Introduction to Fungi, Third Edition

412 HYMENOASCOMYCETES: ERYSIPHALES
implicated, such as exclusion or reduced uptake
of the fungicides, altered membrane lipid
composition with reduced sterol content, or
mutation of the fungicide-binding site on the
cytochrome P-450 enzyme.
One of the most important recently introduced
classes of fungicides are the strobilurins.
They are based on strobilurin A (Fig. 13.15e),
a natural product from the basidiomycete
Strobilurus tenacellus initially described by Anke
et al. (1977) as a strongly and selectively antifungal
substance. This was derivatized to give
kresoxim methyl, the first commercial strobilurin-type
fungicide (Fig. 13.15f). The mode of
action is based on an inhibition of complex III
of the mitochondrial respiratory chain (Anke,
1997). Resistance of Strobilurus tenacellus to its
own product is based on a mutation of three
amino acids in the target polypeptide, and resistance
based on similar mechanisms has arisen
in many fungal plant pathogens. In consequence,
strobilurins are not currently recommended for
control of powdery mildews on cereals and
cucumber/melon crops (Hollomon & Wheeler,
2002).
A recently released compound with exclusive
activity against powdery mildews is quinoxyfen
(Fig. 13.15g). The mode of action is interesting
because this compound selectively inhibits
morphogenetic events related to infection, such
as germination or appressorium formation, but
not vegetative growth. It is therefore non-toxic to
other fungi. Quinoxyfen is not systemic but is
distributed in the vapour phase and binds to
the surface waxes of the epidermis. It is thus
ideally placed for activity against the germinating
powdery mildew conidium (Hollomon &
Wheeler, 2002). Non-fungicidal compounds
with such a highly specific mode of action
have fewer side effects against non-target organisms
such as saprotrophic or mycorrhizal fungi,
and are likely to increase in importance in
future.
Activators of systemic acquired resistance
(SAR; see p. 116), especially the salicylic acid
derivative benzothiadiazole (Fig. 13.15h), may
prove their worth in the prevention of powdery
mildew infections as well as many other plant
diseases (Salmeron et al., 2002), although they
cannot cure existing infections. Since SAR activators
trigger the expression of a multitude of
defence-related proteins and other mechanisms
in the crop plant, powdery mildews are unlikely
to develop resistance against them, like they
have done against most of the currently used
fungicides, many of which target a single site in
the physiology of the pathogen. Therefore, fungicides
must be used as cocktails containing
chemicals with different modes of action, and
following strict recommendations (Hollomon &
Wheeler, 2002).
13.8.3 Biological control
Their exposed habitat on the leaf surface renders
powdery mildews potentially susceptible to parasitism
by phylloplane fungi. Perhaps the bestknown
of them is Ampelomyces quisqualis, which
produces conidia within pycnidial fruit bodies
associated with the hyphae, conidia and, less
frequently, chasmothecia of powdery mildews.
It is commonly found in nature (Falk et al., 1995;
Kiss, 1997). Attempts are being made to develop
it as a biological control agent (see Bélanger &
Labbé, 2002), but success is limited by the ability
of powdery mildews to grow at lower humidities
than A. quisqualis. This may be alleviated by
the application of the fungus in paraffin oil
(Bélanger & Labbé, 2002). One positive aspect is
that A. quisqualis is tolerant of several fungicides
used against powdery mildews, so that integrated
control is possible in principle (Sundheim,
1982).
In general, however, the biological control of
powdery mildews, like that of most other
airborne pathogens, would appear to be limited
to greenhouses because there the environment
can be controlled to a certain extent. With powdery
mildews, humidity appears to be the crucial
parameter not only in interactions with
A. quisqualis, but also other potential control
fungi such as Verticillium lecanii or basidiomycete
yeasts, e.g. Tilletiopsis and Pseudozyma (summarized
in Bélanger & Labbé, 2002). All these, in
contrast to A. quisqualis, control powdery
mildews through the production of biologically
active substances rather than by direct parasitism.
Fatty acid derivatives seem to be the main
active compounds, and these may act by