Soybean Disease Update: August 16, 2014 (UPDATED)

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist August 16, 2014 15:22

Soybean Disease Update: August 16, 2014 (UPDATED)

Severe frogeye leaf spot.

Severe frogeye leaf spot.

Frogeye leaf spot

Frogeye continues to be the most widely observed disease.  I’ve observed frogeye as far south as Lamar County and as far north as Tunica County in the west and as far north and east as Prentiss County this season.  Susceptible varieties are heavily infected and in one case in Lamar County severe blighting was observed in a small area of the field.  Severe yield loss can be the result of foliar blighting since the fungus will cover the leaf and reduce the potential photosynthetic capability of the leaf tissue.  Fungicides provide some ability to reduce the yield losses attributed to the disease.  However, most consider a fungicide application will work similar to an herbicide application when applying the fungicide in the presence of a disease.  Fungicides work by either killing spores that are present (triazoles) or preventing fungal growth throughout the leaf (strobilurins).  If you make a fungicide application when a tremendous amount of disease, or inoculum or environmental conditions are extremely conducive for disease development the fungus will overwhelm the ability of the fungicide.  Overwhelming the fungicide is clearly one of the issues we’ve observed this year in susceptible varieties. Practically speaking, fungicides have slowed frogeye leaf spot down, but that’s about all we should expect on extremely susceptible varieties when challenged with this fungus.

Over the past week I’ve received numerous telephone calls regarding late foliar fungicide applications.  Based on the trials conducted last year and the observations that go with those trials (see: http://www.mississippi-crops.com/2014/07/12/managing-frogeye-leaf-spot-with-fungicides-2013-ms-trial-data/) I feel pretty safe stating that a fungicide application made as late as R5.5 would be economically beneficial.  However, that statement is made based on a single (ONE) fungicide application made that late and doesn’t include a fungicide application made prior to that particular timing.

From a resistance standpoint, the total number of counties exhibiting strobilurin-resistant isolates climbed to 52 on Friday and as of Saturday the number had increased to 54.  Stay tuned for additional information regarding new counties exhibiting fungicide resistance as we continue to collect isolates from counties where we observe frogeye leaf spot.  See the “Disease Monitoring” tab for the most up to date map regarding fungicide resistance within the frogeye leaf spot fungal population.

Soybean rust

At present (8/16/14), soybean rust has not been detected in MS.  However, active soybean rust has been observed in Alabama on kudzu and soybean (as far north as Prattville, AL) and in several parishes in Louisiana on kudzu.  Scouting continues throughout the southern parts of MS since conditions have been extremely conducive for the development of soybean rust this season.  However, the cooler winter we experienced likely slowed the disease down.  For current information regarding the location of soybean rust positive counties please refer to the information included on: www.sbrusa.net .

Sudden death syndrome. Note the severe interveinal necrosis associated with SDS.

Sudden death syndrome. Note the severe interveinal necrosis associated with SDS.

Sudden death syndrome

On Thursday I was able to observe some SDS in a field in eastern Bolivar County.  SDS is generally observed in light soil classes and will not be observed in field soils that contain a heavy clay content.  In addition, be mindful that the specific symptoms associated with SDS will appear similar to those that can occur when some fungicide products are applied to soybean.  So if you are observing SDS in an entire field check to make sure a fungicide product that contains one of the ingredients that could produce a phytotoxicity symptom (e.g., Proline, Stratego YLD, Muscle ADV, Overrule) has not been applied prior to the observation.  In addition, numerous root-based issues can manifest symptoms similar to SDS: Southern blight, red crown rot, Phytophthora root rot (seems rare in MS as I’ve only observed the disease twice), general stress, general root rot, and root-knot nematode.

In general, plants exhibiting symptoms of SDS will not have observable signs of the disease on the main stem, but leaves in the upper most plant canopy will have considerable interveinal necrosis.  If you grab the main stem and remove the plant from the ground by simply pulling on the plant normally the entire root will come out of the ground.  Keep in mind the ability to remove the entire root is one major difference between SDS and the general root rot that has previously been presented on the blog (see: http://www.mississippi-crops.com/2014/08/01/soybean-disease-update-august-1-2014/).  In addition, large areas of SDS may be the result of a large soybean cyst nematode population so make sure to soil sample for nematodes in situations where a large, circular pattern of SDS are observed.

Soybean stem canker. Note brown to maroon main stem lesion.

Soybean stem canker. Note brown to maroon main stem lesion.

Stem canker

In the field the presence of stem canker can be easily confused with SDS.  Make sure you’re observing the base of the stem for the characteristic canker rather than making a determination based on the aboveground leaf symptoms.  Stem canker will be present if a brown to maroon lesion is present anywhere on the stem.  In most cases the lesion will begin at the soil line and likely move up in the plant in susceptible varieties.  However, lesions can be initiated at any point on the main stem where a branch of the plant joins the stem.  Aboveground, foliar symptoms can appear as interveinal chlorosis with some interveinal necrosis that will appear quite similar to SDS.  In the past I’ve more often observed stem canker in east MS; however, I observed one variety in a strip trial in the Delta on Thursday with clear symptoms of stem canker marking the second time that I’ve observed stem canker in the Delta.  Keep in mind that detecting stem canker in a field should mean that you attempt to find a resistant variety for the next season as stem canker can be a devastating disease.  Variety trial ratings for those varieties included in the MSU soybean variety trial screening program are included online annually.

Aerial web blight

I continue to receive telephone calls regarding the presence of aerial blight in the Delta.  To date I have not observed aerial blight in the Delta, but I have received a few phone calls from consultants in fields that border the hills where aerial blight has been observed.  Just because a white fungal growth is present somewhere in the canopy doesn’t mean that aerial web blight is present.  Some secondary fungi can produce a white fungal growth in addition to diseases such as Southern blight that will produce a white fungal growth at the base of the plant and the soil line.  On Thursday I observed white fungal growth on dead plants as a result of stem canker.

Aerial blight can be a difficult disease to diagnose.  While I realize that we oftentimes suggest that aerial blight can be more of an issue in a rice/soybean rotation that doesn’t always necessarily mean the disease will be present in those fields.  In the 7 years I’ve been in MS I’ve observed aerial blight twice in the Delta.  Both of those field situations were small soybean fields in extremely protected areas with one of the fields being a narrow row spacing between two corn fields in a situation where rice had been planted the previous season.  The majority of the fields where aerial blight is a problem are in small field settings where the field is surrounded by trees on either three sides or all four sides.  Restricting air flow can increase the severity of a disease such as aerial blight.

 

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist August 16, 2014 15:22
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