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Organic contact (protectant) fungicides

Organic fungicides were developed in the 1930s and
quickly replaced the inorganic fungicides for most
purposes. They are termed “protectant” or “contact”
fungicides because they act only near the site where
they are applied – to protect the plant surface or to
control an established infection. Thus, they need to
be applied over the whole plant surface, and must be re-
applied to protect any new growth. They are, however,
extremely durable fungicides and many of them are still
in use today. Fungi do not develop resistance to them
easily because they interfere with basic metabolic
processes and often have multiple sites of action in
fungal cells.
Examples of these protectant fungicides include
the dithiocarbamatessuch as maneb(Fig. 17.3) and
thiram(Fig. 17.3). These contain one or more thiol
(sulfur-containing) groups and they inactivate sulfydryl
(-SH) groups in amino acids, proteins, and enzymes.
They are used to control many leaf diseases. Other pro-
tectant fungicides include the dinitrophenols such as
dinocap(Fig. 17.3), which uncouples respiration from
ATP synthesis.

Systemic fungicides

Systemic fungicides are absorbed by plants and are then
distributed internally, usually by upward movement in
the xylem, where they can help to protect new growth.
The development of these compounds in the 1960s re-
volutionized crop protection because these fungicides

can eradicate existing infections and protect against

subsequent infections. They are now used widely,

although most of them do not move downwards in

the phloem, so they provide little protection against

the major soil-borne pathogens. Their use has also

revealed another problem: the active ingredients are

not general toxicants but instead have highly specific

sites or modes of action – for example, on a single step

in an enzymic pathway or by binding to a specific

component of a fungal cell. Because of this, fungi

can develop resistance to them quite easily, often by a

single-gene mutation. When this happens a fungus

often shows cross-resistance to all other members of that

fungicide group. For this reason, the systemic fungicides

need to be used in conjunction with a resistance-

management strategy. This can involve the alternat-

ing use of fungicides with different modes of action,

or the combination of systemic and broad-spectrum

protectant fungicides. We consider some representative

examples below.

Benzimidazole fungicides

The benzimidazole compounds were the first systemic

fungicides to be introduced – thiabendazole in 1964,

benomyl in 1967, and then others such as carbendazim

(1970) and thiophanate-methyl (Fig. 17.4). Ironically,

thiabendazole was developed and released in 1962

as an antihelminthic, and only later was its fungicidal

role discovered. This empirical approach to fungicide

development is still common. It involves the screen-

ing of thousands of potentially active compounds

against a few major pathogens, typically including a

344 CHAPTER 17

Fig. 17.3Examples of protectant fungi-

cides: maneb, thiram, and dinocap.

Fig. 17.4Examples of two systemic

benzimidazole fungicides.