Certain chemicals are toxic to pathogen. Such chemicals check the growth and development of pathogens and kill them. Based on the nature of the pathogen against which these chemicals are used, the latter have been classified into fungicides, bactericides, nematicides, viricides etc.
Chemical or physical agents (Heat, UV rays etc.) having the potential to kill fungal pathogen come under this category. Although the term fungicide is generally used for chemical agents only. Certain chemicals temporarily check the growth of fungal pathogen and do not kill them.
Such chemicals are called fungistatic chemicals. Chemicals which check the production of spores in fungi are called antisporulant although all such chemicals which protect the plants from fungal pathogen come under the broader definition of fungicide.
1. Sulphur Fungicides:
Sulphur since ancient times has been used as a fungicide, e.g. – Elemental Sulphur and lime sulphur
2. Copper Compound Fungicides:
The fungicidal nature of bone stone, copper sulphate was first of all discovered by Prevost (1807) against Bunt disease.
i. Bordeaux Mixture:
Prof. Millardet of France in 1882 accidentally discovered Bordeaux mixture from the mixture of copper sulphate and lime. This he successfully used to control Downey mildew of grape. Millardet first of all prepared Bordeaux mixture by mixing copper sulphate, lime and water in the ratio 8:15:100 (8kg of CuSO4 + 15 kg lime + 100 litre water). But this mixture was extremely powerful and caused foliar injury.
Bordeaux mixture is now prepared in following concentrations:
(a) 4 : 4 : 50 = 0.8% (4 pounds copper sulphate + 4 Pounds lime + 50 gallon water)
(b) 5 : 5 : 50 = 1.0% (5 pounds copper sulphate + 5 pounds lime + 50 gallon water)
Nowadays 5:5:50 (1.0%) ratio is often used.
Thus Bordeaux mixture contains three ingredients:
(i) Copper sulphate/Blue stone = CuSO4. 5H2O
(ii) Quick lime, and
(iii) Water.
The following reactions take place in Bordeaux mixture:
CuSO4. 5H2O + CaO → Cu (OH)2 + CaSO4
Because of the lime content High volume spray is used for the application of Bordeaux mixture.
ii. Burgundy Mixture/Soda-Bordeaux Mixture:
In this mixture lime is replaced by sodium carborate or washing soda or soda ash, the rest composition being similar to Bordeaux mixture. In ancient Europe because of the non-availability of Quick lime Mason in 1887 used Sodium carbonate in its place. This he did in the city of Burgundy and therefore the name Burgundy mixture.
iii. Chestnut Compound/Cuprammonium Fungicide:
Use of chestnut compound as a fungicide was first of all done by Bewlery in 1921 against Damping off disease of seedlings.
iv. Chaubattia Paste:
This was first of all used at the Government Fruit Research Centre, Almora. This was used as a wound dressing fungicide against diseases of apple and pear such as Black stem, brown stem etc.
3. Thiocarbamate Fungicides:
The importance of Thiocarbamate in the field of Fungicides is comparable to the discovery of DDT as an insecticide. Though Spergon was the first Organic fungicide to be used commercially. In 1931 Tisdale first observed about the possible use of Thiocarbamate as a Fungicide.
Thiocarbamate has been grouped into three categories:
i. Thiuram Disulphides:
Among its different compounds only tertramethyl thiuram disulphide is used as a fungicide. This product is called Thiuram/thiram. It was included in the list of fungicides in 1934 basically as a seed protectant.
But later on it was started using as a Foliage protectant and in certain conditions for soil treatment also. Its general formulation for its use as spray is 80% W.P. and 50% colloidal suspension. For seed treatment usually 50% powder is used. For better results 250 g.a.i. of Thiram is dissolved in 100 litre water and the seeds are soaked in it. For soil treatment 10- 15 kg a.i. is used.
ii. Metallic Dithiocarbamates:
a. Ferbam:
It is a Fe (Iron! containing fungicide.
It was developed by Tisdale and Williams in 1931. Its commercial use as a fungicide started in 1943. In temperate countries it is used against disease of apple and other fruits. For spray its formulation is 0.2 – 0.3% a.i. Its effect decreases on mixing with copper, lime and mercury.
b. Ziram:
It is a Zinc containing fungicide. This is the most stable among the group of Dithio-carbamate fungicides. It has been found to be an effective fungicide to control diseases caused by Fungi Imperfect in fruits and vegetables and also early blight of potato. It provides additional advantage in soil deficient in Zn. For spray purpose formulations of 0.2 – 0.3% a.i. and 50% W.R are available in market.
iii. Ethylene bisdithiocarbamates:
a. Nabam:
Nabam is a sodium containing fungicide. Because of its phototoxicity and instability it is not usually used in fields. Although it gives better results on mixing with ZnSO4 and lime.
b. Zineb:
It is a Zn-containing fungicide. Except certain sensitive plants Zn is usually not harmful for crops. It is used to control many diseases in plants. It is used for soil treatment to control Damping off and other soil-borne diseases. It has nematicidal property and is used to control Meliodogyne spp.
c. Maneb:
This fungicide is similar to Zineb but in Maneb Zn is replaced by Mn i.e., it is Mn-containing fungicide. It was included in the list of fungicides in 1950. It is very effective against Foliar diseases especially Blight diseases of potato and tomato.
d. Mancozeb:
It contains both Zn and Mn.
e. Vapam/Metham-Sodium:
It is a soil Fumigant which was enlisted in 1954. It is used in partial sterilization of soil for the control of Soil Fungi and nematodes. It also acts as herbicide.
S (Sulphur) is an important component in the structural formula of thiocarbamate fungicide.
4. Other Organic Fungicides:
i. Glyodin:
Wellman and Mecallan (1946) discovered about the fungicidal property of glyoxalidine derivatives which is now available under the trade name Glyodin in market. In temperate countries this fungicide is used to control Apple scab and other fruit diseases.
ii. Dodine/Cyprex:
Dodine as a protectant fungicide became available in 1959. It does not harms bees. It can be safely mixed with ordinary fungicides and insecticides. It also has some eradicant property. Dodine is used to control Apple Scab and other foliar and fruit diseases.
iii. Quinone Compounds:
a. Chloranil seed treating fungicide became available in the market under the trade name ‘Spergon’ in 1940. Because of its disintegration in light, it is not used for foliar application.
b. Dichlone: After the success of Chloranil, Dichlone .as a seed treating fungicide was launched in market in 1943 under the trade name ‘Phygon’. Besides its seed protectant property it is also used as a foliar protectant.
iv. Captan:
As a protective fungicide captan was enlisted in 1949. It has been proved successful in the treatment of many diseases of fruits, vegetables and ornamental plants. Alongwith its use as a seed treatment fungicide it is also used to reduce post-harvest losses caused by fungi in storage. Captan is also used for soil drenching @ 0.5% for protection against damping off.
v. Folpet:
Its physical and chemical properties are similar to captan and is sold under the trade name ‘Phaltan’
vi. Difoltan:
It is quite similar to Folpet and captan.
vii. Karathane (Dinocap):
It was first of all enlisted in 1954 to control powdery mildew disease in plants. Because of its solubility in oil, it is not used in oil based spray. It is a good substitute of sulphur and therefore used to control powdery mildew in sulphur – sensitive or Sulphur shy plants. Upto a certain extent it is also successful in controlling mites.
viii. Pentachloronitrobenzene (Quintozene):
It was enlisted in 1930 for the control of Soil borne pathogens and dry rot of potato.
Later on it came to be used to control Rhizoctonia, Selerotium and other soil fungi. It is persistant in soil and cucurbits and tomato plants are sensitive to it. PCNB (Penta Chloro Nitro Benzene) is a nematicide also. It has mainly antisporulant and fungistatic property.
ix. Organic Tin Compounds:
TPTA (Triphenyl Tin acetate) was enlisted under the trade name Brestan whereas TPTA and TPTH (Tripheryl tin hydroxide) under the trade name Denter. TPTA also has insecticidal property. It was enlisted in 1954 to control Potato blight.
x. Oils:
Nowadays oil-mist spray is used to control Sigatoka disease of Banana. Fungicidal property of oils is only due to physical reasons.
5. Antibiotics:
i. Streptomycin:
Streptomycin at 100 ppm or more concentration has been found to be effective against bacterial seed-borne pathogen. But as there is greater possibility of development of resistant strains in bacteria, streptomycin is mixed with Oxytetracyclin (terramycin). This mixture or combination is called Agrimycin. Streptomycin is effective against both Gram- positive and gram-negative bacteria. It does not possess fungi- toxic property.
ii. Cycloheximide:
It is obtained as a byproduct in the production of Streptomycin. Its trade name is Actidione. It is active against Phycomycetic fungus Pythium debaryanum but is ineffective against bacteria.
Cycloheximide is a systemic fungicide and is easily absorbed by roots & leaves. Its effect persists for about 5 weeks after spray. But its production is expensive and there is little difference between effective concentration and phytotoxic concentration. It can also cause mutagenic effect in plants. It is therefore not used for disease control in agricultural crops.
iii. Griseofulvin:
It was first isolated in 1939 from Penicillium griseofuluum. It is obtained as a byproduct of several species of Penicillium. It is a systemic fungicide. Griseofulvin does not check the germination of spore but affects the growth of hyphae. Thus it is not a true fungicide. It does not harm bacteria. Griseofulvin is active against Powdery mildew.
iv. Aureofungin:
It is a broad-spectrum systemic antifungal antibiotic. It is used for seed treatment to protect from rot in storage.
v. Blasticidin-S:
It is produced from Streptomyces griseochromogenes. It is effective against many species of bacteria and certain fungi including Pyricularia oryzae. Because of its growth inhibition property, it is also used as a Chemotherapeutant.
vi. Kasugamycin:
It is obtained from the culture broth of Streptomyces kasugaensis. It is selectively active against Pyricularia oryzae and Pseudomonas spp. Kasugamycin is more effective as an eradicant than as a protectant.
6. Systemic Fungicides:
i. Oxathiin:
Oxathiin was developed by Von Schmeling and Kulka in 1966.
a. Carboxin or DMOC/DCMO:
Its trade name is Vitauax. It is a proven chemical against Basidiomycetes fungi (Smuts and Rusts). Fungi such as Verticillium albo-atrum and Monilia cineraria f. Americana can also controlled by carboxin as they are sensitive to this systemic fungicides.
Vitavax is used for seed dressing and soil drenching in order to control loose smut of cereals (internally seed borne) and Rhizoctonia disease of cotton and sugarbeet.
This chemical is not very stable & completely degrades in soil within 10-30 days. Vitavax is used to control Loose smuts (Ustilago nuda, Ustilago auena) and bunts.
b. Oxycarboxin/DNOCD/DCMOD:
Its trade name is Plantuax. It is used in controlling diseases related to fungi imperfecti. The diseases caused by members of fungi imperfecti are – Early blight of potato, Alternaria leaf spot of crucifers, leaf blight of wheat, Leaf spot of groundnut, stripe disease of barley, leaf spot of rice (Helminthosporium) blast of rice, Ripe fruit rot end Dieback of chillies, Wilt of pigeon pea, wilt of cotton, wilt of linseed, fusarium wilt or Panana disease of banana, Wilt of sugarcane.
ii. Pyrimidine Derivatives:
a. Methyrimol:
It is effective against powdery mildew of cucurbits, cinarea, chrysanthemum and sugarbeet.
b. Ethirimol:
It is like methyrimol and effectively controls powdery mildew of Crucifers.
iii. Benzimidazole:
It comes under the name of Benomyl oxBavistin or Benlate. It is a superior systemic fungicide which acts as a good eradicant and protectant. It has ovicidal action against mites eggs. Benzimidazole is effective against certain fungal diseases of crops such as rice blast, diseases caused by cercospora; verticillium of cotton, powdery mildew and black spot of rose, but has less effect on Helminthosporium spp. & Phycomycetes.
It is almost completely fungistatic and relatively non-fungicidal in action. It is easily absorbed by plant roots and its concentration is more in root and lower part of the stem. It is therefore used in supplemented seed treatment and soil treatment to protect plants from seedling disease during planting. Chloroneb is active against Rhizoctonia spp. It can be mixed with most of the pesticides and organomercurials used in seed treatment.
v. Thiobendazole (TBZ):
Although TBZ was enlisted as an antihelminthic but it is a broad spectrum systemic fungicide. This fungicide is transported from the roots to the leaves of plant and sometimes in the reverse direction also. It does not undergo disintegration in plant tissues. It is effective against Blue and green moulds of citrus fruits. When applied @120g/100kg it successfully controls seed borne bunt, Fusarium niuale and Septoria nodorum. Verticillium wilt can be successfully controlled if soil treatment is undertaken @30kg/ha.
vi. Terrazole/OM-2424:
It is an effective fungicide for the control of seed and seedling disease of Maize, Cotton, Sorghum, Soybean, bean, potato, tomato and cucumis.
vii. Organo-Phosphorus Compounds:
This group includes compounds like Hinozan, Kitazin, Inazin etc. which are active against blast disease at a concentration of 400-500 ppm concentration. It becomes toxic at higher concentrations. It is extensively used in Japan. Among them Kitazin is an effective fungicide because of its excellent systemic behaviour.
7. Organo-Mercurial Fungicides:
Mercuric chloride is an effective Fungicide and bactericide. Because of its high toxicity it is not used to control plant diseases.
The general formula of organo-mercurial derivative is:
The examples of Organo-mercurial fungicides are:
Agrosan GN, Agrox, Aretan/Agallol, Ceresan, Mergamma, Parrygen, Panogen, Puraturb, Semessan. Because of their highly toxic nature great care is taken in their use. The treated seeds should never be used for human or animal consumption. Dry seed dressing contains 0.6-15% Hg whereas liquid dressing contains 0.6-2.0% Hg. Organomercurials mixed with γ-BHC or dieldrin is used to control insects attacking young seedlings.
8. Formaldehyde:
It was for the first time used in Germany for seed treatment during the last decade of 19th Century. Because of its lesser toxicity it was used as a substitute for CuSO4. Inspite of this, use of Formaldehyde for seed treatment has several harmful effects.