels to impact yield. The SBR fungus arrived relatively early in Virginia this year, but it is unlikely to impact the soybean crop in the state.

The standard recommendation for control of SBR once it is confirmed within 100 miles of a crop is to spray soybeans that have not yet reached the R6 stage with triazole or pre-mix fungicides. However, an important caveat to the rule this late in the season is that the crop should also have good yield potential. Otherwise, there may be little or no economic benefit to fungicide sprays. Both Dr. Holshouser and Dr. Rideout concur that the threat of SBR to Virginia soybeans has likely passed.

As always, feel free to contact me if you have any specific questions or concerns.

Hillary L. Mehl, Ph.D.
Assistant Professor of Plant Pathology
Virginia Tech Tidewater AREC
6321 Holland Road
Suffolk, VA 23437
Telephone: (757) 657-6450, ext. 423
Cell: (530) 906-0807
email: hlmehl@vt.edu

Soybean Rust Update

Posted on October 4, 2013 by David Holshouser

Wow, leave the country for a couple of weeks and all hell breaks loose. Actually, it wasn’t that bad. But, the day that I arrived in Beijing, I received notice from Dr. Hillary Mehl, Extension Plant Pathologist, that soybean rust was discovered in Suffolk. This may have been the earliest that we’ve discovered the disease in Virginia. And it came in a year that many of our soybean had not yet reached the R6 (full-pod) stage. So, Dr. Mehl went into attack mode and begin letting everyone know of the discovery. Overall, I think that our communication system worked well.

Then the question of whether to spray or not began to arise. We took the conservative approach and recommended that all soybean that had not reached the R6 stage and was within 100 miles of the Tidewater AREC should be treated with a triazole fungicide. This recommendation really only affected our late-planted double-crop soybean as our full-season soybean crop and much of the double-crop soybean that were planted in late June had already reached the R6 stage.

Was this recommendation correct? Yes, I believe it was. But, only because we cannot predict future weather conditions. In actuality, the dry weather slowed down the disease’s spread. It really never got worse in Suffolk, where we first found it. Yes, it has now been confirmed in several other counties (see the map below for the latest update), but the incidence and amount of leaf coverage is very low. So, in the end, a fungicide application would probably have done little good. But, we will verify this with our research plots.

Will the disease continue to spread? If we get the rainfall early next week, I think we’ll see it start sporulating again and begin spreading a little faster. But, I’d think that most of our soybean are safe now. If the soybean hasn’t yet reached R6, then a frost may make the disease irrelevant. Plus, it’ll take a few weeks before the disease will get to yield-reducing levels. By that time, our crop will like be physiologically mature.

We’ll keep looking an tracking the disease’s spread and incidence. We should be able to learn much from this year. And this knowledge will help us to make better decisions in the future.

Soybean Rust Found in Suffolk, VA

Posted on September 18, 2013 by David Holshouser

Soybean Rust Update: September 18, 2013

On September 17, 2013 soybean (SBR) rust was observed and confirmed on soybean leaves collected from the Tidewater AREC sentinel plots on September 5. Suffolk is the only county in Virginia thus far with confirmed soybean rust, but we will now intensify our scouting efforts throughout the soybean-growing regions of the state. Extension agents and growers should continue scouting for SBR in their respective counties and submit soybean leaves to the Tidewater AREC Plant Disease Clinic for evaluation of SBR and other foliar disease.

So far in 2013, SBR has been confirmed on soybeans in 185 counties/parishes in 10 states in the U.S. (AL, GA, FL, MS, LA, SC, AR, NC, TN, and VA) including two counties in North Carolina and one county in SE Virginia.

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. Since August, soybean leaflets from sentinel plots at the Virginia Tech Tidewater AREC in Suffolk, VA have been evaluated on a weekly basis, and as in prior years, the first report of SBR in Virginia was from one of these sentinel plots. Other diseases in soybeans at this time include Cercospora blight, brown spot, frogeye leaf spot, and anthracnose.

Risk of yield loss in soybean is minimal if SBR is detected following the R6 development stage. However, growers with soybeans that have not yet reached the R6 stage should consider spraying fungicides for control of SBR (triazole or pre-mix fungicide). This is particularly relevant for late-maturing/double-cropped soybean.

As part of the scouting effort and to track the spread of SBR throughout Virginia, extension agents are encouraged to submit soybean leaf samples (50 leaflets per sample) for evaluation of soybean rust and other diseases. Moisture is required for infection and development of SBR, so it is best to collect leaves from fields shaded from the sun in the morning since these areas will hold moisture within the canopy longer. A detailed protocol for scouting SBR is attached. Samples should be submitted to the Tidewater AREC with the attached SBR diagnostic form.

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

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

Soybean Rust Moves into North Carolina

Posted on September 6, 2013 by David Holshouser

Asian soybean rust was confirmed earlier today in Scotland County, NC. Scotland County is on the NC/SC border (see map below). There were sporulating pustules on 5 of 50 leaves examined. No soybean rust was found on leaf samples taken from Lenoir County, which is closer to Virginia. This puts soybean rust approximately 140 miles from South Hill, 160 miles from Emporia, and 190 miles from Suffolk.

In general, we will only recommend a fungicide spray if soybean rust has been confirmed within 100 miles of our soybean fields. Therefore, there is no need to apply fungicide for control of soybean rust in Virginia at this time. Applying fungicide too early will only reduce its effectiveness once the disease arrives. Furthermore, soybean yield will not be affected if rust infects the crop after the R6 development stage (seed touching each other in the pod); therefore, fungicides are not recommended after the R6 stage, even if soybean rust is found close by.

Although soybean rust is not close enough to Virginia to initiate fungicide sprays at this time, the disease has moved faster than previous years. Most of our full-season (May-planted soybean) have reached the R6 stage (full-seed); therefore, these soybean are “safe” from any yield loss that may result from soybean rust infestation. However, much of our double-cropped soybean are still susceptible; they are anywhere from the R3 (early pod) to R5 (late pod) development stages. We will continue to monitor soybean rust movement across NC, continue to check soybean fields in Virginia, and let everyone know immediately if soybean rust is found in or close to Virginia.

For more details on Asian soybean rust and its movement, see the following website:

http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi

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.

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.