Some Principles Of Fungicide Resistance IV: FRAC Codes
LEXINGTON, KY.
The previous two articles
in this series showed how
fungicide resistance develops,
and why over-reliance
on fungicides is risky. This
third installment “switches
gears” by considering the submicroscopic
world of fungicide
resistance.
Although they are too small to see with the
naked eye, cells of fungi are quite complex (Figure
1). Fungicide manufacturers take advantage
of this complexity by creating chemicals
that poison the biochemical activity of one or
more of these cell structures.
In order to understand how fungicides poison
fungi, it is important to understand the normal
metabolism of a healthy fungal cell. See Figure
2 for an example. In a healthy cell, enzymes
turn a certain molecule (called “substrate”) into
another molecule (called “product”). Without the
simultaneous, furious activity of many thousands
of enzymes, the cell would be unhealthy
or dead.
A fungicide is merely a chemical that interferes
with the normal function illustrated in Figure
2. One way it might interfere is
illustrated in Figure 3: by binding
the active site of the enzymes. Other
fungicides interfere in other ways,
but in all cases, they interfere with
some molecule normally present in
healthy fungal cells.
Thus, each fungicide has a particular
biochemical way of poisoning
the cell. Why is this important for
users of fungicides? This is the key
point: if two fungicides poison the
cell in precisely the same way, they
are the same fungicide, from the
point of view of the fungus. It does
not matter:
• Whether the active ingredients
have different chemical structures
• Whether the active ingredients
have different names
• Whether they are sold under different
trade names
• Whether they are made by different
manufacturers
• Whether they are formulated differently.
From the point of view of the fungus,
if they poison the cell in the
same way, they are the same fungicide.
Here is a classic example. Benomyl
was the first systemic fungicide.
It was sold under various trade
names, including Agrocit®, Benex®,
Benlate®, Tersan 1991®, and others.
Another fungicide that continues
to be important today is
thiophanate-methyl, which has been
sold under a wide variety of trade
names, including Cleary’s 3336®,
Fungo®, Topsin M®, and many others.
Benomyl and thiophanate-methyl
have different chemical structures
(Figures 4). However, they both poison
the fungal cell in exactly the
same way. Therefore they are both
considered to be benzimidazole
fungicides, a name which communicates
this shared mode of action.
This means that, even if you alternate
between fungicides within a
fungicide group, the fungus “sees”
them as the same fungicide. It also
typically means that if resistance develops
to one member of the group,
usually resistance is present for all
members of that group.
So which group a fungicide belongs
to is really important for crop producers
wanting to steward fungicides
wisely. Our producers have a lot “on
their plate”, so fortunately, you don’t
have to learn any biochemistry, or
even learn the names of fungicide
groups. Several years ago, members
of the global Fungicide Resistance Action
Committee (FRAC) decided to
represent fungicide groups using
numbers. So the benzimidazole group
(which includes benomyl and thiophanate-
methyl) is represented as
FRAC Code 1. Any fungicide with
FRAC Code 1 poisons fungi in the
same way. This also means that any product
with any other FRAC Code poisons a different
biochemical target, so any fungicides with a different
FRAC Code truly are different from the
members of FRAC Code 1.
FRAC Codes are presented on the labels of
most fungicidal products sold in the USA. See
Figure 6 for an example. This makes it easy to
alternate products having different biochemical
modes of action – just look for products having
a different FRAC Code.
Bottom line: Active ingredients within the
same “fungicide group” poison fungi in exactly
the same way. From the point of view of the
fungus, such fungicides are identical, regardless
of who markets the product. Fungicides in
the same fungicide group share the same FRAC
Code. This code makes it easy for growers to
use fungicides in ways to reduce the risk of fungicide
resistance.
More information on FRAC Codes is available
at http://www.frac.info/index.htm. Watch for
additional installments in this series of articles
on fungicide resistance. Δ
DR. PAUL VINCELLI: Extension Professor and
Provost’s Distinguished Service Professor, University
of Kentucky
Figure 1. Structure of a typical eukaryotic cell. From:
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect06.htm.
Figure 2. Normal molecular function within a living cell. From:
http://waynesword.palomar.edu/molecu1.htm
Figure 3. The red chemical is a toxin that interferes with the normal functioning of
the enzyme pictured in Figure 2. It binds to the active site of the enzyme, preventing
enzymatic activity. From: http://waynesword.palomar.edu/molecu1.htm
Figure 4. Chemical structure of benomyl fungicide. From:
http://webbook.nist.gov/cgi/cbook.cgi?ID=C17804352&Mask=200