West Lafayette, Indiana
May 22, 2007
Indiana is at greater risk for
developing soybean rust this year than in the past two years,
said a Purdue University
expert. Plant pathologists reported finding soybean rust 53 days
earlier this year than last on kudzu in southern Louisiana.
"This is not good news for Indiana soybean growers because it
opens the window of time for rust to develop in the Hoosier
state," said Greg Shaner, Purdue Extension soybean rust
specialist.
Soybean rust first appeared in Indiana last year, but it arrived
in October - too late to damage the crop, Shaner said. If the
rust had arrived earlier in the growing season, as it threatens
to do this year, it could have resulted in yield losses.
It is still too early to predict the likelihood and potential
intensity of soybean rust in Indiana this year, but the early
breakout has researchers on the lookout. Timing is important
because the soybean plant is most vulnerable in the flowering
stage, which occurs in late June and early July.
"To the best of our knowledge all of our soybean varieties are
susceptible to rust, and the only way to control it is to use a
fungicide," Shaner said.
If soybean rust is found in Indiana, all the agricultural and
natural resources Extension educators will be notified, and
efforts will be made to get information to growers.
One thing growers can do to prepare is to check with their local
chemical dealer and make sure a fungicide will be available in
late July or early August, if needed. However, Shaner does not
believe there is a need to stockpile fungicides.
Soybean rust overwinters in Florida and along the Gulf Coast,
and the spores migrate north as the weather warms, drifting on
the wind like smoke from a smokestack.
"Given the right weather conditions, soybean rust can travel up
to 500 miles in two or three days," Shaner said.
Rainfall scrubs spores from the air and deposits them on the
plants below. Depending on rainfall intensity, some spores may
escape being washed down and will continue migrating, so
deposition can occur over a wide area.
To help determine the path of these spores, scientists look at
the wind trajectory and where rain falls, which helps determine
where spores will be deposited. Last week's heavy rains in
Louisiana created ideal conditions for the development of rust,
and more rain is expected, Shaner said. However, the rust in
Louisiana does not pose an immediate threat to Indiana because
soybeans are just being planted here.
If it were fairly dry immediately north of a source of rust
spores in Louisiana, there may be a gap in the rust outbreak.
The disease would leapfrog, perhaps skipping 100 or 200 miles
and appear in Arkansas or Tennessee. If the scrubbing wasn't
thorough enough over that area, some spores could keep moving
north, all the way to Indiana.
Indiana's summer weather is probably favorable for the
development of soybean rust in most years. Once the spores land
on a leaf, they need moist conditions for at least seven hours
or more to germinate.
Soybean rust can move fast - it only takes nine days from the
time a spore infects for that infection to start producing
pustules. Initial infections that develop from spores coming
from the south will most likely be few, and they are hard to
detect at this stage, Shaner said. But, the very early stages of
disease development, when no more than 10 percent of leaves show
any infection, is the threshold for effective disease control
with a fungicide.
During the summer months, scientists will be out collecting 100
leaves from each of Indiana's 20 sentinel plots each week,
Shaner said. The leaves will be shipped overnight to a Purdue
lab and examined by scientists for signs of rust. Each 50-by-50
square-foot plot is just a small fraction of the total soybeans
planted in the state, but the intensive scouting of these plots,
along with examination of similar plots in all states south of
Indiana, provides an effective early warning system for the
disease, Shaner said.
In addition to monitoring the soybean sentinel plots, several
kudzu sites in the state will be monitored. |
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