Soybean Rust Detected in a Sentinel Plot and Commercial Soybean Fields in George County

Soybean rust distribution within the southern U.S. as of July 16, 2012.

On Friday, July 13, 2012, soybean rust was detected in a soybean sentinel plot and 3 commercial soybean fields in George County near Lucedale, MS (geographically east and south of Hattiesburg, MS).  For several months soybean rust has been present in the Mobile, AL area so detecting the disease in this particular county is not a shock and is the main reason that sentinel plots have been located in this particular area.  The 3 commercial soybean fields were in the early R5 growth stages (≈ R5.2-R5.3).  To put this into a historical perspective, the 2012 soybean rust detection would mark the earliest in both date and soybean growth stage that soybean rust had been detected on soybean in MS.  In 2007, soybean rust was detected on kudzu in Wilkinson County on July 11, 2007 and was the earliest we have detected the disease in MS.  Typically, if we observe soybean rust in soybean the disease has already been detected in the state on kudzu or soybean growth stages closer to physiological maturity (R6 or beyond).

The sentinel plot monitoring network that now includes monitoring the state for additional important soybean diseases that can result in an economic yield loss (e.g. aerial web blight, frogeye leaf spot), was funded for the 2012 season by the Mississippi Soybean Promotion Board as well as the United Soybean Board.  Since 2005 the network has used early planted soybean plots containing several different soybean maturity groups to monitor for soybean rust throughout MS and other states where the likelihood of the disease overwintering on kudzu (a close relative of soybean and additional host of the fungus) could occur in areas along the Gulf Coast.

Following the mild winter and the overwintering of soybean rust in AL, FL, LA, and TX the early find is not a surprise.  However, the early hot and dry conditions encountered throughout much of the state during June means this is an interesting observation at this time of the year.

Soybean rust scouting

Soybean rust pustules on the underside of a soybean leaf. Note the cluster-type pattern of the disease. When observed through a hand lens the pustules will appear raised like little volcanoes.

Several foliar soybean diseases can easily be confused with soybean rust.  First and foremost, observing the pustules on the underside of the leaf can be used as a diagnostic tool but caution should be taken before declaring that a field is infected with soybean rust.  A 20x hand lens (or greater magnification) is the best tool to observe for the presence of pustules.  Keep in mind that pustules appear like little volcanoes on the underside of the leaf.  But, bacterial pustule, downy mildew, Cercospora blight, and Septoria brown spot can all be confused with soybean rust since they can also produce a raised lesion on the underside of the leaf that can be mistaken as soybean rust.  Attempting to observe the presence of soybean rust without a hand lens should not be done and if there is any question please call and we can positively identify the presence (or absence) of soybean rust together.  In addition, the symptoms produced by soybean rust on the upper leaf surface are difficult to describe and only a handful of people in MS are able to scout a field by relying on the upper surface lesion symptomology.

Management alternatives

Presently, we are not suggesting that fungicides be applied to a widespread area of the state in response to the detection of soybean rust in George County.  The farmer who manages the three fields that contained soybean rust was alerted to the situation and will likely make a fungicide application of a product containing a strobilurin + triazole (preventive + curative, since the disease was detected in his fields) as either a pre- or tank-mix.  The situation encountered in George County is isolated from the majority of the rest of the soybean production in MS.  In addition, regardless of the environmental conditions encountered, several weeks are generally necessary for soybean rust to produce enough inoculum in a field situation for there to be the potential for yield loss.  Even though there are some acres of young (≈ V3-V5) soybean fields present in the vicinity of the infected fields in George County, vegetative stage soybean plants are more tolerant to the fungus that causes the disease and a substantial amount of inoculum is required for infection to occur in vegetative stage soybean plants.  Since 2005, vegetative stage soybean infection has only been observed in two specific situations: 1) when a field of heavily infected soybean plants was harvested and volunteer soybean plants emerged during 2008 and 2) at the end of the 2009 season when entire fields of volunteer soybean plants were observed with soybean rust.  One situation that plays to our benefit this year has to do with reduced levels of inoculum in the U.S. following the cold winters of 2009 and 2010.  Remember, soybean rust was only detected in 5 counties in 2010 in MS and not at all during the 2011 season due to reduced levels of inoculum to our south.

We’ll outline several different scenarios below regarding management alternatives:

A table of the currently labeled fungicides to manage soybean rust is included2012 soybean rust fungicides.  Base fungicide application decisions on the potential to reduce the likelihood of a yield loss as the result of soybean rust.  However, realize that the conditions for soybean rust to develop within MS are extremely conducive at this time.  Scenarios will differ based on geographic location within the state, planting date, growth stage at this particular time, maturity group planted, and the specific environment encountered since planting as well as irrigation practices.

-Young soybeans (vegetative stage soybean to early reproductive (< R2)) – Late-planted soybean fields in the aforementioned growth stages likely don’t need a fungicide applied at this time especially if they are still in vegetative growth stages.  Farmers in this specific scenario should continue to monitor the blog, the public website (www.sbrusa.net) and/or the soybean rust telephone hotline (1-866-641-1847) to determine where soybean rust has been identified within MS and whether or not specific fungicide suggestions will change over the next few weeks.

-Wheat beans (vegetative stage soybean) – A fungicide is not likely needed at this time.  However, knowing that the environmental conditions are conducive to the development of soybean rust, monitoring the situation over the next few weeks will be beneficial in determining whether or not a fungicide application will be economically beneficial at more advanced growth stages.

-Full-season soybean sprayed at R3/R4 and now at R5.5 (or beyond) – Soybean fields in this specific scenario would likely not see a yield loss as a result of soybean rust regardless of the specific geographic location (NOTE: yield loss will likely not occur in fields that fit this specific scenario unless a tremendous amount of soybean rust was detected at R5.5 as was observed in some localized fields in Noxubee County during the 2009 season).  Generally, several weeks are required for soybean rust to infect, develop and based on the yield loss estimates generated by the entomologists, soybean plants at or near R6 don’t typically suffer economic yield loss as a result of defoliation.  However, keep in mind that seed rot is a completely different scenario since rotting soybean grain can occur as a result of specific environmental conditions in the absence of defoliation.

-R3/R4 soybean that have not received a fungicide – Applying a fungicide to a field of soybean at the R3/R4 growth stage (whether irrigated or non-irrigated) could be beneficial in a year when soybean rust threatens.  However, several other factors should be weighed before applying a fungicide and specific factors (number of years in soybean, presence of a particular disease) may alter the particular fungicide product prescription on a field-by-field basis.  Hypothetically speaking, if soybean rust were to be detected in a soybean field (at the R3/R4 growth stage) either a triazole or a strobilurin + triazole (pre- or tank-mix) would be the most effective fungicide choice.  Strobilurin fungicides applied as the sole active ingredient are preventive and best applied in the absence of disease.  Either pre- or tank-mix choices that contain a strobilurin + triazole will provide preventive and curative activity in the event that soybean rust or any other yield limiting disease is detected except in situations where aerial blight are detected.

When deciding to apply a fungicide to any soybean field keep in mind that soybean rust is easier to prevent than cure.  Even though members of the triazole class of fungicides are loosely considered to be “curative” the application of a triazole will likely only reduce any fungal population to a more manageable level especially when an environment conducive for disease occurs for an elongated period of time.  Yield, in general, is the best method to determine if a fungicide application was beneficial rather than attempting to determine the impact on the fungal population or overall disease rating post-application.

Environmental conditions and scouting situation

Appearance of soybean rust pustules as they would be viewed through a hand lens. Note the appearance of the pustule and the sporulation that erupts through the top of the “volcano”. Spores appear as tiny grains of clear sand.

We will continue to scout for the presence of soybean rust throughout the state.  At present, soybean sentinel plots are still being observed but will likely begin to be phased out over the next few weeks as some of the varieties planted have reached R6/R7.  Once we move from sentinel plots to commercial soybean fields we typically try to find soybean fields in advanced growth stages (R6-R8) to attempt to determine if the soybean rust fungus is present within a particular area of the state.  In general, soybean fields in advanced stages are easier to scout due to a reduction in leaf material.  Kudzu at this point in this season is becoming a good indicator plant; however, we generally only monitor a few kudzu patches since kudzu can be immune to the soybean rust fungus.

The rain that we have received throughout the state over the past 7-10 days has produced environmental conditions that are much more conducive for the development of soybean rust.  Generally speaking, soybean rust prefers high humidity and moderate temperatures less than 90F as optimum conditions for disease development.  Optimum temperatures for development and spread of the disease can occur at night especially during this time of year and more so if daytime temperatures are well into the 90s.  The forecast throughout much of the state has continued moderate temperatures with a 40-50% chance for rain.