untitled

develops from a small nonmelanized region of the
wall in contact with the host surface. If the fungus can-
not synthesize melanin then there is a broad zone
of contact with the host surface, and the attempted
penetration fails. Melanin-deficient mutants of these
pathogens show a similar inability to penetrate the host.
The main target pathogens, including rice blast, can
rapidly develop resistance to these fungicides, whose
mode of action is based on a specific enzyme – either
a reductase or a dehydratase in the melanin biosynthetic
pathway (Wolkow et al. 1983).

Strobilurin fungicides

Strobilurin Aand Oudemansin Aare natural pro-
ducts found in the toadstools of two wood-rotting
Basidiomycota, Strobilurus tenacellusand Oudemansiella
mucida(Fig. 17.8). They were discovered independ-
ently but later were found to be chemically identical,
with a common mode of action – they inhibit mito-
chondrial respiration in fungi, by blocking the oxida-
tion of ubiquinol at a specific site of the cytochrome
bc1 complex in the electron-transport chain. Thus,
they inhibit the generation of ATP. Although these
compounds had been known for some time, they only
started to be developed commercially in the early
1980s when agrochemical companies produced syn-
thetic analogues, known as the strobilurins, which were
photochemically stable and had other desirable prop-
erties such as low mammalian toxicity, appropriate
mobility within plants and acceptable crop safety.
The first commercial strobilurins – azoxystrobinand
kresoxim methyl(Fig. 17.9) – were launched in 1996,

and there are now over 700 patents filed on these com-

pounds by the major agrochemical companies.

The strobilurin fungicides show an astonishingly

wide range of activity against all the major taxonomic

groups of plant pathogens. For example, azoxystrobin

is registered for use against more than 400 plant

pathogens. It strongly inhibits spore germination,

shows excellent preventative activity, and has eradic-

ant and antisporulation properties. Specific formula-

tions are marketed for many different types of crop,

including all the major diseases of cereals. The ques-

tion arises as to why these compounds are toxic to fungi

but have little effect on plant hosts, which have the

same mitochondrial target sites. The proposed explana-

tion is that there could be differential penetration

and degradation of these fungicides in plants compared

with in fungi. Like all fungicides that act on specific

metabolic targets, there is strong evidence that fungi

can develop resistance to the strobilurins. So they need

to be used in mixtures or as alternating treatments with

other fungicides, as part of a resistance-management

strategy.

Antifungal antibiotics used for plant disease

control

There are several antifungal antibiotics (Table 17.3)

but only a few are selective enough to be used for dis-

ease control. For example, cycloheximideis a broad-

spectrum antifungal agent and is widely used as an

experimental tool in laboratory studies, but it acts by

blocking protein synthesis on 80S ribosomes and is

therefore toxic to all eukaryotes. It is not used for

disease control. The few antibiotics that have been used

PRINCIPLES AND PRACTICE OF CONTROLLING FUNGAL GROWTH 347

Fig. 17.8Oudemansiella mucida, the “porcelain fungus”
that grows on rotting wood and is one of the original
sources of strobilurin-type fungicides. (Courtesy of Marek
Snowarski, Fungi of Poland; http://www.grzyby.pl))

CH 3

CH 3

CH 3

O

C

C

CH 2 O

O

O

N

C N

O

O

CH 3

C

C

O

O

O

HC

N

N

Kresoxim-methyl Azoxistrobin

Fig. 17.9Structures of two strobilurin fungicides.