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Friday, August 15, 2014
Note: click on photo to enlarge, see captions or see other pictures.
SDS in Western Illinois. SDS symptoms began to develop this week in a soybean planting date study at the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) (Figure). The symptoms have become more severe and have been observed on more and more plants seemingly before our eyes as the week progressed.
Soybeans planted on April 17 exhibit the most severe symptoms on the greatest number of plants, followed by those planted on May 7. Those soybeans planted on May 22 have only very mild symptoms, while those planted on June 9 have no symptoms at this time (Table).
Table. Planting date, average air temperature and average soil temperature at a 4 inch depth under bare soil and sod for the 14 days following planting, current plant growth stage, and SDS symptom severity in a soybean planting date trial at the NWIARDC in 2014
Average temperature (in degrees F)
during the 14 days after planting
Growth stage: Aug 14
Relative SDS symptom severity
4" soil (bare)
4" soil (under sod)
R4 (full pod)
R5 (beginning seed)
R5 (beginning seed)
R5 (beginning seed)
The NWIARDC received ½ inch of rain between August 4 and 11, and distinct foliar symptoms began appearing soon after. The rain that has been forecasted for the weekend will likely speed symptom development.
The soybean variety in this trial is listed as having an SDS rating of "7" on a 1 to 9 scale, where a "1" is considered "poor" and a "9" is considered "excellent". This variety is also listed as being "highly" suited to growth in soil that contains the soybean cyst nematode as it carries resistance.
Although the variety planted in this study is considered to have a high level of partial resistance, it is not yet clear why severity is as high as it is at this point in the season. Perhaps conditions were perfect both at planting and now to overcome this variety's genetic resistance. Perhaps the soybean cyst nematode population in this particular field has exploded. Stay tuned for additional information about this trail as the season progresses into harvest.
Infection process takes place early. The pathogen that causes sudden death syndrome (Fusarium virguliforme) survives in soybean residue as thick-walled resting spores called chlamydospores. Infection takes place in the spring when chlamydospores germinate near a soybean seedling soon after planting. After infection, during vegetative growth stages, the fungus colonizes the layer of root cells that lies between the outer epidermis and the inner vascular tissue called the cortex.
The weather conditions to which germinating seeds and emerging seedlings are exposed can influence whether seedling infection is likely to occur. Cool soils that slow seedling emergence increase the chance of infection by prolonging plant-pathogen contact. This is why early planted soybeans are often more susceptible.
Symptoms most often occur later. There have been reports of very high F. virguliforme populations leading to root rots, although this is relatively rare. Most often, symptoms of sudden death syndrome (SDS) occur above ground, in soybean leaves.
When soybean plants reach reproductive growth stages and begin forming flowers and then pods, the fungus is able to make its way deeper into the root, colonizing vascular tissue. The vascular system is essential as it is responsible for moving water and nutrients from the roots to the rest of the plant and sugars from the leaves into roots. It is not yet known why the switch from vegetative to reproductive growth triggers vascular colonization.
Diagnosing SDS. Plants suffering from SDS exhibit foliar symptoms that can often be confused with stem and root diseases caused by other pathogens. Foliar symptoms of SDS most often begin to develop after a significant rain event when plants are in the reproductive growth stages.
F. virguliforme has not grown up and into soybean leaves. In fact, the pathogen continues to reside in the taproot and travels no farther into the plant than the first inch or so into the stem. The fungus produces compounds that are toxic to leaves. These toxins are moved as water is moved from the roots into the leaves.
Foliar symptoms. Foliar symptoms begin as blotchy yellow areas between leaflet veins on one or more plants in patches throughout the field (Figure). Symptoms become more severe with time as the tissue between the veins on leaflets turns brighter yellow, then brown (Figure). Leaves can eventually dry up and die (Figure) and may fall off, leaving the leaf-stem, or petiole still attached to the stem (Figure).
Stem and root symptoms. In order to distinguish this disease from other stem and root diseases and confidently make an SDS diagnosis, one must take a look at the inside of stems and taproots. Plants that have foliar symptoms, no internal stem symptoms and brown discoloration in the taproot are most likely suffering from SDS (Figures).
SDS and yield loss. Yield loss can result from flower or pod abortion if foliar symptoms begin during the early reproductive growth stages and in fewer and/or smaller seeds if symptoms begin in later stages. SDS-associated yield losses can be as high as 100 percent. In 2014, is not yet known how severe symptoms will get and how much yield may be affected.
Management options. There is no guarantee that fields in which there is a severe SDS epidemic this year will experience an epidemic next year. However it is important, particularly in those fields that have had an SDS epidemic, to keep this potentially devastating disease in mind and to take action to reduce the risk of disease in future years.
Unfortunately, there is no in-season rescue treatment to reduce yield loss. Management practices aimed at reducing SDS risk, must be completed at or before planting time.
Planting date. Planting into soil conditions that encourage rapid germination and emergence can reduce the risk of SDS. Prioritizing planting activities to make sure that those fields that have had a history of SDS are planted last, into warmer, drier soils is essential.
Genetic resistance. Check-off funds are going to several universities to fund SDS resistance breeding efforts. Seed companies often provide information about the level of SDS partial resistance in their varieties. For many years before the 2014 growing season, the Illinois Soybean Association funded unbiased, university research on the level of resistance or susceptibility in many of the soybean varieties grown throughout the state. Information from previous years continues to be available on the Variety Information Program for Soybean website. However as many of the varieties tested in years past may no longer be available for purchase, this resource will lose value over time.
Planting soybean varieties that also have resistance to the soybean cyst nematode can also help in SDS management (see below).
Seed treatments. The fungicide seed treatments that often come standard soybean seed have proven ineffective against SDS. However, a new seed treatment that has shown promise in university SDS research trails is currently undergoing the regulatory approval process with hopes of release in the 2015 growing season by Bayer CropScience.
Soybean cyst nematode management. SDS can be more prevalent and often more severe in fields that also have high soybean cyst nematodes populations. The soybean cyst nematode (SCN) overwinters as cysts (dead, swollen, pregnant female nematodes). SCN juveniles emerge from eggs in the spring to feed on soybean roots, creating small wounds. These wounds can help the fungus to enter roots. The fungus can also use the protective environment inside cysts to survive the winter. It is important to continue to make SCN sampling and management a priority.
Managing soil compaction. Some tillage operations, particularly when carried out under non-ideal soil conditions for tillage can lead to plow pan layers which restrict water drainage and root growth. Depending upon the soil type, breaking up plow-pan layers and speeding water percolation with tile drainage may reduce SDS severity.
References and additional information:
Rupe, J.C. and Hartman, G.L. Sudden death syndrome. 2008. In Compendium of soybean diseases. 4th ed. Hartman, G.L., Sinclair, J.B., and Rupe, J.C. eds. APS Press. St. Paul, MN.
Westphal, A., T.S. Abney, L.J. Xing and G.E. Shaner. Sudden Death Syndrome of Soybean. 2008. The Plant Health Instructor. DOI:10.1094/PHI-I-2008-0102-01
North Central Soybean Research Program: Sudden Death Syndrome.
Do you still or know of any other university's that do SDS variety testing?
by Dave on Friday 8/22/2014
Unfortunately, it seems as if the number of university researchers or other impartial groups that had routinely screened entries in variety trials for their level of resistance to SDS is shrinking.
In the past, this list has included Southern Illinois University (as part of VIPS), Iowa State University, University of Kentucky, and others.
Although I am not sure about 2014 trials, VIPS should contain data up to and including 2013 and the University of Kentucky has data from 2013 (MGs 3.9-5.6): http://www2.ca.uky.edu/cmspubsclass/files/pss/soybean/2014/pr672.pdf.
ISU also has a fact sheet, although it may be dated: http://soybeanresearchinfo.com/pdf_docs/SDS_vartest_ISU_2011.pdf.
If obtaining this type of information is a priority for you and your friends that farm, it wouldn't be a bad idea to let your state soybean association know that you feel that this type of basic farm-practical research is of value.
by Angie Peltier on Monday 8/25/2014