Though the south to north movement of Asian soybean rust has been blunted in the Southeast this year, it appears the potentially lethal disease has made its way into the Carolinas.
Clemson University Plant Pathologist John Mueller says a glitch in the testing sentinel plots also may have given the disease a few days head start.
Rust was found on 25 of 25 leaves sampled from maturity Group V soybeans in a sentinel plot just outside of Estill, S.C., which is about 50 miles northwest of Savannah, Ga. These beans were sampled on Sept. 11, and the degree of infection is typical of a very severe outbreak of rust, according to Mueller.
The plot had been sampled on a weekly basis up to August 20. Then due to logistical problems the plot was not sampled for two weeks. This 20 day gap obviously was enough for rust to go from being not detectable to severe levels.
A sentinel site near Cummings, S.C., which is less than 25 miles from Estill, was sampled Sept. 12 and both the maturity Group IV and Group V cultivars were rust free.
All other soybean sentinel plots in the state were sampled and no sign of rust was found in any of these fields, either in soybeans or kudzu.
Based on the severity of rust at the Estill site, Mueller says it is likely other fields were also exposed to rust in this area. Therefore, application of a fungicide containing a triazole is warranted in southeastern South Carolina, including areas normally referred to as the Savannah River Valley plus the areas just south of the Santee river.
Addition of a strobilurin or other fungicides for the control of pod and stem diseases is probably a good idea, Mueller says.
This rust find activates Section 18 labels for rust in South Carolina. These fungicides are labeled only for control of rust.
The list of new Section 18 labels for S.C. includes: Alto, Caramba, Punch, Charisma, and Topguard. The list of previously labeled Section 18 fungicides for rust includes: propiconazole (Tilt, PropiMax and Bumper); Tebuconazole (Folicur, Uppercut, and Orius); myclobutanil (Loredo); propiconazole + trifloxystrobin (Stratego); pyraclostrobin + tebuconazole (Headline SBR); and azoxystrobin + propiconazole (Quilt). Domark 230 ME (a triazole) is now labeled along with the nitrile fungicides Bravo, Echo, and Equus, and the strobilurins Headline and Quadris with section 3 lables.
These fungicides are active against rust and the pod and stem diseases.
Topsin has a section 3 label for soybeans but does not have activity against rust. It is active against pod and stem diseases.
Mueller stresses that fungicides may not be applied after 50 percent of the soybeans in a field reach growth stage R-6. This is when a pod is present containing a green seed that fills the pod capacity at one of the four uppermost nodes on the main stem.