untitled

against powdery mildew fungi by inhibiting an
enzyme (adenosine deaminase) involved in purine
metabolism. In this case, fungicide-tolerant mutants
often show no alteration of the enzyme but are
thought to circumvent the inhibition by obtaining
purines from the host plant.
The acylalaninefungicides (e.g. metalaxyl, Fig. 17.7)
act specifically against Oomycota by inhibiting the
nuclear RNA polymerase, thereby blocking RNA
synthesis. They have no effect on the true fungi
but are highly effective in controlling Pythiumand
Phytophthora spp., including Phytophthora infestans.
Resistance to the acylalanines can develop rapidly
in field conditions, so these fungicides are used in
combination with protectant fungicides such as the
dithiocarbamates. Fosetyl-aluminium (Fig. 17.7) is
another compound that acts specifically against
Oomycota, and it has the remarkable property of
being fully mobile in phloem. So it can be applied
to the shoots and it moves into the roots, making
it extremely valuable for control of root-infecting

Phytophthora spp., such as Phytophthora cinnamomi.

The mode of action of fosetyl-Al is still unclear. It shows

little or no activity against Oomycota in laboratory cul-

ture, and this led initially to the view that it might act

by inducing host resistance. Subsequent work showed

that fosetyl-Al decomposes readily in plants to yield

phosphorous acid (H 3 PO 3 ) and this simple mineral

acid can suppress the growth of Oomycota in culture

media of low phosphate content.

A few fungicides act as melanin synthesis

inhibitors. They have no effect on fungal growth

in culture, but interfere with the infection process,

especially when fungi infect from melanized appressoria

(see Fig. 5.2). These fungicides have found a limited

application in the control of Colletotrichumspp. and

especially of Magnaporthe grisea (the cause of rice

blast). Appressoria attach strongly to a host surface,

then become melanized, enabling them to build-up

an astonishing osmotic pressure equivalent to about

8 atmospheres (8 megaPascals). This is essential for

“focusing” of the narrow penetration peg which

346 CHAPTER 17

CH 2

Cl Cl

O

N

N

O

CH 2

Propiconazole

CH 3 CH 2

N

Imazalil

Cl

Cl CH

CH 2

CH 2 CH CH 2

O

N

N

Fig. 17.5Examples of azole fungicides: propiconazole is a triazole with three nitrogen atoms in the heterocyclic ring;

imazalil is an imidazole, with two nitrogen atoms in the heterocyclic ring.

Fig. 17.6Systemic fungicides of the carbox-

amide and 2-aminopyrimidine classes.

Fig. 17.7Metalaxyl and fosetyl-aluminum,

two fungicides that act specifically against

Oomycota.