🛡️ Inorganic Fungicides

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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Fungicides

29. Fungicides - Classification - Inorganics - characteristics, preparation

and use of sulfur and copper - Mode of action - Bordeaux mixture and

copper oxychloride

Fungicides are chemicals that have the ability to reduce or prevent

the damage caused by fungi in plants and their products. Fungicides are

classified based on mode of action as protective, curative and eradicant

fungicides.

Protective fungicides prevent fungal infection by sporicidal activity.

These arrest the germination of spores or kill the fungal hypae as they

penetrate the leaf or prevent their penetration. E.g. Sulphur

Curative fungicides penetrate cuticle and kill young fungal mycelium

growing in the epidermis and this prevents further development of fungal

growth. E.g. Organomercurials.

Eradicant fungicides are agents that make control of fungus even after

the symptoms become visible and that kill both newly developed spores

and the mycelium. E.g. Systemic fungicides.

The early fungicides were inorganic materials like sulphur, lime sulphur,

copper and mercury compounds.

Sulphur compounds

Elemental Sulphur is available in dust, wettable powder and colloidal

forms. The efficiency of S dust increases with fineness of the particle size.

A high proportion should pass through 200 – 300 mesh sieve. Colloidal

sulphur is formulated with kaolin (diluent) with 40 per cent S and a particle

size of < 6 µ. Sulphur is a contact and protectant fungicide used to control

powdery mildew in fruits, vegetables, flowers and tobacco. It is also

effective against apple scab and rust of field crops. Lime sulphur : It is

aqueous solution of calcium poly sulphides. It is prepared by sulphur

solution in calcium hydroxide suspensions under pressure in the presence

of air. Calcium penta sulphide and calcium tetra sulphide found in the

mixture are the active materials of fungicidal value which on exposure to

air release elemental S.

Mode of action: At first it was observed that S could not be the toxic

agent. Sempio (1932) reported that the action was due to the production

of various S derivatives. Another theory was that the fungal spores reduce

S to H2S which has shown to be toxic to the spores. However in 1953 this

theory was disproved as colloidal S was more effective than H2S. Another

hypothesis ascribed to various oxidation products such as SO2, H2SO4

and thiosulphuric acid. Then no S derivative is responsible and came to

Sempio and finally that S itself is fungi toxic.

Among the heavy metals only Cu and Hg have been widely used as

fungicides although silver is most toxic metal cation. The relative toxicity

is in the order of

Ag > Hg > Cu > Cd > Cr > Ni > Pb > Co > Zn > Fe > Ca

Copper compounds

Copper sulphate has been used since 18 [th] century as seed treatment

against cereal bunt later replaced by organomercurials. Cu ions in

solution are toxic to all plant life. Selective fungicidal action can therefore

be achieved by application of insoluble Cu compound on the foliage.

E.g. COC, Copper carbonate, Copper hydroxide, Bordeaux mixture etc.

Bordeaux mixture

Millardet in 1882 discovered the Bordeaux mixture effective against

powdery mildew. A mixture of copper sulphate and lime was initially applied as a

paste and thus gained wide recognition as “Bovillie Bordelaise” (Bordeaux Mixture). As

the initial mixture continuing 8 kg of blue stone dissolved in 100 litres of water and 30

litres of lime suspension prepared with 15 kg lime was found to cause foliar injury.

Various combination were tried. Now a 4-4-50 mixture (copper sulphate in 1 b; lime in 1

b; water in gallons), is used but the concentration of the ingredients is varied depending

on the purpose. However, the proportion of copper sulphate to lime usually remains

constant. In India Bordeaux mixture is being made by preparing a solution of copper

sulphate and quick like (or hydrated lime) in finely ground form in separate containers

and then mixing them simultaneously into a third container with copper sulphate like

copper, wooden and earthern vessels. Wide variation in the composition of the resulting

mixture will be observed due to chemical reactions between calcium hydroxide and

copper sulphate in solution when the ratio between the components is changed.

Bordeaux mixture named from the locality of its origin consists of Cu SO4

(4.5 kg) and Ca (OH) 2 (5.5 kg) in 454 litres of water. It is prepared under

a wide range of formulae. Once the mixture has been prepared it should

be sprayed immediately on the crop since the toxicity decreases on

standing. It is rather difficult to apply because the precipitate tends to

block the nozzles. Jaggary or sugar is added to prevent crystallisation.

Mode of action is complex. The ai is probably not Cu (OH) 2 but rather

basic Cu SO4 approximately to the formula [Cu SO4. 3 Cu (OH) 2].

Bordeaux mixture is almost insoluble in water. So how is Cu mobilized in

plants to kill the fungus? The exudates both from the surface of leaf and

from the fungal spores can dissolve sufficient quantities of Cu from the

dried deposits due to the presence of certain compounds like amino and

hydroxy acids which can form chelates with copper.

B) Copper oxychloride

(Blue copper 50: Fytolan) Cupramar, Blimix 4%, Blitox 50%.

Copper oxychloride is one of the low soluble copper fungicides produced by the

action of air on cupric chloride solution or scrap copper.

4Cu + O2  2Cu2O

Cu2O+2HCl  2CuCl + H2O

2 CuCl + O + 2 HCl  2CuCl2 + H2O

CuCl2 + Cu  2 CuCl

4CuCl2 + 3CaCo3 + 3H2O  3Cu(OH)2 CuCl2 + 3Cl2

It is marketed in the form of wettable powder containing 50 and 90% copper

oxychloride and dusts containing 4 to 12% metallic copper. The 50% formulation

contains a diluent (Kaolin) and a surface active agent.

Burgandy mixture

This was introduced by Mason in 1887 by mixing copper sulphate (1 part)

with sodium carbonate crystals (1 part) and is less effective than

Bordeaux mixture.

Cheshnut compound

This is suggested by Bewley in 1921 consists of 2 parts of Cu SO4 and 11

parts of (NH4)2 CO3. The two compounds are well powdered thoroughly

mixed and stored in air tight containers for 24 hours before use.

Chaubattia paste

This was developed at Government Fruit Research Station, Chaubattia,

Almora district, UP. It is prepared by mixing copper carbonate (800 g) and

red lead (800 g) in 1 litre of linolin or raw linseed oil. It is used as wound

dressing agent on pruned parts.

Copper oxy chloride approximately Cu Cl 2. 3 Cu (OH) 2 is marketed as

colloid and wettable powder. It is a protective fungicide used to control

potato blight, several leaf spots and effective against several diseases of

horticultural crops.

COPPER HYDROXIDE Cu (OH)2 IS A NEW FORMULATION

INTRODUCED.

Mode of action is the denature of proteins by free copper ions. Since

enzymes are made up of proteins, the Cu inactivates the enzymes. The

Cu kills the fungal spores by combining with the sulphohydril groups of

certain enzymes.

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Summary Cheat Sheet

Topic	Key exam point
Main class	Inorganic/contact fungicides in this lesson context
Common examples	Bordeaux mixture, copper oxychloride, and sulphur compounds
Sulphur group	One of the oldest fungicidal groups used for disease control
Copper group	Classic protective fungicides widely used in crop protection
Bordeaux mixture	Standard exam example of copper-based fungicide preparation
Use mode	Mostly protectant/contact rather than systemic
Related preparations	Chaubattia paste and similar copper-based preparations appear in applied questions
Agronomic use	Used mainly to prevent infection on plant surface
Exam distinction	Inorganic fungicides differ from organic and systemic fungicides by chemistry and action pattern
Trap	Do not classify Bordeaux mixture as a systemic fungicide

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website