GLOSSARY

GLOSSARY

CLASSIFICATION OF FUNGICIDES

The term "fungicide", strictly taken, refers to

chemicals capable of preventing or eradicating diseases

caused by fungi. Bacteria, the pathogens of bacterial

plant diseases, may sometimes be controlled with

bactericides or antibiotics. However, in the

classification below, this distinction is not rigidly

applied. Fungicides are generally not effective against

virus diseases. However, because many virus diseases

are transmitted by insects, mites or nematodes, their

control is sometimes possible by using insecticides,

miticides and nematicides to destroy these vectors.

Classification of fungicides by chemical nature

Fungicides can also be classified into main chemical

groups consisting of inorganic or organic synthesized

compounds.

1. Inorganic fungicides

Inorganic fungicides are derived from elemental sulphur

or rather simple metal salts and do not contain carbon.

Generally, they are stable, almost insoluble in water

and persistent.

Sulphur, in its elemental form, is probably the oldest

known cure against plant diseases. Originally, finely

ground flowers of sulphur (the form of sulphur

resulting from sublimation following the heating of

crude sulphur and rapid cooling of the fumes) was

applied as a fungicidal dust. It proved to be effective

against powdery mildews and spider mites. Currently,

elemental sulphur is mostly formulated as a wet paste

or a "flowable" prepared from colloidal sulphur

particles of 0.2 - 10 micrometers in diameter. The

third form is a wettable sulphur powder which is mixed

with a wetting agent during the milling process.

Previously, a liquid lime-sulphur mixture was widely

used for orchard spraying against powdery mildews,

anthracnose leafspot and brown rot diseases. This

mixture is prepared by boiling sulphur and slacked lime

in the ratio of 2:1 in five parts of water. Lime

sulphur, however, cannot be sprayed in combination with

organo- phosphates, copper compounds or other metal-

containing pesticides.

The efficacy of sulphur against powdery mildews and

mites is by direct contact and also by fumigant action;

it has no systemic effect. For good fumigant action,

the temperature should be above 20C; however, above

32C, sulphur vapour becomes phytotoxic. Because spores

of powdery mildew germinate in the absence of a water

film on the leaves, this fumigation effect is important

for killing the fungus.

Sulphur is a cheap and safe fungicide, but outside

temperatures need to be monitored during the time of

application. Formulations of sulphur are Elosal,

Kumulus S and Thiovit.

Copper fungicides

Another early discovery was the fungicidal effect of

Bordeaux mixture (a watery solution or slurry of copper

sulphate and hydrated lime). Bordeaux mixture, with 12

per cent copper, is a safe fungicide with low mammalian

toxicity; it controls a wide range of diseases such as

downy mildews and late blight of potatoes. The product

has been largely replaced by "fixed coppers" in which a

copper compound of low-solubility is packed in a form

that is stable in storage and readily dispersable in

water. The copper compound is usually a basic cupric

salt with a bright red, blue, green or yellow colour.

The solubility and the rate of release of the toxic

copper ion (Cu-radical) are decisive for providing

efficacy against the fungi and for preventing

phytotoxicity to the host plant.

In general, protective fungicides have a low rate of

ionization. However, if a film of water is present at

the spot of infection where the fungal spore

germinates, a portion of the fungicide residue may go

into solution. The small quantity of Cu-ions absorbed

by the germinating spore is then replaced in the

solution from the residue. The spore accumulates the

toxic Cu-ions and is killed in the process. Such

compounds are less phytotoxic than Bordeaux mixture,

but their fungicidal efficacy tends to be less. Copper

oxychloride is the most widely used compound against

Phytophthora diseases. Examples of fixed coppers are:

cupric carbonate copper sulphate

copper hydroxide cuprous oxide

copper oxychloride

Inorganic fungicides containing heavy metals, such as

mercury, nickel, zinc and chromium ions, have strong

fungicidal properties and have been in wide use over

the past 35 years. These metal salts are generally very

toxic to all forms of life and they are rather

persistent in the soil and on treated surfaces. For

these reasons, inorganic mercurial and other heavy

metal fungicides have been banned in many countries.

Manufacturing of such fungicides has been discontinued,

except for some special purpose formulations, e.g.

mercuric oxide is used as a paint against bark cankers

and for sealing bark injuries, and insoluble mercurous

chloride is still used for seed treatment.

2. Organic fungicides

Since the development in 1931 of the first organic

sulphur compound, thiram, a wide range of synthetic

organic fungicides have gradually replaced the less

selective inorganic compounds. Currently, about 150

synthetic fungicides are available or under

development. A common feature is their high efficacy at

rather low dosages, their long residual activity and a

greater safety to crops, animals and the environment.

Generally, the organic fungicides are broken down by

soil micro- organisms and leave no persistent toxic

metabolites.

2.1. Dithiocarbamates

are derivatives of the sulphur- containing

dithiocarbamic acid. For half a century, these

fungicides have been among the most effective and

popular ones in use. In combination with metallic salts

such as zinc salt (ziram), ferric salt (ferbam) and

manganous salt (maneb), these organic sulphur compounds

offer a particular effectivity, better stability and

less phytotoxicity than elemental sulphur. They have no

systemic activity. Since they provide a good cover,

several dithiocarbamates are used as seed protectants

against soil-borne diseases. Their toxic effect to

fungi probably stems from the isothiocyanate-radical

(-N=C=S-) which is formed as a break-down component.

Additionally, chelates are formed within the fungal

cells when dithiocarbamates or heavy metal fungicides

are applied. When an excess quantity of such chelates

is present, they may interfere with enzymatic and

metabolic processes within the fungal cells. Heavy

metal dithiocarmates thus have great killing power.

Examples are:

ferbam propineb

maneb thiram

metham-sodium zineb

nabam ziram

mancozeb: a complex of zinc (2-5% Zn) and maneb (20%

Mn). This ready-to-use fungicide combines the benefits

of both maneb and zineb.

2.2. Organometallic compounds

Mercury fungicides were popular because of their

efficacy against fungi, their good disinfective and

protective action, and often considerable volatility

which properties made them very well suited for seed

treatment. However, because of their high mammalian

toxicity, all organic and inorganic mercurial

fungicides have lost their registration and are no

longer available for any purpose.

2.2.1. Organocopper compounds

include organic salts of acetate, naphthenate, oleate

and quinolinate. Copper acetate was first developed in

1889 and became the first factory-made basic copper

fungicide. Copper compounds are not easily washed from

leaves by rain, since they are relatively insoluble in

water, and thus give longer protection against disease

than do most of the organics. They are relatively safe

to use and require no special precautions during

spraying. The currently accepted theory for the mode of

action of copper's fungistatic action is its

nonspecific denaturation of protein. The Cu++ion reacts

with enzymes having reactive sulfhydryl groups -- which

would explain its toxicity to all forms of plant life.

copper 8-quinolinate cuprobam

2.2.2. Organotin compounds

are triphenyl tin (fentin) salts, which are both toxic

and phytotoxic. They demonstrate good control of blast

diseases in rice, late blight in potatoes, coffee berry

disease and brown spot disease in tobacco. The most

widely used organotin fungicides are:

fentin acetate fentin hydroxide

fentin chloride

2.2.3. Substituted aromatics

are derivatives of the benzene or phenol ring with

hydrogen atoms replaced by chlorine, nitrogen or oxygen

atoms. Possessing fungicidal properties, most of them

are suited for seed treatment and soil treatment to

control soil-borne fungi such as smut and damping-off

diseases of seedlings. Examples are:

chloroneb hexachlorobenzene, HCB

chlorothalonil pentachloronitrobenzene, PCNB

dicloran, DCNA pentachlorophenol, PCP

etridiazole tolchlophos-methyl

Chlorothalonil and dicloran are used as protectants on

foliage and fruits; PCNB and HCB for seed treatment;

etridiazole and tochlophosmethyl for soil- treatment.

2.2.4. Dicarboximides

constitute a group of fungicides which have the basic

chemical structure shown in the diagram below; they

contain a sulphur and nitrogen atom at the central

position. The group is also referred to as

"sulfenimides". In general, they are considered to be

among the safest pesticides and are used for seed

treatment and for protectant sprays against Sclerotinia

diseases:

chlozolinate procymidone

iprodione vinchlozolin

metomeclan

2.2.5. Phtalamides

and related compounds are non- systemic fungicides with

broad spectrum activity against many foliar diseases in

fruit crops, vegetables and ornamentals. Captan is used

on a large scale in tropical cultures; its use for food

crops, however, is progressively being restricted:

captafol folpet

captan tolylfluanid

dichlofluanid

2.2.6. Dinitrophenol fungicides with a

non-systemic but specific action against powdery

mildews are:

binapacryl dinocap

2.2.7. Triazines

comprise a group of compounds with herbicidal activity;

they are reviewed below. Only anilazine is a fungicide;

it is used for protectant application on vegetables.

3. Systemic fungicides

These compounds are absorbed by the plant by

translocation of the active ingredient through the

cuticle of leaves; hence they are taken into the

vascular system. These toxicants are transported via

the sapstream towards the foliage and the apical

(growth) point. The direction of transport is generally

towards the leaf tips, but not downwards to the stem

base and the roots. Some systemics can be applied to

the soil and are slowly absorbed through the roots to

give prolonged disease control. Systemic fungicides

give a much better protective effect than the

non-systemic protectant fungicides which need to be

sprayed evenly onto the plant surface where they

remain, essentially where they are deposited. Most

systemics also exert a curative (therapeutic) effect

against pathogens that have already invaded the plant.

There are many systemic fungicides on the market, which

may be classified in one of the groups below:

* Oxathiins

carboxin methfuroxam

furmecyclox oxycarboxin

Carboxin and related oxathiin fungicides control mainly

pathogens of the Basidiomycetes, a class of fungi which

includes such important pathogens as smuts and rusts of

cereals, and the soil fungus Rhizoctania solani.

Methfuroxam and furmecyclox control R. solani in

cotton and potatoes.

* Benzimidazoles and thiophanates

benomyl thiabendazole

carbendazim thiophanate-methyl

fuberidazol

This group contains highly effective, systemic,

broad-spectrum fungicides which are widely used in many

tropical crops. However, intensive and exclusive use

has caused resistance in major crops. Fuberidazol is an

important replacement for organomercury compounds as a

seed dresser. Thiabendazol controls post-harvest

diseases in fruits. These fungicides are not effective

against Phycomycetes (Phytophthora, Pythium and

Perenospora spp.)

* Pyrimidines

bupirimate ethirimol

dimethirimol fenarimol

* Acylalanines

furalaxyl metalaxyl

* Ergosterol biosynthesis inhibitors (EBIs)

a. Imidazoles:

fenapanil prochloraz

imazalil triflumizole

b. Piperazine, pyridine- and pyrimidine compounds:

buthiobate pyrifenox

fenarimol triforine

nuarimol

c. Morpholins:

aldimorph tridemorph

dodemorph trimorphamide

fenpropimorph

The morpholine fungicides demonstrate a systemic,

specific activity against powdery mildews of cereals,

bananas, rubber and ornamental plants.

d. Triazoles:

bitertanol myclobutranil

diclobutrazol penconazol

etaconazole propiconazol

fluotrimazol triadimefon

flusilazol triadimenol

flutriafol triflumazol

The EBIs are a chemically heterogeneous group of

systemic fungicides, grouped together because of a

similar mode of action. They are also called sterol

biosynthesis- inhibiting fungicides (SBIs) or

demethylation inhibitors (DMIs). Most of these

compounds have systemic, protective and curative

properties, and are effective against powdery mildews

and rust fungi which are important diseases of cereals

and many other crops. They are also effective against

many other leaf spot pathogens such as Pyrenophora

spp., Venturia spp. and Septoria spp.

* Organophosphates

ESBP pyrozophos

iprobenfos tolclofos-methyl

edifenphos trimiphos

ditalimfos

These compounds display protective as well as curative

activity because of their systemic property. ESBP

(Inezin), iprobenfos (Kitazin P) and edifenphos

(Hinosan) are applied against rice blast; the others,

against powdery mildews. Toclofos-methyl controls

soil-borne diseases, especially Rhizoctonia solani.

* Phenylamides and other fungicides against

Oomycetes

Phenylamides:

benalaxyl metalaxil

cyprofuram ofurace

furalaxyl oxadixyl

Others (including carbamates):

cymoxanil propamocarb

fosetyl prothiocarb

hymexazol

This is a heterogeneous group of fungicides sharing the

property of resistance against Oomycetes, the causal

fungi of important root and foliar disesases such as

Perenospora, Plasmopara and Phytophthora. Owing to

their highly specific and systemic activity, resistance

of this class of fungi to phenylamides may occur.

* 2-Aminopyrimidines

bupirimate ethimirimol

dimethirimol

A small group of systemic fungicides with high efficacy

against powdery mildews (Erysiphaceae).

* Quinones

benodanil futonil

chloranil mepronil

dichlone

4. Other organic compounds

There are important fungicides which do not belong to

one of the chemical groups above; they comprise a

random collection of fungicides with non-related

chemical structures. Examples are:

anilazine etridiazol

chlorfentezin fenaminosulf

chlorothalonil guazatine

dichlofluanid pencycuron

dazomet quinomethionate

thiocyclam

Most of these compounds are non-systemic protective

fungicides. Pencycuron is specifically active against

Rhizoctonia solani, sheath blight of rice, in

particular. Dazomet is a soil fumigant with a broad

spectrum activity, whereas etridiazole and fenaminosulf

are soil fungicides with restricted use, mainly against

Phycomycetes spp.

Guazatin and oxine-copper are applied in seed

dressings, whereas the main use of sec-butylamine is

for the control of post-harvest fruit-rotting fungi,

for which purpose guazatin is also used.

5. Antibiotics

There are certain chemical substances produced by

micro-organisms, such as bacteria and fungi (molds),

that have the capacity, in very dilute concentrations,

to inhibit the growth of or to kill bacteria and

certain fungi that cause diseases in animals and

plants.

To date, several hundred antibiotics have been

discovered, but relatively few have been developed into

commercial products. The Actinomycetales (an order of

lower plants) constitute the largest source of

fungicidal antibiotics. In particular, the family of

Streptomycetaceae has become widely known for its

members:

* S. griseus from which streptomycin and

cycloheximide are produced that are used as

bactericides for fruits, tobacco, maize and

ornamentals.

Their use is limited for toxicological reasons.

* S. avermitilus, from which abamectin is

produced.

* S. griseochromogenes, which after

fermentation yields the active substance of

blasticidin-S.

* S. kasugaenis, from which kasugamycin

is produced.

* S. cacaoi, which after extraction

yields polyoxins.

Of the latter three Japanese antibiotics, the first two

are effective against blast diseases of rice and the

third against rice sheath blight. Abamectin has

insecticidal, miticidal and nematicidal, but very

little fungicidal, effectivity. Other antibiotics, such

as penicillin, bacitracin and tetracycline, are used

against bacterial diseases in man and livestock.

Tetracyclines appear also to help control some

mycoplasma-like plant diseases.

Cycloheximide, the least complicated antibiotic

compound, affects the growth of yeast and fungi by

inhibition of protein- and DNA-synthesis. Its growth

regulating properties are used for promoting the

abscission of fruits such as oranges and olives.

Streptomycin specifically disturbs the build-up of the

vital nucleic acids DNA and RNA. Cycloheximide

(Acti-dione) is the most toxic fungicide (LD50, oral,

rats, is 2 mg/kg) and an extremely strong repellent to

rats (it is a restricted use product).

Antibiotics having fungicidal and bactericidal activity

which are used for agricultural purposes are:

cycloheximide streptomycin

Antibiotics that have fungicidal action only are:

blasticidin-S polyoxins

kasugamycin validamycin