Soybean Seedling Disease and Fungicide Seed Treatment Issues to Consider Following Flooded Soils

Pythium seedling diseases and fungicide seed treatments

Little if any information seems to exist regarding the specific fungicide seed treatment suggestions that would follow extended periods of soil flooding.  In the past, most of the published research has been conducted in situations where the crop was planted and a flood followed for short periods of time (i.e. hours not days/weeks).  In almost every case, root rots caused by species of Pythium were significantly increased depending upon flood duration.  One of the most destructive genera of fungal plant pathogens, Pythium, is a water-loving organism and will likely be present at greater concentrations following a flooded situation.  Moreover, the fungus produces motile spores that require water for release.  Most importantly, Pythium spp. are “pioneers” and would be one of the first microbes to re-inhabit soil that received anaerobic conditions for an extended period of time.  Typically, seedling diseases caused by Pythium are more problematic when cool wet conditions prevail.  However, over the past few years the incidence of non-lethal, or sub-lethal, Pythium seedling disease has been on the rise particularly during periods of warm weather.  However, this is not to suggest that other seedling diseases would not occur in limited situations.  Diseases caused by Rhizoctonia solani may also occur depending on environmental conditions.

Fungicide seed treatments are a good insurance application.  More often than not they serve their purpose in years when the environment cooperates and “the planets don’t align for a complete disaster”.  By applying a fungicide seed treatment you are protecting the seed in the ground for a relatively short period of time.  Changing environmental conditions can wreak havoc following planting especially after seed has remained in the soil for an extended period of time or shortly following seedling emergence.  However, this year in particular, and in those areas that have been affected by the flood, verify that the seed treatment to be used will prevent seedling disease caused by Pythium.  The particular products that are labeled to prevent seedling diseases caused by species of Pythium contain the active ingredients azoxystrobin, mefenoxam, or metalaxyl.  Several different fungicide products contain the specific active ingredients.  If you have a specific question regarding a fungicide or the organisms managed by the active ingredients contained within that product please feel free to call and double check.  Furthermore, keep in mind that just because a seed treatment fungicide is applied a seedling disease can still occur if the seed remains in the ground for an extended period of time, moist soil conditions are encountered (either due to rain OR post-flooding soil conditions), the environment shifts, or excessive soil fungal populations of more than one particular fungus results in stand failure.     

Typically in situations where I have observed fields with stand failure, even when a seed treatment fungicide was applied, the environment was to blame.  2011 has provided good examples of this particular situation.  In some cases, producers that planted soybeans early in warm temperatures watched the environment turn to cooler, wetter weather.  Ultimately this increased the amount of seedling disease that occurred in some locations.  However, with the current environment throughout the state, the high temperatures if mixed at some point with either moist soil conditions or rain may result in just as much of an issue from seedling disease.  Over the past several years sub-lethal Pythium, also referred to as non-lethal Pythium, has been prevalent in MS.  Generally this particular disease is more of a problem during warm to hot weather but generally appears to occur in situations when planting is followed by heavy rainfall.  Typically plants are not killed, but damaged terminals can result from infection and plants exhibiting this particular symptom will not likely produce the yield that could be expected from non-infected plants. 

Soybean inoculants

Most published information regarding soybean inoculants suggests that populations of Bradyrhizobium japonicum don’t survive long in a flooded situation.  But, data suggests that populations rebound over time following a flood because the bacteria enter a period of “dormancy” when anaerobic conditions prevail.  However, based on observations from rice/soybean rotationals, nodules are still present on roots following the flooded field soil associated with rice fields.  Keep in mind, as a general rule of thumb, the specific suggestion for soybean inoculants in MS states that if the ground hasn’t been in soybean for 3 or more years than an inoculant should be included.  Remember that using an inoculant is a much cheaper alternative than having to apply urea at a later date. 

Soybean inoculants can be marketed in several different formulations including dry powder, liquid, and peat-based powders.  Below, the information focuses on liquid inoculants with a single peat-based product that have been most recently researched by MSU so as to at least limit the list. 

Make sure the seed that you purchase, if it has already been treated with a fungicide or will be treated with a fungicide and an inoculant, will have a seed applied fungicide product that is compatible with a specific inoculant.  At present, Mississippi State University does not have first hand data with regards to the specific compatibility of seed treatment fungicides and inoculants.  The table included in this update was assembled based on information provided by each of the companies that market a specific inoculant product (NOTE: specific insecticide seed treatment compatabilities were not included since this update ONLY includes seedling diseases and seed treatment fungicides).  In some cases, seed treatment fungicides have a negative impact when mixed with inoculants, especially those products that contain “moly”, that is considered to be toxic to rhizobia bacteria.  Please confirm compatibility with the attached table, check the specific company website or feel free to contact personnel within the MSU-ES.  However, in no way does the attached table promote the use of these specific products and I realize there are many other products available.  Moreover, as additional research is conducted with more products in MS those will be added to the attached table so as to provide the best picture of available products.

In general there are two specific methods that can be used to apply the products to seed:

Simultaneous Application: Seed treatment fungicide and inoculant are mixed together as a tank mix and applied to the seed at the same time.  Combining the treatments in a single seed treatment application will not only save money but will save time on the turn row. 

Sequential Application: Seed treatment fungicide and inoculant are maintained in separate mix tanks and applied to the seed at different times or the inoculants is added to seed that has already received a fungicide treatment.

In most cases the inoculant products that are available for pre-treatment have a different duration of survivability on the seed post-application.  The specific period should be considered when ordering products to be applied especially if you are planning on storing the seed prior to planting (refer to specific timings in the attached table) regardless of the duration.  Specific periods should be expected to differ depending on the fungicide product applied as well as method of application.

An extensive research project was initiated while Dan Poston was present at the DREC to consider commercially available liquid inoculants, predominantly on lighter soil types without a prior soybean cropping history.  The work was funded by the Mississippi Soybean Promotion Board and continued by Brewer Blessitt before his departure.  The results have been widely presented throughout MS and include information on some of the most commonly applied inoculants in our production system, specifically: Excalibre, Optimize, and Vault.  Moreover, the interest in inoculants has increased with the shifting crop acreage in MS over the past 5 years.  The products considered were:

Cell-Tech Soybean – Available from EMD Crop BioScience – Specifically formulated with 2 billion spores per gram of product (Bradyrhizobium japonicum).  The product can be applied to the seed or in furrow.  However, if applied with seed treatment fungicides there is a 4 day window within which the seed should be planted.  One important point to note is this product is probably better suited to cooler environments. (Suggested Retail Price: approximately $1.50/unit) http://www.emdcropbioscience.com/products/product_detail.cfm?PROD_ID=26

Excalibre + Extender – Available from Advanced Biological Marketing (ABM) – Should be applied to soybean seed in non-chlorinated water.  Composed of two separate components (3 with water) to be applied as a slurry.  Three separate strains of bacteria are present within the formulation.  The product is safe to be applied with fungicides up to 120 days prior to planting. (SRP: approximately $2.37/unit) ‘http://treatyourbeans.com/excalibre.php

Launcher – Available from Precision Laboratories – The product delivers two strains of bacteria to produce nodules in a minimum of 4 billion spores per milliliter of product.  (SRP: NA/unit)http://www.precisionlab.com/ProductSelect.pli?Parent=540-400Y1

Launcher Pro – Available from Precision Laboratories – A three product mix of two rhizobium bacterial strains, bacterial protector and a growth promoter.  The bacteria are present in high quantities exceeding 4 billion spores per gram of material.  The product is safe to be applied with seed applied fungicides for 45 days prior to planting. (SRP: approximately $3.45/unit)http://www.precisionlab.com/ProductSelect.pli?Parent=540-PAK

Optimize 400 – Available from EMD Crop BioScience – As compared to Cell-Tech, there is a 120 day planting window post-treatment with seed applied fungicides and the inoculants.  Is a two component product with the inoculants and a promoter.  (SRP: approximately $3.50/unit) http://www.emdcropbioscience.com/products/product_detail.cfm?PROD_ID=24

Vault NP + Extender – Available from Becker Underwood – The specific extender on this particular liquid inoculant extends the “on seed” window by approximately 30 days.  (SRP: approximately $2.31/unit) http://www.beckerunderwood.com/en/inoculants/nodpeus

Vault SP – Available from Becker Underwood – Is a peat-based inoculant product rather than the liquid products listed above.  Seed to be treated with this particular product should be planted within 24 hours after treatment if applied as a hopper-box style treatment as opposed to the products listed above.  It is not known if there are particular fungicide compatibility issues with Vault SP; however, representatives from Becker Underwood simply state that seed that receives a fungicide should be dry prior to adding the inoculants to the hopper-box (SRP: approximately $2.16/unit)http://www.beckerunderwood.com/en/inoculants/ussus