Physiological Scorch of Soybean:  Is it SDS, black root rot (CBR), brown stem rot, Phytopthora root and stem rot, or just weather?

from Steve Koenning, Extension Plant Pathologist

and Jim Dunphy, Extension Crop Scientist

NCSU 

Some soybeans exhibit a symptom referred to as “Physiological Scorch”.  When there is extensive chlorosis (yellowing) between the veins of the leaf, or necrosis (dead tissue) between the veins, which may occur on the top of the plant or throughout the plant, we refer to this symptom as physiological scorch.  It typically occurs when the roots and vascular system aren’t effectively doing their job, such as when root and/or stem pathogens restrict the vascular system when soybean is in the reproductive phase, especially during pod filling.  We suspect in some cases no disease may be involved or specifically to blame for these symptoms.  Wet or saturated soils through much of the state have resulted in root systems that are poorly developed.  Now that things are drying out a little, the root systems may not be able to meet all the demands of the shoots and pods for nutrients and water.  What we view as a “burning up of the plant” is really just the plant response to a water and nutrient shortage when demand is greatest.  Once plants enter the reproductive stage, they will add only a few new roots and will not replace those that have died, thus the plant is limited in what it can do.

A number of pathogens can cause this symptom.  Most commonly this symptom is associated with “SDS” (sudden death syndrome) or “CBR” (cylindrocladium black root rot) of soybean.  Lab and or visual analysis are needed to distinguish between the two diseases.  Other potential causes for these symptoms include dectes stem borer, Phytophthora root and stem rot, stem canker, and charcoal rot.  Regardless of which disease is present, fungicides are unlikely to provide a remedy, since these are a result of root rots or other vascular disease.

For more information see: http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy007/soy007.htm and http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy005/soy005.htm.