Saturday, August 23, 2014

Got Sudden Death Syndrome (SDS)? What was Your Soybean Planting Date?

Sudden Death Syndrome (SDS) is caused by the fungal pathogen, Fusarium virguliforme and is considered, by some, to be the “#2” most devastating disease to soybeans.  It survives in soybean residue as thick walled, fungal survival structures.  It actually can infect soybean seedlings early in the season, if environmental conditions are conducive for pathogen infection.  During the soybean vegetative growth stages, this fungal pathogen that causes SDS can colonize the root cortex (outer root) if soil conditions are cool and wet.  In some cases infection can still occur in warmer soils, but infection may not be as severe. During the soybean reproductive stages, when soybeans are flowering or putting on pods, this fungal pathogen can grow further into the center of the root or into the vascular tissue, and into the lower stem.  This is where the pathogen that causes SDS can disrupt the water and nutrient uptake by the soybean plant.  This fungal pathogen then produces toxins that move as water to the top of the plant and cause leaf symptoms to appear.  These leaf symptoms often appear after a significant rainfall.  After leaf symptoms appear, it is possible that leaves may defoliate from the plant.  

SDS symptoms may first appear as a yellow patch out in the soybean field, but a closer look will reveal yellowing or browning between the leaf veins.  Many different diseases such as root rot or canker diseases can cause similar leaf symptoms.  Brown stem rot is the most commonly confused disease and can be easily distinguished from SDS if the soybean stem is cut down the center.  The pith or the inner part of the stem will be brown if the soybean is infected with the pathogen that causes brown stem rot.  The inner part of the stem will be white if the soybean is infected with the pathogen that causes SDS.  The roots of a plant with SDS will be rotted and may contain a bluish mold on the outer roots, but this may be hard to find, so it is not a reliable diagnostic characteristic.
SDS symptoms just appearing after a heavy rain in compacted areas of a field.

Pictures of SDS symptoms provided by John Howell, Burrus Account Manager

Pictures of SDS symptoms provided by John Howell, Burrus Account Manager
Even though some soybean varieties are rated as having good resistance against this disease, there is no soybean that is totally resistant to SDS.  Often times, but not always, SDS infects soybeans that have been infested with soybean cyst nematode (SCN).  Soybean fields that are most at risk are low lying areas or compacted areas that are poorly drained, which can collect water.  However, the key to the impact of this disease is the environmental conditions early in the season, when this fungal pathogen infects soybean roots.  SDS pathogen infection can be more severe if soybeans are planted within cool, moist conditions.  
Angie Peltier, U of I Commercial Agriculture Extension Educator, illustrated this point best at the Northwestern Illinois Agricultural Research and Demonstration Center.  There she planted a soybean variety that consisted of a good disease rating of 7 for SDS (1 being the worst and 10 being the best) and was listed as “highly suited for soils with soybean cyst nematode (SCN)" at several different planting dates.  The following chart was taken from Angie Peltier’s blog, which can be found at:
Planting date
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)

April 17
R4 (full pod)
May 7
R5 (beginning seed)
May 22
R5 (beginning seed)
June 9
R5 (beginning seed)
Chart taken from Peltier U of I Extension Blog
In Peltier’s chart above, you can see that the earlier that this soybean variety was planted, the greater the severity of SDS symptoms observed on August 14th.  Most of the fields showing SDS symptoms, which I have had the chance to evaluate in the last week were planted May 6, 7, 8, 9, 10.  The “magic temperature” for the SDS pathogen to infect soybean seedling roots is around 60 degrees.  Soil moisture may also be conducive for infection to occur.  
Another observation was that tillage could have been a factor when it came to the onset of SDS. Remember earlier in this article that I said that the SDS pathogen may favor soils that are cooler, compacted, or not as easily drained. Early in the season, no-till fields may have had cooler soil temperatures, when compared to those that were tilled.  Depending on soil types, compaction can be reduce by practicing no-till, because there are less passes over the field with equipment.  On the other hand, significant tillage can also help to break up field compaction and allow vertical drainage to occur.
Ultimately, soybean yield loss will depend on SDS onset and severity.  SDS is usually more severe when symptoms appear before soybean podfill.  When symptoms occur this early, yield loss occurs by way of reduced seed number, because flowers and pods can be aborted.  The good news (if there can be good news) is that most disease observations indicate that the onset of SDS symptoms became apparent during the R4 or R5 soybean growth stages.  This means that symptoms began to appear after soybean podfill.  In my opinion, based on previous research, I think it is too early to assess yield loss due to SDS.  Unfortunately, SDS symptoms can become more severe over time, and soybeans stressed by SDS can be more vulnerable to other root, stem, and seed diseases.  Therefore, soybeans that show symptoms after podfill can suffer yield loss due to reduced seed size, reduced pods, and seed weight, because seed quality could be at risk.  As soybeans reach the later growth stages, such as R7, the final yield loss can be observed.  
For now, if soybeans are showing SDS symptoms, we can focus on future disease management.  Of course, we know that there is no “in season” disease treatment.  Currently, soybean seed treatments will have no effect on the pathogen that causes SDS.  However, there is a Bayer seed treatment currently pending approval that may provide some protection against this soil borne fungal pathogen.  The most important thing to consider is planting soybeans in warm, dry soils.  Often times, this means that you may need to plant soybeans fields that have been previously affected by SDS at a later date.  Planting soybeans with higher level of resistance to SDS and SCN will also prove to be helpful.  For more information, you can refer to Burrus soybean disease ratings at the following link: Improving soil drainage and eliminating compaction will also help to reduce risk of the onset of SDS.  Unfortunately, a rotation from soybean to corn will not help to combat SDS, because the fungal pathogen that causes SDS has also been found to survive on corn roots.

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