Soybean Disease Update: August 18, 2013

Downy mildew of soybean.

Soybean rust

I’ll begin with the soybean rust situation since I had numerous calls regarding “false” information this past week.  First and foremost, ANY and ALL information regarding the presence of soybean rust in the Mississippi soybean production system will be released either on this blog or on the website www.sbrusa.net.  If you suspect you have soybean rust please call the MSU Extension Service and we can either confirm or deny the presence of soybean rust on leaf samples.  Listening to rumors or someone other than an individual that works on the disease monitoring situation (Billy Moore, Malcolm Broome) will likely result in panic that is quite unnecessary.  Information regarding the particular counties where rust has been identified will be available on the blog in the form of maps.  We continue to scout for soybean rust in MS and even though the disease is present in Alabama and Arkansas as well as Louisiana to our south the disease has only been identified in 8 counties (Adams, Amite, Hancock, Harrison, Jackson, Newton, Pearl River, and Wayne) to date.  Stay tuned to maps provided in the “Disease Monitoring” section (https://www.mississippi-crops.com/disease-monitoring/) for continued updates. Parts of the state continue to be dry and this is likely limiting the development and spread of the disease from the few isolated kudzu and soybean locations where infected plant material has been observed.

Septoria brown spot of soybean can be easily confused with soybean rust due to the upper leaf symptoms.

Numerous diseases can easily be confused as soybean rust so keep this in mind as you scout your soybean fields.  Some of the specific diseases include: bacterial pustule, downy mildew, and Septoria brown spot.  This season in particular I have observed fields with above normal Septoria brown spot at advanced reproductive growth stages.  A proper disease diagnosis is most important when dealing with soybean rust since early detection is extremely important.  Observation of the symptoms on the top of the leaf is not an effective method to diagnose the presence or absence of soybean rust.  Few people can detect the disease at such levels as to effectively suggest the application of a proper fungicide treatment.  Quite frankly, this is one of the main reasons why we continue to scout for soybean rust (as well as other diseases of economic importance).  The Mississippi Soybean Promotion Board and United Soybean Board understand the importance of this particular disease and have continued to fund sentinel plots as well as monitoring for economically important soybean diseases.

Frogeye leaf spot

Frogeye leaf spot continues to be an extremely hot topic this season.  Farmers that have planted susceptible varieties (see: https://www.mississippi-crops.com/2013/07/28/soybean-frogeye-leaf-spot-management-decisions-for-the-disease-following-an-r3r4-strobilurin-application/ for information regarding a particular varieties’ response to frogeye leaf spot) and then applied either 4 or 6 fl oz/A of a stand-alone strobilurin fungicide have observed firsthand what frogeye leaf spot can do.  I am not prepared to claim that we have widespread fungicide-resistance, but, we have certainly collected as many infected leaves as we could from more than 25 counties throughout the state.  Information regarding the presence or absence of fungicide resistance will likely be available in the late winter or early spring as we have a graduate student in Starkville working on that particular project to determine how much fungicide resistance we have in the frogeye leaf spot fungal population.  The project was funded by the MS Soybean Promotion Board.  If you have a field of soybean that has received a fungicide and still has a tremendous amount of frogeye leaf spot please contact us so we can retrieve a sample for analysis.

As a reminder, if you have MG V soybeans in the field, and have not made an R3/R4 fungicide application determine the specific variety planted and find out whether or not it is susceptible to frogeye leaf spot before making a fungicide application.  In fields of susceptible soybean I have observed moderate levels of frogeye following a fungicide application.  Strobilurin products should not be considered to be “curative” as they will only prevent the spread of the fungal mycelium in the leaf parts that have not previously been infected.  Susceptible soybean varieties should be sprayed with either a pre-mix or tank mix fungicide that contains a preventive (strobilurin) and a curative (triazole or carboximide).  Applying a stand-alone strobilurin is acceptable to fields of frogeye-resistant soybean varieties.

Sudden death syndrome present in a small clump.

Sudden Death Syndrome

Numerous reports of SDS have occurred over the past several weeks.  In some instance, the application of fungicides that contain specific fungicide active ingredients, especially those products in the triazole family, can produce a symptom similar to SDS when applied at hot times of the day or in full sunlight (see: https://www.mississippi-crops.com/2013/08/09/not-everything-is-as-it-seems-fungicide-phytotoxicity-and-plant-diseases/).  The typical symptoms associated with SDS include interveinal chlorosis, interveinal necrosis, and a darkening of the vascular tissues that can be revealed by cutting through the stem of the plant to observe the pith.  In general, SDS will occur in clumps of small plants.  However, in situations where the soybean cyst nematode is present in numbers above economic threshold (> 1 cyst per pint of soil) there is an increased likelihood for SDS to become economically damaging.  In situations where large, oval areas of SDS occur, submit a soil sample to the nematode diagnostic laboratory in Starkville.  Collect a soil sample by mixing 10 to 15 soil cores in a bucket from the area where symptoms are observed and place in a ziplock bag.  Also collect a similar sample from the areas of the field that appear “healthy” for sake of comparison.  Keep the soil samples cool by placing them in a cooler or keeping them out of sunlight.

Keep in mind that several other diseases, in addition to phytotoxicity, can be confused with SDS.  At some point in the future I will likely include information on each of the diseases as a means of improving field-level diagnoses.

Black root rot foliar leaf symptoms.

“Mystery disease”

For the better part of the last six years I have referred to a particular root disease as the “mystery disease”.  Sometime during 2008 we moved to calling the particular disease “black root rot”.  The particular disease can produce leaf symptoms that appear similar to SDS except that the veins will generally stay green and leaves of affected plants yellow and don’t generally produce any necrosis between the veins as is observed with SDS.  Plants that are extracted from the soil by grabbing the stem and pulling will typically leave a portion of the taproot in the soil.  Observation of the roots that are extracted from the soil reveals some blackness of the taproot and the root system is typically quite degraded.  Roots can easily be broken as the entire belowground portion of the plant has rotted and becomes brittle.  Culture work in the laboratory on the Starkville campus by Dr. Maria Tomaso-Peterson has revealed Thielaviopsis basicola from infected plant material.  The particular fungus, Thielaviopsis basicola, causes black root rot of cotton as well as soybean.  Additional work will be done in the laboratory to continue to confirm the presence of the fungus.  At present, and once the disease is observed, no management alternatives are necessary.

Black root rot root symptoms. Notice the blackened discoloration on the main root. In addition, note the broken taproot.