cereal rust fungus, a powdery mildew fungus, a downy
mildew pathogen (e.g. Phytophthora infestans), and a
rice pathogen (Magnaporthe grisea). Compounds that
show promise in this primary screen are chemically
modified to produce a range of isomers for further test-
ing, to enhance the fungicidal activity, and to minimize
undesirable side effects. This type of approach led to
the separate patenting of benomyl and carbendazim
from the initial compound, thiabendazole. It is now
believed that they have similar or identical modes of
action: they are converted to methyl benzimidazole-2-
yl-carbamate (MBC) which binds to spindle micro-
tubules and blocks mitosis. They also affect fungal tip
growth, by binding to the cytoplasmic microtubules
which deliver components to the growing hyphal tip
(Chapter 4).
Benzimidazole fungicides are selectively antifungal,
with little or no effect on plant and animal cells,
despite the fact that plants and animals also have
microtubules. The reason for this was shown by in
vitrostudies. Microtubules are formed when β-tubulin
forms a dimer with α-tubulin, and the microtubule
grows by successive additions of these dimers. MBC
binds strongly to the β-tubulin of fungi, preventing its
association with α-tubulin, and therefore preventing
the self-assembly of microtubules. MBC does not bind
strongly to the tubulins of higher animal or plant cells,
nor to the tubulins of Oomycota. So it is a selective
antifungal agent (Davidse 1986).
The benzimidazoles have been used successfully to
control several plant pathogens, but fungi can develop
tolerance (resistance) to them if they are used re-
peatedly. This seems to be caused by point mutations
at various sites in the β-tubulin gene, sometimes invol-
ving reduced binding of the fungicide to β-tubulin
and sometimes affecting the interaction of β-tubulin
with α-tubulin or with the tubulin-associated proteins,
all of which are necessary for functional microtubules.
Despite the use of benomyl (Benlate) as a fungicide
for more than 30 years, it has been linked to a birth
defect in which mothers exposed to benomyl in the
very early stages of pregnancy can give birth to children
with empty eye sockets. According to The Observer
newspaper (21 December 2003), “more than 40% of
pregnant rats fed high levels of benomyl produced
foetuses with severe eye defects.” The manufacturer
recently withdrew Benlate from the global market.
Sterol synthesis inhibitors
Sterols are found in the cell membranes of all eukary-
otes and of some archaea (the methanotrophs), but are
absent from bacterial membranes. Sterols insert into the
phospholipid bilayer and help to maintain membrane
stability and fluidity. Different groups of organisms have
different types of membrane sterol. Fungi characterist-
ically have ergosterolas their membrane sterol, whereas
cholesterol is the characteristic sterol of animals, and
sitosterol and similar phytosterols are found in plants
and Oomycota.
All sterols are synthesized by a complex, multistep
pathway, described in Chapter 7 (see Fig. 7.15). In
outline, they are derived from the condensation of
three molecules of acetyl coenzyme A, to form isoprene
units(5-carbon compounds), then three isoprene units
condense to form the 15-carbon compound, farnesyl
pyrophosphate. Two molecules of this combine to
form the 30-carbon compound, squalene, which then
undergoes a series of cyclization reactions and ring
closures, resulting in the sterol, lanosterol. This is
the precursor sterol from which all other sterols are
produced. There are several intermediate steps from
lanosterol to the final sterol, but the key step leading
to ergosterolis the removal of a methyl group from the
C-14 position of the molecule. This step is catalysed
by the enzyme lanosterol 14 αα-demethylase, which
has an iron-containing cytochrome P-450 as its co-
enzyme. This demethylation step occurs only during
synthesis of the fungal sterol, not during the synthesis
of plant or animal sterols, so it provides an ideal target
for antifungal agents.
Several systemic fungicides act by inhibiting sterol
demethylation, with the result that the fungus can-
not synthesize its normal sterols, and instead other
sterols such as lanosterol are incorporated in the fun-
gal membrane. This leads to membrane leakage and
ultimately to cell death. The imidazoles(e.g. imazalil,
Fig. 17.5) and triazoles(e.g. propiconazole, Fig. 17.5)
are important examples of fungicides that act in this
way. Their common feature is the possession of a
five-membered heterocyclic ring containing either
two (imidazoles) or three (triazoles) nitrogen atoms.
These azole fungicides are thought to act in the same
way as the azole drugs used to treat fungal infections
of humans (see below). The lipophilic part of the
fungicide is thought to bind to the demethylase
enzyme, while nitrogen in the heterocyclic ring asso-
ciates with an iron-containing coenzyme, blocking
demethylation.Group-specific systemic fungicidesSeveral systemic fungicides act more or less specifically
on particular fungal groups. For example, the carbox-
amidefungicides such as carboxin (Fig. 17.6) act
against Basidiomycota (rusts, smuts, Rhizoctonia solani)
by interfering with respiration; they inhibit the step
in the TCA cycle where succinate is dehydrogenated
to fumarate (see Fig. 7.2). The 2-aminopyrimidine
fungicides such as ethirimol(Fig. 17.6) act specifically