General Soybean Disease Update: August 13, 2011

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist August 13, 2011 17:04

Southern MS

On Monday and Tuesday (August 8 & 9) the soybean disease scouting crew made a tour through southern MS to scout soybean sentinel plots as well as commercial fields and kudzu.  Little if any disease was identified; however, even with the high temperatures and abnormally low rainfall some diseases were observed.  Aerial web blight was observed in isolated areas of a few fields, black root rot, Cercospora blight (late-season Cercospora), frogeye leaf spot, sudden death syndrome (SDS), and target leaf spot were all identified at low levels.  In addition, kudzu in the southwestern corner of the state (geographically near Pond, MS), typically one of the first locations where soybean rust is observed annually, has experienced many weeks of drought so leaves were being dropped from the plant.  The other states in the region are reporting similar instances of the diseases listed above.  Moreover, with the continued hot, dry weather throughout much of the southern U.S. soybean rust is not considered to be a threat at this time.  At present, the closest soybean rust to MS is in Iberia Parish, LA, which is along the coast of southern LA.  Infected kudzu is typically present at that location throughout the year until the past two years when colder than normal winter temperatures killed kudzu to the coast thus eliminating the opportunity for the fungus to overwinter on green plant material.  As of August 13, 2011, soybean rust has NOT been identified in MS.

Soybean rust situation as of August 13, 2011 in the southern U.S.











Target spot of soybean with mature lesions along top and bottom of leaf.

Presently, black root rot, Cercospora blight, frogeye leaf spot, southern blight, and SDS have all been observed at low levels throughout the Delta.  Additionally, over the past few weeks I’ve received numerous calls regarding spots on leaves as well as pods in conjunction with leaf shed in the lower canopy, particularly in MG IV soybean fields that were planted early and have received a fungicide (either 4 or 6 oz of a strobilurin) application.  The particular disease responsible for the leaf shed begins as small spots, typically 1/8 inch in circumference with a dark brown to black center and can be present on both leaves and pods.  Lesions on pods are typically reddish brown and not as large as lesions on leaves.  However, the lesions on pods do not penetrate the pod wall and appear to simply be on the outside surface of infected pods.  Most of the lesions on leaves will have a greenish yellow halo (that in situations where the lesions are older may appear light brown to tan) around the lesion (see photos).  In severe cases infected leaves turn yellow and drop off the plant prematurely and in extreme cases the bottom 2/3 of the canopy will defoliate.  At present it is believed that this is target spot since in all of the cases that have been observed over the past two seasons the more mature, target-type lesions are plentiful on adjacent leaves.  In the most mature stages of the disease, target spot lesions look like a target, are typically round and have a bright yellow halo around the outside of the lesion.  However, the smaller lesion size would be considered to be atypical by most in MS, but based on the description of the disease from several sources, the presence of the more mature target spot lesions, and the manifestation of the disease from the bottom of the canopy and moving up appears to be a consistent description of target spot.  Cultural work is underway in the laboratory in an attempt to isolate the fungus and verify the presence of the target spot causal agent.  In 2007, many of us were convinced this particular leaf spot was caused by a bacterium; however, following more careful observation in subsequent years and based on the fact that the disease defoliates the lowest leaves first a fungus was thought to be responsible.  More often than not, target spot is identified late in the season (post R5) and thus should not be a threat to yield as long as the upper most leaves stay on the plant. 

Eastern MS

Aerial web blight continues to be a concern in eastern MS.  Typically, once the soybean pod becomes woody, generally post- R5.5, a soybean plant is less susceptible to a yield loss caused by aerial web blight.  If soybean plants have reached reproductive growth stages (R1) and aerial web blight has been identified in the field a fungicide is an economical management practice since the disease can move quickly and greatly reduced yield.  Aerial application of a fungicide is not the best management practice since coverage will be the greatest issue as well as canopy penetration.  For more information regarding trials conducted last season (2010), as well as specific fungicide application methods please see:

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist August 13, 2011 17:04
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