On Saturday, Aug. 15, Asian soybean rust was confirmed in multiple Mississippi counties. With two exceptions, the disease is at very low levels of infestation. Fungicide applications have been recommended only with major caveats.
“There are numerous factors that will apply to applying a fungicide that will include management objectives, presence of rust, environment (temperature and expected rainfall), product choice, particular soybean growth stage, and proximity to rust infection areas,” said Tom Allen, assistant Extension/research professor at the Delta Research and Extension Center in Stoneville, Miss., in an e-mail alert.
The announcement continues:
“This is not intended to cause a panic.
“On Thursday afternoon (Aug. 13), soybean rust was detected in a commercial soybean field in Washington County, just west of the Stoneville experiment station. Soybeans were at the R7 growth stage. Incidence (number of leaves) and severity (level of rust on each leaf) was low. I found approximately 20 leaves with probably (less than) 1 percent surface area covered on each. This field did not receive an R3/R4 fungicide application.
“On Friday (Aug. 14), soybean rust was detected in Carroll, Grenada, Humphreys, Leflore, Montgomery, Sunflower, Yalobusha, and Yazoo counties. I will go through the levels of rust, and growth stages below but I want to state just how low the levels of rust in almost all of these locations truly was.
“I am very serious when I say that on some leaves rust was detected (in the field) with a hand lens covering an area no larger than the period at the end of this sentence. In my opinion some of this was likely a secondary infection from within the state based on the age of some of the pustules.
“There is no doubt in my mind that rust was likely distributed across a wide geographical area during the rains that occurred in July”
The following counties have “very low levels of rust, less than 1 percent incidence and severity: Carroll County (soybeans at R7), Grenada County (soybeans at R6), Humphreys County (soybeans at R8), Sunflower County (soybeans at R7), Yazoo County (soybeans at R7), and Leflore County (soybeans at R7/R8).”
The Yalobusha County find (soybeans at R6) has moderate levels of rust.
As for the Montgomery County find (soybeans at approximately R6), there is “high levels of rust within this particular field. An interesting note regarding this location is the field did receive a mid-R5 strobilurin fungicide for seed quality. This likely did not get the product deep enough into the canopy and rust was not stopped.”
However, “it would not be stopped from a strobilurin fungicide anyways. (The application) would only protect uninfected plant material that received the fungicide. Please be mindful of this type of situation.”
Currently, suggestions regarding fungicides are as follows: “If you have soybeans in any of the above listed counties (with the addition of Holmes County that went positive last week) that are now in the R3/R4 growth stage and have not received a fungicide application prior to this, then either a pre-mix (strobilurin plus triazole) or a tank mix (strobilurin plus triazole) fungicide may be warranted. This is likely also the case for dryland beans since after the rainfall over the past month they likely have very good yield potential.
“However, this situation is left completely up to the producer. At this time, we feel that once soybeans have reached the R5.5 growth stage they are likely out of the woods.
“However, this is all up to Mother Nature at this point and if there are any changes we will get the information out as fast as possible.
“Soybeans that have reached physiological maturity are not impacted from soybean rust since it is a foliar (leaf) disease. With all of this said if your specific management objective includes a fungicide for seed quality we are not suggesting that you change this.”