Aberdeen, Idaho
August 9, 2004
When late blight struck a western Bingham County potato
field on July 26, University
of Idaho Extension potato pathologist Jeff Miller was
waiting for it.
Two different weather-based
forecasting models he and graduate student Donna Henderson
had been analyzing predicted that the potentially disastrous
fungal disease would strike this year. Funding from the USDA
Risk Management Agency has allowed Miller to work on the
models for the past 3 years, after an initial grant from the
Idaho Potato Commission.
The first model-pioneered in
Washington-uses AgriMet weather data collected all over
southern Idaho to predict the probability of late blight in
a given growing season. It bases the risk of an outbreak on
the amount of precipitation that falls in April and May and
on an additional "favorable hours" factor that Henderson
identified and developed: the number of hours in April and
May that temperature is between 50 to 80 degrees Fahrenheit
and relative humidity is over 80 percent.
By June 1, this probability
model was predicting that late blight outbreaks were "highly
likely" in southern Idaho.
The second model, based on one
widely used in the Midwest, is much more complex. At weather
stations in Wendell, Twin Falls, Oakley, Aberdeen and
Rupert-Minidoka, it measures how long potato leaves are wet
and the temperatures occurring during that timespan. Each
day, the model assigns a "severity value" of 0 to 4. Once
these numbers add up to 17, late blight should occur within
10 to 14 days.
Miller says this leaf wetness
model predicted that late blight would develop by July
28-31-just two days later than it was first reported.
By Aug. 6, UI scientists had
confirmed late blight in five different areas-three in
western Bingham County, one in Minidoka County and one in
Cassia County. The source of the disease could be infected
seed, infected carryover potatoes-volunteers or cull
piles-from last year or spore-laden thunderstorms.
"Now that it's around, growers
near infected areas should be spraying with a protectant
fungicide on a weekly basis," Miller says. Fortunately, many
growers tipped off by the first model have been proactively
treating their fields since row closure and others have
responded quickly enough to prevent crop losses.
Mother Nature, however, gets
much of the credit for restraining damage to just a few
hotspots, Miller says. "The weather turned hot and dry. When
that happens, late blight spores aren't produced in great
numbers and don't move very far. But that doesn't mean
growers can stop worrying. If it turns wet again, it will be
all we can do to get to harvest."
Henderson, who combined her
interests in biology, statistics and modeling when she began
working on the project last year, calls the results of her
efforts "very satisfying." With no late blight reported in
Idaho in 2002 and only a few fields in 2003, the disease's
sporadic nature demands a well-informed defense by
producers, she says. "We thought it would benefit farmers to
have an advance warning system so they could aggressively
manage their fields before late blight occurred."
To track the progress of late
blight in Idaho, growers can click on
www.ag.uidaho.edu/potatopath/alerts.htm, call the UI late
blight hotline at 1-800-791-7195, sign up for e-mail updates
at www.pnwpestalert.net or subscribe to the Dow
Agrosciences' Blight Risk Alert fax service at
1-888-395-7378.
More information on late blight is available on the Web site
of the Idaho Center for Potato Research and Education,
www.ag.uidaho.edu/potato.
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