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"Based on the new discovery in Florida,
the center has issued its forecast for a serious threat," said
Dennis Bowman, crop systems educator with University of Illinois
Extension. "The site indicates that favorable conditions exist for
spore production at the source, for long-range transportation and
for effective deposition. There is also a high-risk warning for
susceptible plants in southeast Louisiana, the Florida panhandle,
Mississippi, Alabama and Tennessee."
According to the forecast, there is a
moderate risk for the remainder of the Ohio Valley region, including
Kentucky, western West Virginia, and the southern fringes of Indiana
and Ohio, plus the central and southern Appalachian Mountains. There
is a low risk for areas beyond, due to the massive washout of
airborne spores expected Friday and Friday night and the cold
temperatures to the north.
Bowman notes, however, that making
sense of the forecast requires an understanding of exactly how the
words "threat" and "risk" are defined.
"The term 'threat' refers to disease
development factors located at or very near the source," Bowman
said. "The term 'risk' refers to the potential spread and
development at the source and along the forecast trajectory. Threat
is related to weather conditions, disease level and the source area,
which determine the level of spore production."
The term "threat" also includes the
potential for transportation to other growing areas, based on
favorable overhead and nearby sky conditions.
"The risk focuses on the danger of
disease development for growing areas along the course of a
trajectory," Bowman said. "The principal factors evaluated in the
forecast are spores being produced at a source of infection, the
potential for spore survival during transport, the potential for
spore deposition and the potential for successful infection at the
site of deposition."
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"Low risk" means that almost all
conditions are unfavorable for rust survival. "High risk" means all
or almost all conditions are favorable for the successful spreading
of the disease. In the middle there is "moderate risk."
"The forecast center has divided
that up into three levels of moderate risk," Bowman said. "Those are
weakly moderate, moderate and strongly moderate. As an example, a
forecast with mostly cloudy weather and a 30 to 40 percent chance of
rain along the projected path of the spores would fall into the
range of strongly moderate risk. Cloudy weather improves spore
survival, and rain will wash the spores out of the atmosphere and
onto the crop."
Although this information will be
very useful in determining how best to manage soybean rust in
Illinois, Bowman emphasizes that the alert is only a forecast.
"Just like with a weather forecast,
it is a prediction about what may happen, based on the best
information available," Bowman said. "As we learn more, the forecast
will get better. For now, it is important to remember that high risk
does not guarantee infection and low risk does not mean there is no
risk at all."
[University of Illinois news release]
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