Guess the Pest! Week #23 Answer: Sudden Death Syndrome of Soybean

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to Lamar Witmer for correctly identifying the disease as sudden death syndrome of soybean and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #23 Answer: Sudden Death Syndrome of Soybean
by Nancy Gregory, Plant Diagnostician; ngregory@udel.edu

Sudden death syndrome of soybeans (SDS) is caused by the fungus Fusarium virguliforme. We started seeing this disease in Delaware in 2002 in cool and wet seasons, but have seen it more often in the past few years. SDS can be confused with other stem diseases such as Phomopsis stem canker and charcoal rot. Leaf symptoms of yellowing and browning between the veins are typical, and leaves shrivel and fall off, leaving petioles still on the stems. If stems are pulled up and placed in a plastic bag overnight, blue spore masses of the fungus may be seen at the base of stems. The internal stem tissue (cortex) may show dark discoloration. There is a toxin produced by the fungus that is responsible for the symptom pattern showing up at the top of the plant. The fungus overwinters in debris, and disease is most severe when infection occurs early. Improving drainage, alleviating compaction, and treating seed may help get seedlings established.

Soybean Sudden Death Syndrome in DE and MD

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu; @Delmarplantdoc

Over the past three weeks there have been increasing reports of soybean sudden death syndrome (SDS) in full season soybeans. SDS is one of the top ten most important diseases affecting soybeans, but is still a minor player in the Mid-Atlantic. Yield loss can occur when foliar symptoms show up before R5.

Sudden Death Symptoms on a soybean leaf.

Infection of soybeans typically occur early in the growing season, when conditions are cool and wet and plants are in the vegetative stage of growth. The fungus responsible for SDS then infects the lower portion of the stem. When plants reach the later reproductive stages, the fungus can produce a toxin that moves with the water conducting tissues into the foliage, where it accumulates. Accumulation of the toxin results in the development of foliar symptoms. Leaves of infected plants develop yellow spots, which expand, while the leaf veins remain green. Roots will often be rotted, and the lower stem, when split, will be brown on the outside while the inner tissue will remain unaffected. This stem symptom can help differentiate between SDS and Brown stem rot (BSR) and BSR will rot the entire central stem column, resulting in a brown, dry rotted appearance. Foliar symptoms alone are not diagnostic of SDS. Occasionally blue fungal growth can be observed on the root or soil surface, especially after heavy rains. In Delaware and Maryland, we see SDS in small patches, often 15 yards across or less. Many times affected areas are found in low lying areas that hold more water.

Blue growth on the root and soil line is indicative of Sudden Death Syndrome.

Foliar symptoms of SDS can cause leaf curling and death.

When split, lower stems will be necrotic and brown, while the inner tissue will remain relatively untouched.

Management of SDS starts with crop rotation to a non-host, such as corn for at least two to three seasons. Unfortunately the pathogen can still overwinter and survive on corn and other residues for quite some time, so in severe cases, you will need at least three years out of soybeans. Thusfar, I have yet to see a field where infection was severe enough to warrant this approach. When rotating, even short term, rotations with snap or lima beans should be avoided.

If you have soybean fields with known SDS issues, select a variety with tolerance to the disease to minimize potential yield losses. Tolerant varieties are much less likely to suffer severe yield losses if the disease does occur.

Although the seed treatment iLeVO has been shown to help reduce SDS, it may not be economically sound to plant entire fields with this seed treatment if only sections or small portions of the field are affected. If considering this seed treatment, one potential route would be to scout and map out affected areas and plant treated seed in the field sections or areas where SDS is problematic, and plant your untreated or regularly treated seed in unaffected areas of the field.

The pathogen infects when it is cool and wet. Therefore, double cropped soybeans are less likely be infected by the SDS pathogen. They can still develop the disease, especially if persistent cool wet weather occurs during the vegetative stages, but the likelihood for this is far less than in full season beans.

Finally, if you have a field with SDS and are working the ground, ensure that you clean the equipment before utilizing it in another field or till SDS impacted fields last. This will minimize potential movement of the pathogen to additional

Corn and Soybean Disease Update

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu

Until this week conditions have not been favorable for significant levels of disease development in corn and soybean. Most full season soybean is between R1 and R2, and low levels of Septoria brown spot and Downy mildew are present at many sites. Double crop beans are just getting out of the ground and are perhaps at V5 is some of the early fields. In corn, Grey leaf spot is now present in many fields, although levels have been very low in the majority of fields I have examined. Potential irrigation burn was observed in one field, and some smut has been found at a couple of sites. Things to keep an eye out for in the next couple of weeks are Sudden Death Syndrome in full season soybeans and Anthracnose top dieback in corn. Many full season beans were planted into cool, wet soils, which favor infection by the SDS pathogen. As the beans mature the fungus will produce a toxin, which results in the characteristic interveinal necrosis often associated with SDS (Figure 1). Blue fungal growth may be seen on the base or the roots of infected plants. SDS may be associated with Soybean cyst nematode, so full season beans planted into fields with a history of SCN may have a greater potential to show disease symptoms.

SDSsoybean

Figure 1. A leaf with onset of SDS symptoms. Several other disorders and diseases can cause similar symptoms.

With the amount of anthracnose we have seen this year there may be a greater potential to see anthracnose top dieback in some corn fields. Typically the top dieback phase starts two or three weeks after pollination. Top dieback caused by the anthracnose pathogen is characterized by yellowed, purple, or dead/dying flag leaves on plants scattered throughout the field. When the leaf sheaths are peeled back at the top of the affected plants, shiny black lesions can be observed on the outside of the stalk. Under appropriate conditions a salmon-colored gel can be seen on the stalk. This gel contains spores of the fungus. If the stalk of the top is split, the pith will often appear discolored and rotted in the upper internodes. Factsheets for SDS and Anthracnose can be viewed on the University of Delaware Field Crops Factsheet site (http://extension.udel.edu/factsheet/), hard copies will be available at your county Extension office soon.

A final note- it is too early to see symptoms of red root rot and stalk rots in corn. These diseases occur much later in the season. Remember, many things can cause red brace roots.

Soybean Disease Updates – August 23, 2013

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu

Sudden Death Syndrome in Soybeans
There have been a couple of reports of SDS in Delaware and Maryland, and the UD Plant Diagnostic Clinic confirmed one sample earlier this week. This disease is rare in Delaware, but does pop up from time to time. SDS is caused by the soilborne fungus Fusarium virguliforme. Plants infected with F. virguliforme typically do not present symptoms until after flowering (R1). Early symptoms of the disease include mottling and crinkling of the leaves. Tiny yellow to white flecks appear between leaf veins. As the disease progresses the leaf tissue between the veins turns yellow/brown, while the veins remain green (Figure 1). Soon thereafter the leaves shrivel and fall from the plant, but the petioles remain intact. If the plant is removed from moist soil tiny blue structures may be visible at the base of the stem. These are spore masses produced by the fungus. In most cases diagnosis is completed by sectioning the lower portion of the stem lengthwise. The cortex of a stem infected with SDS will be streaked with tan/light brown lesions, whereas healthy plant stems remain white. The pith of infected plants also retains a white coloration. Symptoms of SDS can be confused with other pathogens of soybean including charcoal rot. Stems infected with charcoal rot fungus contain round black/gray structures that look like tiny bits of coal (hence the creative name) or zones delineated by sharp black margins. The roots of SDS infected plants will also be rotted. You will not see the combination of symptoms with plants with nutrient issues, chemical burn, or insect damage.

F. virguliforme overwinters in the residue or free soil as recalcitrant spores, which are resistant to a wide range of temperatures and stresses. The pathogen typically infects plants while they are in the seedling stage, and infect the cortex between V1 and V6. When plants begin flowering the pathogen colonizes the cortex more thoroughly, and toxins are produced that are translocated to foliage. These toxins are responsible for the characteristic interveinal necrosis. Symptoms often appear during heavy rains during the reproductive stages and disease is favored by high soil moisture. Consequently, the disease typically is present in areas of the field that are poorly drained (low lying or compacted areas). The disease is spread short distances on mechanical equipment, workers boots, etc., and spores can be disseminated very short distances in rain.

An interesting twist to the story is that SDS is often found in association with Soybean Cyst Nematode and the pathogen can be isolated from within cysts of SCN. Thus the nematode may play a role in pathogen spread or infection. Consequently, if you detect SDS in your field you should consider sending a soil sample to the UD Diagnostic clinic to determine SCN levels and drive future planting considerations.

The options for management of SDS are limited. Some soybean cultivars are more tolerant to SDS than others and resistance ratings can be obtained from seed dealers or directly from seed companies. A variety should be used that has both SDS and SCN resistance. Avoid planting soybean into cool, wet soils. If you have a field with a history of SDS consider planting it last to minimize the chance of infesting additional soybean fields. If you have a chronic issue with poor drainage in areas of your field, consider improving drainage in the area. A combination of rotation to corn and minimal tillage has been shown to reduce SDS levels in some cases. Overall SDS is not a major issue in Delaware. Plants can recover from this disease and I have only heard about isolated parts of the field having disease.

 SoybeanSDSFigure 1. A leaf with symptoms of SDS. Foliar symptoms are not diagnostic of the disease and can be caused by other agents.

More Reports of Frogeye Leafspot on Soybean
Frogeye leafspot has been detected throughout the region, and may be more prevalent than in normal years. I detected the disease at low levels in full season soybeans in mid-July (http://extension.udel.edu/weeklycropupdate/?p=6038). Symptoms of the disease include tan to brown spots with a purple/dark red halo (Figure 2). When inspecting lesions with a hand lens, it may be possible to see black dots with grey/silver-colored spores at the center. The pathogen infects young leaves more readily than older leaves, but it can take up to two weeks for symptoms to develop. As a result, plants may have a layered appearance. Warm wet conditions, especially warm nights with heavy dews, favor disease development.

Typically this disease moves in late in the season and within season management is not required. However, this year we have many fields of late-planted double-crop beans. These fields should be scouted for frogeye. In general, if the disease is detected in the mid canopy at early reproductive stages (R1-R3) and your risk level is high, a fungicide may be warranted. Factors that increase disease risk in double crop beans include: 1) the use of a Frogeye susceptible soybean variety; 2) presence of a significant level of disease in the field; and 3) favorable weather. Research indicates that applications made at R3 tend to be most effective in states where this disease is common. Economic factors should also be weighed carefully when making any decision to spray fungicides.

Frogeye can be managed between seasons by selecting varieties with resistance to Frogeye leaf spot. Ratings can be obtained from seed companies or dealers. Most varieties with resistance to Frogeye leafspot are group V or higher, so your selection may be somewhat limited. Rotation with non-hosts, such as corn and small grains will help reduce inoculum levels and disease severity. Residue management, when practical, can also help reduce the impacts of this disease.

SoybeanFrogeyeLeafspotFigure 2. A leaf with symptoms of Frogeye leaf spot

Update on Soybean Rust
Several people have asked about the status of soybean rust this year, particularly because planting of double crop soybeans was delayed due to inclement weather. Earlier in the season there was some concern from the South that soybean rust may be a greater concern than in years past, mainly because the environment was conducive to the development of the disease. Consequently, the pathogen was detected in sentinel plots and commercial fields 2-3 weeks ahead of schedule. The disease has developed more slowly than anticipated, although disease progress is still ahead of pace for many Southeast soybean growing regions and has spread more rapidly in August. As of August 19, 80 counties in 8 southern states have reported soybean rust (Figure 3). At this time last year 50 counties reported soybean rust, and during the dry season of 2011, only 5 counties reported the disease. Currently the disease has not been detected in our neighboring states.

Soybean rust moves via spores blown from the south. Recent forecasts from the SBR ipmPIPE website (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi) indicate that Delaware is not likely to receive any spores in the near future. It is not likely that rust will make it here in time to cause any damage to our soybeans, even late-planted double-crop beans, but I’m not quite ready to throw in the hat quite yet. The take home message: we are not at risk for soybean rust at the current time. We are closely monitoring the situation and will keep you informed of the status throughout the remainder of the soybean growing season.

SBR19Aug2013

Figure 3. Soybean rust confirmations as of August 19, 2013

Field Crops Disease Blog
Check out my field crops disease management blog article for the week: Faux Cercospora leaf blight (AKA sunscald): http://extension.udel.edu/fieldcropdisease/. You can now subscribe to my blog and receive updates via email by clicking the “contact me” button under my picture and entering your information into the pop up that appears. Make sure that you check off the box at the bottom to subscribe to blog updates.

Spraying Soybeans with Fungicides After Hurricane Irene

Arvydas (Arv) Grybauskas, Extension Plant Pathologist, University of Maryland; arvydas@umd.edu

The question has come up after the tremendous rainfall and damage caused by hurricane Irene whether or not one should spray their soybeans with fungicides. There was also a question just before the hurricane as to whether or not it was advisable to throw in a fungicide since many fields were to be sprayed for insects like podworm and stinkbugs. The answer to both those questions is highly dependent on each individual situation and a blanket recommendation cannot be made, especially because soybean rust is not going to be an issue this season.

Before we discuss the factors that play a part in determining fungicide need let me briefly explain why we rely on statistics and statistically valid research to answer these questions. Most of you are familiar with yield monitors and their output either because you have equipped your combine with one, or the custom combining operation that does the harvesting has one, or you have seen sample maps in a trade publication. If you haven’t then I’m sure you are still aware of the issue of variability in a field because of all the hoopla associated with any precision Ag equipment article or ad. These have been invaluable tools as they have really shown that an apparently uniform 40 acres is really quite variable. The obvious reasons for the variability are variations in soil type, depth, moisture and fertility just to name a few. But there are also many other less obvious reasons associated with pests since they are not uniformly distributed across any field as well. Some fields are more variable than others but every field is to some degree not uniform. The fact is if you took any production field and arbitrarily split it in half the average yield in each half would not be identical. In fact I would suspect that the vast majority in our area would easily have about 3-5 bu/A yield difference between each half. Again some might have greater uniformity and a smaller difference and some might have more. It is this variability that illustrates the need for statistics. Think of it this way. If the same field that we split arbitrarily in half produces yields that are different by 3-5 bu/A without a treatment then how do we know if we put a fungicide (or anything else) on half that it did anything? We use statistical tools and design experiments following certain statistical rules to answer that question. Two of the most important rules are replication and randomization. We will replicate so that we test a treatment more than once and get a measure of the variability in the response. We will randomize where each replicate of each treatment goes in the field to make sure they all have an equal chance of being in a high yield spot or low yield spot in a field. By using these and other tools we can then use the variability that is there to say whether or not a treatment has a certain probability of having an effect. The point is a bigger number doesn’t always mean that the treatment has a real or significant effect for those conditions. Sometimes a 3-5 bu yield difference could just be chalked up to being part of the background variation.

Ok, getting back to should we spray fungicides on soybeans after this heavy rainfall and wind damage. Let’s start with the damage. There are have been no independent scientifically sound experiments that show a consistent and reliable yield advantage to spraying fungicides in the absence of diseases. Physical damage by itself is not going to be corrected by a fungicide. Some fungicides reportedly have a positive effect on plants by counteracting components of plant stress responses. However, there is no proof from independently conducted field trials that this affect is large enough to affect yields in the field in non-diseased plants.

Stands that are lodged, leaning or matted down because of high winds tend to favor the development of certain diseases because the canopy remains closed and therefore remains wet for longer periods. Also, the distance a fungus needs to move either from leaf to leaf or soil to leaf in lodged or leaning stands is generally less. Heavy rains play a more important role than the stand damage in driving disease development. The rain from Irene provided two main avenues for greater disease development: spread of disease-causing agents if they were present; and a wet or humid environment that favors pathogen growth and infection of plants. A key that cannot be ignored is there has to be a pathogen around and in a form capable of being moved.

There are soybean diseases that will not respond to foliar fungicides. Bacterial blight and Bacterial pustule are diseases that are favored by hard driving rains. They look very similar to several fungal diseases, but because bacteria cause these fungicides cannot control them. Don’t get talked into spraying just because there are some leaf spots. There are also some fungal diseases that will not respond to foliar applied fungicides. The stem and root diseases in particular, Charcoal rot, Fusarium wilt, SDS and Phytopthora all are fungal diseases that cannot be controlled either preventatively or curatively at this time of year. One on-line source of brief disease descriptions with pictures is http://cipm.ncsu.edu/ent/ssdw/soyatlas.htm .

The soybean diseases that can occur in Maryland that can have an impact on yield, and can be managed with foliar applied fungicides are primarily: soybean rust, frogeye leaf spot and brown spot. Soybean rust due to the earlier drought in the South is not likely to be a problem this year. Its reestablishment and increase on kudzu has been severely delayed and is at the lowest level of infestation in August since monitoring for this disease began in 2005. Hurricane Irene therefore was highly unlikely to pick up enough spores from Florida and deposit them in Maryland to be an issue. Of the remaining two diseases Frogeye appears to have more potential for damage but is fortunately less common than Brown spot. Brown spot is the most common foliar disease but tends to be a lower canopy problem. Both are much more likely to develop if the previous crop was soybeans. I cannot find any data that would indicate there is much of a chance of getting a yield response from a fungicide by controlling frogeye or brown spot when the fungicide is applied after the R3 growth stage. The reproductive growth stages are: R1 initial flowering, R2 full flower, R3 small pods are formed at upper nodes, R4 full pod, R5 beginning seed, and R6 full seed. One on-line source for soybean growth stages can be found at: http://extension.agron.iastate.edu/soybean/production_growthstages.html . There are other common diseases that tend to form late in the season and tend to be considered seed quality issues. Another way to understand these late season diseases is that fungicides reduce them but without a significant change in yield. Only seed quality is improved. Although seed size is often improved, only rarely does that translate to significant yield increases. The most common of these diseases are purple seed stain, pod and stem blight, and anthracnose. There is a third category of generally late appearing diseases that are reduced by fungicides but we have little evidence that they respond to the fungicide with a yield increase, for example Phyllosticta leaf spot. In short as a general rule it takes a fair amount of foliar disease that starts early in the reproductive period, if not before, to cause enough damage to reduce yields that can be countered with a fungicide.

With regard then to our current post-Irene situation, here are my thoughts on fungicides in soybeans.

● Scout first. If you know there is a specific problem that is starting then you can much more easily decide on the need for a fungicide.

● If you are a seed producer and the soybeans are between R1-R3 then consider spraying. You will protect some seed quality and have the potential for managing brown spot or frogeye if present and in that case may see a yield benefit. If the beans are between R4 and R5 then consider spraying if you haven’t sprayed earlier. However, expect only a seed quality improvement.

● If you are growing soybeans for general production and are between R1-R3. Scout first! If you have symptoms of frogeye leaf spot or of brown spot in the middle of the canopy then a fungicide application may be warranted. This is much more likely to occur if the soybeans were planted no-till into soybean stubble. If the field is at R4 and symptoms of frogeye or brown spot are evident then spraying is much less likely to improve yields. If the soybeans are at R5 then there is little evidence that you will get your money back from a fungicide application especially if it is just brown spot. If there is frogeye, it’s R5, and it continues to be wet the disease will get worse but there is no evidence that you will get a return on the fungicide. If you are not comfortable with not spraying then consider spraying in strips. In essence run your own test. It will cost you less and you will have a better basis for making the decision next time.

● Knowing the field history and rotation history are helpful. If you have soybean planted no-till into soybean stubble (it could be wheat-bean or barley-bean stubble) then you have the best chance of having diseases that can be reduced with fungicides and therefore get a yield response with a fungicide. The next level of risk based on crop rotation is when soybeans are just a year apart in the rotation, e.g., full-season beans followed by corn followed by wheat or barley and then double-crop beans. The lowest level of risk is there is a two-year or more rotation to soybeans. The most likely thing to happen with a fungicide application at R3 to R4 is improvement of seed quality (for example reduced purple seed stain or improved test weight) but not necessarily improve yields. The higher the risk based on rotation then the higher the chance a yield improvement will be attained. In my opinion, you have a 30-50% chance of getting a yield boost (3-5 bu/A) in soybeans with the higher chance coming from the higher risk situation and spraying at R3.

● Last point. Spraying for yield protection without knowledge of what is out there has already led to the development of a strain of frogeye that is resistant to strobilurins (Headline, Quadris, etc.). This has been confirmed in IL, KY and TN. It’s not a very good insurance program to spray against diseases when you just spray for yield boost because you are destroying one of the best tools you might need later on.

Soybean Disease Update – August 26, 2011

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Sudden death syndrome was diagnosed on soybean from Sussex County. This was an unexpected find since I associate SDS with cool, wet seasons. The only control for SDS is to plant resistant varieties, or, if susceptible soybeans are to be planted, double crop soybeans are rarely infected. I would not expect this to be widespread but we did see it so be on the lookout for symptoms.

Foliar symptoms of SDS

 Infected root on the left with diseased leaf compared to healthy stem and leaf.

Charcoal rot was also confirmed on soybean as well as corn. Soybeans are killed by the fungus resulting in dead, stunted plants in irregular patterns in the field. Symptoms are worse on full season early maturing varieties (Maturity Group III and possibly early Group IV’s). The diagnostic symptoms are the ashy gray stems and lack of roots. Splitting the lower taproot and stem longitudinally with a knife reveal the best diagnostic feature which is the presence of the small black microsclerotia in the pith and often the cortex. Charcoal rot is favored by hot, dry weather and is common in Delaware soils that have been heavily cropped to soybeans over the years. The tiny sclerotia overwinter in the soil and it just takes the right environmental conditions for the disease to occur. There is no usable resistance in soybeans to charcoal rot. The stress during flowering and early pod set triggers development of symptoms if that coincides with hot, dry, stressful weather. Most initial infections occur during the seedling stage and remain latent until flowering and later. Later maturing soybeans typically flower and set pods most years when conditions are less stressful and escape the yield reducing effects of the infection. Double crop soybeans are rarely affected by charcoal rot here in DE. Two year rotation will help reduce microsclerotia levels in the soil but if corn is used at least three years is suggested. Longer rotations would be better if they could be implemented.

 Split soybean stem showing the charcoal gray discoloration in the pith with microsclerotia.

 Close up of gray pith from the presence of the tiny microsclerotia of the fungus that causes charcoal rot.

Soybean Disease Update

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Powdery mildew is present on some varieties. It is not a yield limiting disease and control is not warranted. See the article in WCU 17:23 for more information on this disease. Downy mildew on soybeans is very common on varieties with no or limited resistance. It too is not thought to be yield limiting here. A few growers have asked about white mold in soybeans. This disease is caused by Sclerotinia sclerotiorum, the same fungus that causes white mold in snap beans, lima beans and peas. Soybeans that are rotated with these crops in fields that have had the disease in the past are most at risk. This disease occurs sporadically in Delaware. Conditions favoring white mold are usually present when we have had lots of rain and the temperatures are moderate during flowering. Crops that are lush and dense with tight canopies are most at risk since the lower parts of the plant stay wet for long periods of time. During dry seasons, growers that over-irrigate soybeans during the late flowering to early pod fill have induced it on fields with a history of white mold. The fungicides that are used on soybeans for disease control and plant health are not effective for white mold. Fungicide applications have not been very successful at controlling white mold.

Sudden death syndrome (SDS) was confirmed from the two finds last week from New Castle and Sussex counties. Several more fields have been diagnosed this week. We have not seen this disease in Delaware since 2000 when it was first identified. The reason we are seeing it again is that we had weather conditions that were very favorable for SDS, just like in 2000. It has to be cooler and wetter than normal for us in the early part of the season for SDS to appear. What does SDS look like in the field? Yellow blotches form between the veins, usually developing first on the uppermost leaves. In a few days the yellow blotches will coalesce and begin to turn brown. The end stage is complete tissue death between the veins, with the only green tissue remaining being that associated with the primary leaf veins. The edges of severely diseased leaves will roll inward. Over time, the diseased leaflets may fall off the leaf stalks (petioles) or they may remain attached to the plant. When you dig up the infected plants primary, secondary and tertiary roots are severely rotted. Nitrogen-fixing nodules are mushy. The exterior of the stem appears healthy but the interior of the stem is a milky-brown to gray color, compared to the yellow-white color of a healthy stem. Serious yield loss usually only occurs when plants are exhibiting serious foliar symptoms BEFORE mid-pod fill. After that time, plants can look pretty rough, but yields may not be affected much. Individual and groups of plants, 10-50 feet in radius, usually show a range of symptoms ranging from some leaf spotting to complete defoliation. Wet or otherwise stressed areas of fields from compaction or other causes, such as along field edges, will usually be the first to develop symptoms. In extreme cases, entire fields may show symptoms. When SDS is severe, symptoms will first develop in “hot spots” and later progress into other areas. This gives the effect that the disease is spreading, but in reality it is not. Rather the time of infection, crop health, and field conditions vary, so disease symptoms are expressed at varying times and rates.

Control of SDS
The only control is reducing plant stress by reducing compaction, and planting resistant or tolerant varieties. Rotation is of little to no value in controlling SDS. Be careful to check plants carefully for these symptoms because stem canker can also produce similar symptoms. The following pictures will give you a good idea of what SDS looks like in the field:

field shot of soybeans with SDSField shot showing the browning leaves and some defoliation

early leaf symptoms of SDSEarly leaf symptoms

later leaf symptoms of SDSLater leaf symptoms showing interveinal necrosis

SDS causes leaves but not petioles to fall off the plantLeaves will fall off but the petioles (leaf stalks) remain attached

stem of healthy soybean (lower) and soybean with SDS (upper)The bark has been removed from the lower stem and tap root to show the brown discoloration of the vascular tissue under the bark of the diseased soybean compared to the healthy white stem on the bottom plant.

Soybean Rust Report
On September 2, soybean rust was reported on soybean from a sentinel plot in Dorchester County, South Carolina just 30 miles from the Atlantic Ocean. This area has received abundant rainfall recently while the rest of the state has been dry. The amount of rust at this site is very low at the present time. On September 1, soybean rust was reported on soybeans in Jefferson and Phillips Counties, Arkansas; Panola County, Mississippi; and Macon and Miller Counties, Georgia. The disease was also observed in soybean sentinel plots in Lee and Macon Counties, Alabama, as well as on kudzu in Crenshaw County in that state. On August 29, soybean rust was detected on kudzu in Jackson County, Florida. On August 28, soybean rust was reported in Drew, Lincoln, Desha and Lee counties in Arkansas; Lafayette, Morehouse and West Carroll parishes in Louisiana; Attala, De Soto and Madison counties in Mississippi; and in Greene County, Alabama.

The total number of counties reporting soybean rust in 2009 has more than doubled the number that had reported rust on the same date in 2008. Rust is moving in areas that have had both the moisture and temperatures that favor rust in the South. Monitoring will continue here in Delaware. Many Group 3 and 4 soybeans planted in mid to late May are approaching R6 or later and would not be at risk if SBR would move north in a tropical storm event. Late double crop soybeans will be the most at risk if the weather pattern should change and we get some tropical storms or a hurricane. Most of the rust activity is in the Mississippi River Valley heading north into Arkansas. Besides the scattered reports of SDS, powdery mildew and downy mildew soybeans look good from a plant disease perspective.

sbrmap3Sep09

Soybean Sudden Death Syndrome

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Sudden death syndrome (SDS) has been tentatively identified from a soybean field in Sussex County and several fields in New Castle County this week. We have not seen this disease in Delaware since 2000 when it was first identified. The reason we are seeing it again is that we have had weather conditions that were very favorable for SDS just like in 2000. It has to be cooler and wetter than normal in the early part of the season for SDS to appear. What does SDS look like in the field? Yellow blotches form between the veins, usually developing first on the uppermost leaves. In a few days the yellow blotches will coalesce and begin to turn brown. The end stage is complete tissue death between the veins, with the only green tissue remaining being that associated with the primary leaf veins. The edges of severely diseased leaves will roll inward. Over time the diseased leaflets may fall off the leaf stalks (petioles) or they may remain attached to the plant. When you dig up the infected plants primary, secondary and tertiary roots are severely rotted. Nitrogen-fixing nodules are mushy. The exterior of the stem appears healthy but the interior of the stem is a milky-brown color, compared to the yellow-white color of a healthy stem. Serious yield loss usually only occurs when plants are exhibiting serious foliar symptoms BEFORE mid-pod fill. After that time, plants can look pretty rough, but yields may not be affected much. Individual and groups of plants, 10-50 feet in radius, usually show a range of symptoms ranging from some leaf spotting to complete defoliation. Wet or otherwise stressed areas of fields from compaction or other causes, such as along field edges, will usually be the first to develop symptoms. In extreme cases, entire fields may show symptoms. When SDS is severe, symptoms will first develop in “hot spots” and later progress into other areas. This gives the effect that the disease is spreading, but in reality it is not. Rather the time of infection, crop health, and field conditions vary, so disease symptoms are expressed at varying times and rates. The only control is reducing plant stress by reducing compaction, and planting resistant or tolerant varieties. Rotation is of little to no value in controlling SDS. Be careful to check plants carefully for these symptoms because stem canker can also produce similar symptoms.

foliar SDS symptomsFoliar symptoms of SDS

SDS root symptomsInfected root on the left with diseased leaf compared to healthy stem and leaf