Introduction to Fungi, Third Edition

410 HYMENOASCOMYCETES: ERYSIPHALES
adult plants. This phenomenon is called adult
plant resistance (Hsam & Zeller, 2002). Thus,
it can be envisaged that the products of horizontal
resistance genes may be involved, for
example, in producing a thicker cuticle, cell
wall or papilla.
Resistance breeding will certainly remain one
of the key methods for controlling powdery
mildews, and it is likely that major and minor
genes will be combined to produce cultivars
with more durable and effective resistance to
B. graminis and other powdery mildews. Further,
genetic engineering may enable breeders to produce
cultivars with overexpresssed pathogenesisrelated
(PR) genes (Salmeron et al., 2002). The
principles of PR genes are explained briefly on
p. 116.
13.8.2 Chemical control
A good overview of fungicides against powdery
mildews has been given by Holloman and
Wheeler (2002). By far the oldest remedy against
powdery mildew is powdered elemental sulphur,
which was mentioned by Homer in about
1000 BC (Agrios, 2005) and was rediscovered in
the nineteenth century and combined with lime
for enhanced efficacy (Large, 1940). Sulphur
lime mixtures saved the French wine industry
from ruin in the mid nineteenth century when
U. necator appeared in Europe, just as Bordeaux
mixture (based on copper sulphate and lime)
played a crucial role in protecting vines against
Plasmopara viticola later that century (see p. 120).
In the early twentieth century, as described
above, efforts to control B. graminis on
cereals shifted to breeding for resistance.
Dithiocarbamate (see Fig. 5.27) was released in
1934 and, like sulphur, had a purely protectant
activity.
The first systemic fungicide against powdery
mildews was the benzimidazole benomyl
(Fig. 13.15a) which is converted to the active
molecule carbendazim inside the plant. Carbendazim
binds to tubulin proteins, interfering
with their assembly into microtubules and
especially with the nuclear spindle during
mitosis (Davidse & Ishii, 1995). Resistance against
benomyl is common among plant-pathogenic
fungi and is usually due to a mutation in the
b-tubulin gene at the site where carbendazim
binds, thereby reducing its affinity (Davidse &
Ishii, 1995).
The next two important groups of systemic
fungicides to be released against powdery mildews
were the morpholines (Pommer, 1995) and
2-aminopyrimidines introduced in the 1960s
(Hollomon & Schmidt, 1995). Both are used more
or less exclusively against powdery mildews.
Fig13.15 Fungicides against powdery mildews. (a) The benzimidazole benomyl.The active substance, carbendazim, is
produced in planta by removal of the side chain at the position indicated by the arrow. (b) The morpholine tridemorph. (c) The
2-aminopyrimidine ethirimol. (d) The triazole triadimefon.The molecule becomes reduced to its more active alcohol in planta and
by fungi at the position indicated by the arrow. (e) Strobilurin A, an antifungal substance from Strobilurus tenacellus. (f) Kresoxim
methyl, the first strobilurin-based fungicide. (g) Quinoxyfen. (h) Benzothiadiazole (¼ benzo(1,2,3)thiadiazole-7-carbothioic acid
S-methyl ester), an inducer of systemic acquired resistance.