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There is an urgent need to find new chemicals (with
novel modes of action) and new cellular targets, to pro-
vide a greater range of options for controlling fungal
diseases.

Fungicides used for plant disease control

The term “fungicide” is used in a broad sense for any
compound that kills or inactivates fungi. Most of these
compounds are chemically synthesized, but some are
modified derivatives of naturally occurring compounds.
All fungicides have to be officially approved for
use by the relevant national regulatory body, which
also specifies the crops on which the fungicide can be
used and any “withholding period” between the last
application of a fungicide and the harvesting of a crop.
Currently, there are about 80 different chemical groups
of fungicides (where a group is defined as a specific type
of chemical such as the pyrimidines, the imidazoles,
the triazoles, etc.). These groups can be combined into
12 categories according to their primary cellular target
or mode of action, as shown in Table 17.2.
In practice, fungicides can be grouped into three
broader categories, reflecting their different roles in
plant disease control – the inorganic fungicides, the
organic contact (protectant) fungicides, and the
systemic fungicides.

Inorganic fungicides

Inorganic fungicides were the first to be used to con-

trol plant diseases, beginning with elemental sulfurin

1846, copper saltsin 1882, and mercuryaround 1920

(although its use has been banned since 1992 because

of its extreme toxicity, and Britain holds the distinc-

tion of being the last industrialized country to accept

this ban!).

Inorganic sulfur always had a limited use because

of its insolubility, but it can be dusted onto leaves to

control powdery mildew fungi, because the hyphae and

spores of these fungi develop on or near the leaf sur-

face. Sulfur is the only fungicide available to organic

growers because it is an entirely natural product.

Fungicides based on various copper salts have been used

extensively for their broad antifungal and antibacterial

spectrum. They were spectacularly successful against

downy mildew of grapevines (Plasmopara viticola,

Oomycota) in the late 1800s, when “Burgundy mix-

ture” and “Bordeaux mixture” were developed. Usually,

the copper fungicides are complexes of copper sul-

fate with lime(Bordeaux mixture) or they are based

on copper hydroxideor copper oxychloride. They

control many leaf and fruit diseases, but are potentially

phytotoxic. Their mode of action is based on the dis-

ruption of several basic metabolic processes, so fungi

do not develop resistance to them easily.

PRINCIPLES AND PRACTICE OF CONTROLLING FUNGAL GROWTH 343

Table 17.2Classification of fungicides according to their main site or mode of action.

Site/mode of action Comments

Nucleic acid synthesis A few fungicides, including the acylalaninesthat control diseases
caused by Oomycota; e.g. Phytophthora infestans
Microtubules (mitosis and cell division) A few fungicides, including the benzimidazoles that act systemically
Respiration Many fungicides that block steps in the mitochondrial electron
transport chain or that inhibit ATP synthesis
Amino acid and protein synthesis A few fungicides and antibiotics such as blasticidin-S and kasugamycin
Signal transduction A few fungicides that affect G proteins in cellular signalling or MAP
protein kinase
Lipids and membrane synthesis Several fungicides that affect lipid peroxidation, phospholipid
biosynthesis, or cell membrane permeability
Sterol biosynthesis Many important fungicides that target different steps in the sterol
synthesis pathway
Glucan and cell wall synthesis A few fungicides and antibiotics (polyoxin, validamycin)
Melanin synthesis A few fungicides that block melanin biosynthesis
Multisite activity Several inorganic fungicides (sulfur, copper) and protectant (contact)
fungicides that disrupt basic metabolic processes
Induced host plant defense A few compounds (e.g. salicylic acid, chitosan) that activate plant-
defense mechanisms
Various; unknown mode of action A few compounds, including phosphorous acid and fosetyl-aluminum,
for controlling Phytophthoraroot rot