Soybean Rust Spreads to Alabama

Scouts have confirmed first case of rust on soybeans in Alabama, Florida also notes rust on soybeans for first time. Compiled by staff

Published on: Jun 30, 2005

USDA has announced that Asian soybean rust has been found in Baldwin County, Ala., which would be the first instance of the disease discovered outside of Georgia and Florida. The Georgia find earlier this year was on volunteer soybeans that have since been destroyed.

The Alabama soybeans were in two different sentinel plots, each in different reproductive stages. Recent weather conditions may also be right for spores to move north.

The ag agency has also reported a find of rust on soybeans in Florida in Marion County. The Florida find was a sentinel site.

ALABAMA REPORT filed 6/30/05

Observations

Soybean rust was found in Alabama for the first time this year. The disease was detected in soybean sentinel plots growing in Baldwin County. Baldwin County is on the east side of Mobile Bay in southwest Alabama. Approximately 10 leaves in the lower canopy of two plants were expressing symptoms of the disease. Samples were collected on June 28 and the pathogen was identified by microscopic examination and serological testing on June 29. The disease was found on one plant at the R6 growth in an early-planted sentinel plot and on a second plant at the R1-R2 growth stage in a neighboring sentinel plot. The University of Arkansas found four soybean rust-like spores in a Syngenta spore trap located in close proximity to these sentinel plots earlier this week.

The spores would have been deposited at this location sometime between June 16 and June 22. Weather conditions in that part of the state have been warm (75-90 degrees) with frequent rain showers and cloudy skies. On June 30, the University of Arkansas informed me that that they found 25 soybean rust-like spores in a Syngenta spore trap in Headland, Alabama in Henry county in the southeast corner of the state. This area is within 40-50 miles of Seminole County, Georgia and Leon County, Florida, two sites that soybean rust had previously been found on soybean. Samples were collected from a soybean sentinel plot at the Headland site next to the spore trap on June 29 but rust was not detected through microscopy or serological testing.

Spore traps have also been established in Escambia, Lee, Elmore, Pickens, DeKalb and Limestone counties. Spores have yet to be detected in these traps. On June 29, no rust was detected in soybean sentinel plots in Calhoun, Etowah, Talladega or DeKalb counties in east central Alabama or Fayette county in west central Alabama. Also, no rust was detected in kudzu sentinel plots in Tuscaloosa, Autauga or Lee counties. No rust was detected in soybean and kudzu sentinel plots in Pickens county on June 28.

Growth Stages

The majority of soybean sentinel plots have reached reproductive growth stages. Kudzu is widespread throughout the state. Commercial soybean fields have reached the R3 growth stage in west-central Alabama (Pickens county) and in the north and northeast soybean production areas. The majority of soybeans in south Alabama were planted in early June, though scattered fields are in reproductive growth stages.

Management

Growers in south and central Alabama with a crop between bloom (R1) and pod fill (R6) should strongly consider applying a premix or tank mix of a strobilurin and triazole-type fungicide. Growers in north Alabama should be prepared to apply a strobilurin or a triazole/strobilurin combination product if the disease appears to be moving into their region. An application at the R3 growth stage, regardless if rust is reported in the immediate area, would provide about 3 weeks protection against rust as well as Cercospora and frog-eye leaf spot.

Forecast Outlook

In the last seven days weather conditions in south and central Alabama have been favorable for rust development. Temperatures of 70-90 degrees and frequent, though scattered, rain showers have been common. Conditions have been relatively dry in north Alabama though temperatures appear to be favorable for the disease. Heavy dews would also provide the moisture needed for rust development.

Scouting Recommendations

Fields should be scouted at least twice-a-week. The first symptoms of rust are very small brown to brick red spots on the upper leaf surface. Eventually, slightly raised pustules form in the spots, primarily on the lower leaf surface. As pustules become numerous, leaves turn yellow and drop prematurely.

Scouting Techniques

Focus scouting efforts in sections of soybean fields that may be shaded for part of the day which may allow for more favorable (wet) conditions for rust development.