"The three major families of fungicides
used to combat this disease are the strobilurins, the sterol
inhibitors and the nitriles, which are usually referred to by the
principal active ingredient of chlorothalonil," said Matt
Montgomery, crop systems educator with University of Illinois
Extension. "It is important in combating rust to understand how each
of these products work and under what circumstances they should be
Montgomery notes that
the three fungicide families can generally be classified into one of
two categories. Strobilurins and chlorothalonil are both categorized
as protectants when used in soybean fields.
"These products inhibit the
establishment of the fungus in plant tissue and exhibit limited
mobility in beans," Montgomery said. "Recommended use is therefore
largely restricted to tissues that lack many soybean rust pustules.
Even the best of these materials only works effectively when less
than 3-5 percent of the leaf surface is covered with rust pustules."
The strobilurins include products
with a number of different active ingredients, including
azoxystrobin, which is used in Quadris; trifloxystrobin, which is
used in a portion of Stratego; and pyraclostrobin, which is used in
"Those active ingredients are
currently in various states of registration, and no endorsement is
intended," Montgomery said. "All product names are property of their
Montgomery points out that the
sterol inhibitors, which are also known as triazoles, are deemed as
curative products and designed for different uses than the products
in the other category.
"The triazoles are more mobile and
kill fungal tissue," Montgomery said. "Recommended use is therefore
targeted toward those tissues that exhibit clear signs of the
disease. This means that growers must shift to triazoles once 3-5
percent of the leaf surface is covered with rust pustules."
Each of the major fungicide groups
works in distinctly different ways.
top of second column in this article]
Strobilurins injure fungi by
disrupting the electron transport chain at work within the cells.
"The active ingredient in this group
shuts down respiration, which spells death for the fungus,"
Montgomery said. "Strobilurins affect a fairly narrow site of
action, causing some real resistance concerns."
Montgomery notes that chlorothalonil
also disrupts respiration, but the method by which it does so is
very different from the strobilurins.
"The active ingredient in this group
inhibits a host of enzymes needed for various cell processes," he
said. "Many of those enzymes are needed to advance various parts of
the respiration process, which therefore results in inhibited
respiration. Fungal tissues therefore die due to energy depletion or
Most researchers express fewer
concerns about the development of resistance to this group compared
with the other rust fungicide families because the material affects
a fairly broad site of action. Montgomery points out that this does
not mean there are no concerns at all, only that there are fewer
worries about resistance with this group of products.
"Sterol inhibitors or triazoles also
inhibit enzymes," Montgomery said. "However, these active
ingredients inhibit enzymes needed to form sterols, which are
sometimes termed lipids. Sterol inhibitors affect a fairly narrow
site of action, causing some resistance concerns."
The sterol inhibitors include such
active ingredients as propiconazole, which is used in Tilt and the
other portion of Stratego; tebuconazole, which is used in Folicur;
tetraconazole, which is used in Domark; and myclobutinil, which is
used in Laredo EC.
"No matter what brand name a product
goes by, it is essential to understand which mode of action it
uses," Montgomery said. "Only then can a grower make an informed
decision about how to effectively use it to combat soybean rust that
appears his field. The results are beneficial to both the
environment and the bottom line of the farmer."
[University of Illinois news release]