General Soybean Disease Update: Soybean Sentinel Plot Situation and Regional Soybean Rust Update, July 16, 2011

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 16, 2011 17:04

The soybean crop is advancing regardless of the extreme environmental conditions encountered throughout MS.  At present, there are no foliar diseases of much consequence to report from either soybean sentinel plots or commercial fields.  A little frogeye leaf spot has been reported from a couple of isolated fields in the north Delta.  In addition, Septoria brown spot has been observed in the lower canopy of some row-watered soybean fields throughout the Delta.  In both situations fungicides were not suggested.  Keep in mind that the majority of foliar fungal diseases do not prefer high temperatures.  In addition, rain and cooler temperatures will most certainly be required at this point for much foliar disease to develop.  However, keep in mind that even with hot and dry conditions it is possible for aerial web blight to occur.    

Appearance of fungal structures in soybean plant tissue when charcoal root rot is present.

Even though the foliar disease situation has been quiet this year, soybean root diseases have been plentiful.  Numerous fields throughout the Delta have clear symptoms of charcoal root rot due to the elongated period of excessive temperatures.  In addition, I have received numerous calls regarding soybean plants that have died following an irrigation event.  In almost all of the cases charcoal root rot was to blame.  Charcoal root rot requires a stress for the disease to manifest itself and produce symptoms.  In situations where irrigation has not been provided in a timely manner or requires an excessive period of time to get across a field plants can stress and succumb to the fungus.  Plants removed from the soil will oftentimes have dry and dead roots with small black structures embedded in the plant tissue.   

On the regional front, following the bi-weekly soybean rust conference call:

Alabama – Environmental conditions continue to be hot and dry throughout the southern part of the state.  The drought monitor suggests that the drought is considered to be exceptional at this time.  Some scattered afternoon showers have started to occur more frequently throughout the southern part of the state.  However, no soybean rust or other foliar diseases to report from AL.

Florida – Yesterday (July 15, 2011), soybean rust was detected at low levels on kudzu in Gadsden County, in the Florida Panhandle.  Historically, this week in July is the week for soybean rust to be detected in the Panhandle.  At present this is the most northern soybean rust in the U.S. and means that rust is present in 4 counties in FL, all on kudzu.  Conditions remain hot and dry throughout the majority of the state.  However, some afternoon rain showers have occurred throughout northern Florida but temperatures are expected to remain hot for the foreseeable future.

Georgia – Conditions remain hot and dry as with the other states in the region.  Soybean planting continues in some parts of Georgia at this time.  Southern Georgia has received more frequent rainfall than previously; however, still no soybean rust to report.

Texas – Soybean was scouted in the lower Rio Grande River Valley following Tropical Storm Arlene.  No diseases to report from that part of the state but 8 inches of rain resulted from the storm.  Soybean in the area ranges from R2-R5.  As can be expected, Cercospora leaf blight is prevalent in some fields.  On a different note, portions of northern Mexico (Tampico) were behind on planting soybean this season.  Fall plantings of soybean are expected in southern TX as well as in northern Mexico.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 16, 2011 17:04
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