Louisiana Soybean Rust Detection Questions
and Answers
Plant Protection & Quarantine
November 2004
Q: Where has soybean rust been detected in the continental
United States?
A: Phakopsora pachyrhizi, or soybean rust, was confirmed
on November 9 in two research plots associated with Louisiana State
University’s research farm located approximately three miles from
the main campus in Baton Rouge, Louisiana.
Q: What methodologies were used to positively identify the
pathogen affecting the suspect samples as soybean rust? And, what does
that
testing process involve?
A: USDA scientists with APHIS’ National Plant
Germplasm and Biotechnology Laboratory in Beltsville, Maryland, confirmed
the presence of
soybean rust using polymerase chain reaction (PCR) and morphological
testing. PCR testing is simply a DNA test. In using PCR techniques,
DNA is
extracted from spores or infected leaf samples, subjected to PCR, and
then ground and purified before being analyzed for the presence of key
diagnostic sequences of DNA that distinguish soybean rust from related
species. Morphological testing is simply a microscopic examination of
the spores or lesions on infected leaf samples. The testing process
is generally completed within 24 hours.
Q: What is soybean rust?
A: Soybean rust is caused by either of two fungal
species, Phakopsora pachyrhizi, also known as the Asian species, and
Phakopsora meibomiae, the New World species. Affected plants are quickly
defoliated, resulting in reduced yields and seed quality. Soybean rust
has devastated soybean crops in many parts of the world, with reported
yield losses as high as 80 percent in some afflicted areas of Africa
and South America. The Asian species of soybean rust is the species
detected in Louisiana.
Q: How is soybean rust spread?
A: Soybean rust is a fungus, spread primarily by windborne
spores that can be transported over long distances. Seed-borne transmission
has not been documented. Clouds of spores are released if infected plants
are disturbed by wind or by individuals walking through rust-infested
areas.
Q: What is believed to be the source of infection for the recent
detection?
A: APHIS has dispatched its Soybean Rust Detection
Assessment Team to work closely with state officials to determine the
nature and extent of infection. An exact source of infection may never
be known, but a probable explanation is the spread of the disease from
South America to the United States during the active hurricane season.
Q: What steps is APHIS taking to control or prevent the further
spread of soybean rust in the United States?
A: At present, APHIS has dispatched its Soybean Rust
Detection Assessment Team to determine the nature and extent of infection.
Because soybean rust is spread via wind currents, quarantining the area
will not be effective to contain the disease. In preparation for the
possible introduction of the disease, APHIS had been working with industry
groups and its sister agencies, Agricultural Research Service and Cooperative
State Research, Education and Extension Service, to educate soybean
producers nationwide on how to identify the disease and report suspected
finds. With the soybean harvest in Louisiana complete and the detection
of the disease in a research plot, this is the best possible time for
the introduction. Now that we have detected the disease, we can better
prepare growers for the upcoming season.
Q: Is the detection of soybean rust expected to impact the
trade of U.S. exports?
A: No. USDA does not anticipate any effect on exports.
The disease occurs in most areas where soybeans are grown: Asia, Australia,
Africa, and South America Hawaii. The New World Species is also found
in the Carribbean. Despite the presence of soybean rust, many of these
countries export
soybeans.
Q: Can soybean rust be controlled to minimize potential impacts?
A: Yes. Soybean rust can be managed with the
judicious use of fungicides. However, early detection is required for
the most effective management of
soybean rust. Fungicide applications can reduce yield loss depending
on the developmental stage of affected plants, the time when soybean
rust is
detected, and the fungicide application method. For efficacy information
on fungicides labeled for use on soybeans, consult university extension
personnel in your state.
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