Soybean
growers in the United States, and especially in the Mid-South,
are being put on alert for the invasion of a plant disease
resident pathologists say could rob Mid-South growers
of as much as 40 to 50% of their annual soybean yields.
The fungal disease hasn’t reared its ugly head in
our homeland yet, but agriculture officials are preparing
for the when, not the if, of the appearance of soybean
rust.
Soybean rust moved from Japan and China into South Africa
and in the past few growing seasons wreaked havoc with
South American soybean yields.
“Soybean rust causes early leaf defoliation, which
affects yields. We know infected fields in South America
have seen losses of 20 to 50 %,” says Benny Graves,
plant pest division director with the Mississippi Department
of Agriculture and Commerce’s Bureau of Plant Industry.
“Unfortunately, all of our soybean varieties are
susceptible to soybean rust. We don’t have any resistant
varieties available for growers.”
Graves, speaking to the Southern Association of State
Departments of Agriculture during their recent annual
meeting in Jackson, Miss., issued the warning for all
Southeastern states to be on alert.
He expressed the probability the disease will find its
way into the United States by natural methods of transmission,
but also says the industry needs to be aware of the possibility
the disease could be brought into the country intentionally.
“We are always concerned about our homeland security
and bioterrorism, and this disease could be used against
us,” he says.
Soybean rust is spread easily by spores, which travel
with the wind or could be brought into the country on
the clothes of someone who visits a South American soybean
field.
“Because it is wind dispersed, there is no way to
quarantine this disease. A hurricane would be a good way
to bring it this way, or it could come by way of the land
bridge up from South America and Mexico,” says Graves.
Plant pathologist Billy Moore, who works with the Mississippi
Soybean Association’s SMART program, says soybean
rust will eventually show up in the United States, and
when it does, it will be an expensive disease to bring
under control.
Moore says fungicides that will control rust are available
to growers, but it could take up to three applications
per season at a cost of about $15 per application, per
acre, for the chemicals alone.
“If it gets here, we want to be able to find it
fast, and with the appropriate fungicides, control it
fast to prevent losses,” he says. “South American
producers have lost 40 to 50% of their yields in some
fields. That would be unbearable if it comes here and
we don’t have resistant varieties and we don’t
have control procedures. This is a particularly devastating
disease. If it makes its way to the United States, it
could be the worst disease we’ve ever been faced
with.”
Multi-level
boundaries for defense
Graves says MDAC in cooperation with other state agencies
and agricultural organizations are on alert for the disease.
Several joint sessions have been held to teach agricultural
consultants, MDAC inspectors, MSU Extension agents and
others how to watch for and identify soybean rust. Similar
coordinated efforts are taking place at the national level.
“We are aggressively checking our soybean fields
for soybean rust,” says Moore. “We feel like
if it does occur, we will find it.”
“All the projection models show the Mid-South has
the right environmental conditions for this disease. The
consensus is it will show up in the Gulf Coast states
first and could cause the most severe yield losses in
those states,” says Graves.
Soybeans grown in the Mid-West aren’t immune from
the disease, but because the environmental conditions
of that area aren’t as favorable for the disease,
yield losses there aren’t projected to be as significant
as they could be in Mid-South.
“If it shows up, one of the aspects of concern is
the psychological affect the disease and possible yield
losses from it would have on the soybean market,”
added Graves. “That’s another reason we are
moving ahead to have a plan in place to control it when
it shows up. If we find it, we will have to be aggressive
in our use of fungicides and in our recommendations to
control it.”
Graves adds that in addition to the labeled fungicides,
MDAC plans to have Section 18 permits, which allow for
non-labeled chemical applications under special circumstances,
in place so time will not be wasted getting clearance
to use the needed fungicides.
Another complicating factor of soybean rust is that Kudzu,
which is prevalent in the Mid-South, is also a host plant
for soybean rust.
“The rust won’t kill Kudzu, so it serves as
a host for the rust to grow and disperse spores,”
says Moore.
Moore says plant breeders are in an accelerated mode in
their search for resistant varieties, but he believes,
even if germplasm with resistance were to be identified
this year, it would take about seven years to bring the
material forward as a variety for growers, and he doesn’t
think soybean rust is going to wait that long.
“The only thing we will have to rely on will be
fungicides,” says Moore.
The
signs of soybean rust
According to USDA’s Animal and Plant Health Inspection
Service, host plants infected with soybean rust first
show small lesions, which gradually increase in size turning
gray to tan or brown. They become polygonal shape restricted
by leaf veins and may eventually reach two to three square
millimeters.
Infection begins on the lower first leaves. Choruses,
or yellowing of infected leaves, progresses in different
degrees, usually not completely until the infection is
well advanced. Lesions may appear on the petioles, stems
or pods, but are most common on the leaves, especially
on the lower surfaces.
As the plant matures and sets pods, infection progresses
rapidly under the right environmental conditions (moisture,
high humidity and heat) to cause high rates of infection
in the middle and upper leaves of the plant.
The symptoms of soybean rust are similar to the symptoms
of two common bacterial diseases: bacterial postule and
bacterial blight.
Photographs and instructions for collecting and verifying
samples from suspect plants are available on the APHIS
soybean rust site at
http://www.aphis.usda.gov/ppq/ep/soybean_rust. DBJ
