Soybean Disease Update: July 5, 2013

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 5, 2013 19:45

Soybean Disease Update: July 5, 2013

New this season, disease monitoring maps will be posted on the blog.  To locate the maps, refer to the top bar on the main page for the “Disease Monitoring” tab (http://www.mississippi-crops.com/disease-monitoring/).  Maps will be updated on a regular basis, generally as needed when information changes, with pertinent scouting information.  Currently, the maps for soybean diseases included a regional map of the soybean rust situation in Arkansas, Louisiana, and Mississippi (similar to the map that can be viewed at www.sbrusa.net) and the most up to date information regarding the distribution of strobilurin-resistant frogeye leaf spot in MS as well as our region (AR, LA).  Please make sure you click on the right map and don’t confuse the frogeye map with the soybean rust map.  Colors for each of the maps match what is used on the general website for disease monitoring throughout the nation (located at: http://sbrusa.net/cgi-bin/sbr/public.cgi).

Typical, characteristic symptoms of frogeye leaf spot.

Typical, characteristic symptoms of frogeye leaf spot.

Frogeye leaf spot

Scattered reports of frogeye leaf spot in reproductive soybean fields have been made over the past several weeks.  However, throughout the season numerous calls have been made regarding look alike maladies on soybean leaves.  Remember, depending on the herbicide chemistry used in some instances herbicide injury and/or drift can produce lesions that appear similar to frogeye.  One key characteristic, frogeye leaf spot will not have a yellow halo around the lesion.  Some herbicide products, such as Dual, can produce symptoms similar to frogeye.

Numerous questions over the past week regarding fungicide applications at the R3/R4 growth stage timing.  Keep in mind that a general, stand-alone strobilurin fungicide (e.g., Headline or Quadris) may not be the best option on frogeye susceptible soybean varieties since resistance has already been documented in MS.  But, with that said, resistance has not been determined to be widespread and last year only two instances were documented in our production system.  To determine the specific fungicide application strategy for your farm or field make sure you know whether or not the specific varieties are either susceptible or resistant to frogeye leaf spot based on the information provided by the seed source.

See: http://www.mississippi-crops.com/2013/06/01/soybean-fungicide-management-considerations-for-2013/ for additional information regarding the management of frogeye leaf spot in susceptible or tolerant soybean varieties.

General leaf tissue characteristic associated with Fusarium wilt of soybean.

General leaf tissue characteristic associated with Fusarium wilt of soybean.

Fusarium wilt

Since 2007 numerous reports of early Sudden Death Syndrome (SDS) have been made throughout the MS soybean production area.  Generally speaking, SDS can become apparent at approximately R5.5 and is not observed at vegetative stages or prior to the R5 developmental growth stages.  But, another disease, Fusarium wilt, is a potential issue as we have observed scattered symptoms caused by Fusarium since as early as 2006.  Symptoms of the disease generally begin during vegetative growth stages and first occur as yellowing of leaves in the lower canopy (see image).  Plants that are unearthed from the soil have a decomposed root system and if pulled from the soil rather than dug with a shovel a good portion of the tap root will remain in the soil profile (this differs from SDS whereby the entire root system will be pulled out of the soil).  As the disease progresses, interveinal chlorosis resembles SDS but is slightly different in that areas of the leaf between the veins don’t generally fall out of the leaf.  In addition to the majority of the root breaking off below the soil line, the remainder of the root will have a dark, black discoloration on the outside and removing the bark on the outside of the plant to reveal the vascular tissue will produce a slight discoloration of the vascular tissues.

Soybean rust

As of today (7/5/2013), soybean rust has not been detected in MS.  However, in LA, soybean rust was detected last week (6/27/2013) in reproductive stage soybean plants in two parishes.  Both parishes are towards the southwestern part of MS.  Hot spots were detected in one of the fields with severity as high as 10% on some plants.  Sentinel plots have been planted throughout MS at 24 locations to protect soybean farmers and serve as an early warning system (see: http://www.mississippi-crops.com/disease-monitoring/ for maps).  In addition to monitoring for soybean rust the sentinel plots will be used to monitor for strobilurin-resistant frogeye leaf spot as well as monitor for the development of additional fungicide resistant organisms.  Disease monitoring was funded by the Mississippi Soybean Promotion Board as well as the United Soybean Board.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 5, 2013 19:45
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