Farthest North Progression for Soybean Rust

No recommendation for spraying yet.

Published on: Aug 7, 2009
EDITOR'S NOTE: In the interest of accuracy, this e-mail from Tom Allen, Mississippi State University Extension plant pathologist, is presented in its entirety, with minimal editing. Cecil Yancy, Editor, Mid-South Farmer.
PLEASE READ THIS ENTIRE EMAIL!!!!!! There is NO fungicide suggestion at
this time, says Mississippi State University Extension plant pathologist Tom Allen.
This is not intended to cause a panic.  Yesterday (Aug. 6, 2009), soybean rust was detected in two adjacent commercial soybean fields near Thornton, Miss.
This marks the first report of soybean rust in Mississippi this season and the farthest north the disease has progressed in the U.S. to date.  This was an irrigated field that had received an R3/R4 fungicide application and was presently at the R5.7 growth stage.  Moreover, rust was detected underneath a power line in this particular field that likely did not receive fungicide during the application.
Immediately across the road in a dryland field rust was additionally detected. In both locations incidence and severity was VERY low.  However, the rust pustules were sporulating.  Based on the level of infection in the sprayed field, under the power line, it is likely that the disease has been present in this particular location for about 4 weeks.  Following the conducive conditions that we have had over the past month this is not a surprise.  At this point this is a VERY localized infection and rust has NOT been detected anywhere else in Holmes County or in Mississippi.  Please continue reading for specific treatment suggestions, which at this time are to NOT make a curative/preventive application.  
There are a few comments I'd like to make about the detection of soybean rust at the field level.  Firstly, you cannot detect rust by driving down the road.  In some cases soybean rust will develop in a particular soybean field and be difficult to detect since it can be present at incredibly LOW levels.  In addition, confirmation of the disease requires finding pustules on the underside of a soybean or kudzu leaf.  Looking at symptoms on the upper leaf surface is NOT enough to confirm the presence of the disease.  Uredinia, the pustule on the underside of the leaf, with urediniospores (spores) present can confirm the presence of the disease.  
In addition, field scouting for the disease must be done with a hand lens to identify the pustules on the underside of the leaf.  Relying on the symptoms as observed by the naked eye is not enough to confirm the presence of soybean rust.  In addition, please keep in mind that there are numerous other diseases in our production system that can look VERY similar at the field level.  These include bacterial blight, downy mildew, cercospora blight (late-season cercospora), and septoria brown spot.  If you have questions regarding whether or not what you are looking at is in fact soybean rust please do not hesitate to call.
Now, I'll try to answer a few of the most common questions:
What does a red county on the public Web site (www.sbrusa.net) map mean?
When a county is turned red on the website this does NOT automatically trigger a fungicide application.  A red county simply states that rust has been detected in a particular county.  Several factors need to be considered to trigger a fungicide application including weather (past and present), local soybean growth stages, and the level of infection (the incidence and severity of the disease at that particular location). ALL of these particular factors will aid in determining the proper fungicide(s) to be used and whether or not they are necessary at all.  
So what does this mean?  
As I said last week in the soybean rust update we knew that rust would turn up and was likely to be detected anywhere in the state.  High temperatures (>90F) are not conducive for the development and spread of soybean rust.  However, with nighttime temperatures in the low 70s the fungus can sporulate and cause disease. 
Essentially, at higher temperatures the disease will shut down during the day, require a period of time to start back up at night, but as temperatures increase throughout the day the fungus and disease will shut down and go dormant until more conducive conditions occur.











Where do we go from here? 
At this time, since this is the ONLY location where soybean rust has been detected there is NO fungicide suggestion.  This particular producer is aware of the situation and has agreed to treat the field to reduce the level of local inoculum and likely prevent further spread throughout the Delta.  Only time will tell when and where more rust will be detected.  Be aware that we are out scouting today and will be throughout the weekend and early next week. 
Further detection of the disease may in fact change this suggestion. Fungicide suggestions?  There are absolutely NO fungicide suggestions at this time based on the fact that rust has been identified in a localized situation and has not been detected outside of Holmes County. However, for soybeans in this particular area (Holmes County near Thornton, MS) that were going to receive an R3/R4 fungicide application the products of choice would be a pre-mix (strobilurin/triazole) if the application has NOT already been made.
We will continue to update the Soybean Rust Hotline (1-866-641-1847) that has graciously been provided by the Mississippi Soybean Promotion Board and BASF as new information becomes available or if anything in particular changes.  Please feel free to contact me with any specific questions or concerns (662-402-9995).  Additionally, if you want to be added to my email update that is sent out regularly send me an email (tallen@drec.msstate.edu)