Volume
3 Issue 9
July
18, 2006
Soybean Rust in the U.S.: An update from a
rust-free state
Soybean rust is in the news, but not in Minnesota.
The aggressive fungal pathogen that causes Asian soybean rust
continues to be of interest, and in the last two years has become
commonly known in the southern US. The chances that it still may
develop into a production issue here during 2006 dwindle with every
week and each soybean growth stage that pass.
Production of rust spores: 2006
The fungus is known to rapidly produce copious amounts of spores
that are spread by wind. This year, spore production and
consequently rust establishment, in the southern states were slowed,
and in some cases stopped, due to widespread and severe drought.
Plant hosts were present, as usual, but the dry, hot weather didn’t
support infection and disease development early in the growing
season.
Where has it been reported as of mid-July?
 As
Figure
1 illustrates, the disease has been detected in Alabama
(5 counties), Florida (12 counties), Georgia (5 counties), Louisiana
(2 counties), and Texas (1 county). Most
detections are the result of leaf tissue examinations on a
susceptible, non-cropped plant known as kudzu. Patches of kudzu, an
aggressive weedy plant species, are regularly scouted for soybean
rust symptoms in those states where the weed is found. Three soybean
rust detections in as many states (e.g.: Alabama, Florida, and
Georgia) have been recorded from this year’s cropped soybeans.
What’s happening in Minnesota?
Researchers, extension educators, and others are monitoring 26
soybean sentinel plot locations across the state for symptoms of
soybean rust. Leaves from plots are collected on a regular basis and
sent to the UM Plant Disease Clinic in St. Paul for disease
identification. Crystal Floyd, UM Soybean Sentinel Plot Coordinator
and Diagnostician, reports commonly identified diseases such as
bacterial blight (Figure
2; caused by Pseudomonas savastanoi pv.
glycinea) and brown spot (Figure
3; caused by Septoria glycines). Bacterial blight
disease development is promoted by extended wet weather and cannot
be controlled with fungicides. Brown spot is usually not an
production issue in Minnesota and fungicide application is needed
only rarely. Crop rotation is a recommended control strategy for
both diseases. Soybean rust symptoms have not been detected.
What do southern states report for current weather
conditions?
Like most other plant diseases, soybean rust is dependent on
environmental conditions for disease spread and development. Disease
development is promoted in areas where (1) leaf wetness occurs at
frequent intervals, (2) warm temperatures (59-86
F) prevail, and (3) the relative humidity exceeds 75%. A
multi-state, coordinated effort is currently in full-swing to
monitor and track development of soybean rust in the U.S.
Current reports on disease status and
weather conditions:
- Alabama remains dry, overall. Rain
showers have been localized and light. Soybean rust is present,
but the weather is not favorable for its spread or development.
- Arkansas remains free of soybean rust. Many areas in
the state haven’t received rain, while some have benefited from
localized rain showers.
- Florida has disease and it’s actively spreading.
Weather conditions promote infection and disease development.
- Louisiana is receiving rain. Weather conditions in
the last ten days have been ideal for soybean rust development.
Spore production is active and the disease is spreading in kudzu
patches that are known as positive.
- South Carolina doesn’t have the disease. Soybean
maturity ranges between R4-R5. The weather has been dry and rain
would be welcome.
- Tennessee does not have soybean rust. The weather was
wet earlier in the growing season, but has been dry for some
time. Current weather conditions don’t promote the disease’s
development.
- Texas destroyed the soybean rust diseased plants that
were detected earlier in the growing season. Additional diseased
sites have not been identified. The weather there remains dry in
much of the state. Soybeans are nearly mature.
for current updates:
USDA Soybean Rust Public Information
http://www.sbrusa.net/
Charla Hollingsworth
Extension plant pathologist
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