The following points highlight the eight main types of chemicals used for plant disease management. The types are: 1. Protectant Fungicides 2. Systemic Fungicides 3. Acylalanines 4. Organophosphates 5. Miscellaneous Systemic Fungicides 6. Antibiotics 7. Soil Fumigants 8. Oils.
Type # 1. Protectant Fungicides:
Protectant fungicides, when applied on plant or plant parts, supplement the defences of the plant by forming a superficial chemical barrier to prevent, or protect against, infection. These fungicides normally do not cure a disease after it has started; many protectant fungicides probably become systemic to a slight extent but not sufficiently to give worthwhile cure. These include salts of mercury, zinc, and iron, sulfonamides, dithiocarbamates, and quinones.
1. Copper Compounds:
Copper is one of the most active metal cations and its action in the field for controlling plant diseases is explained to be due to inactivation of enzymes. The fungicidal activity of copper was mentioned as early as in 1807 by Prevost for treatment of wheat seeds for control of bunt disease, its wide scale use as fungicide came in practice during the end of nineteenth century after the discovery of bordeaux mixture.
i. Bordeaux Mixture:
Bordeaux mixture was discovered by Millardet in 1885 and is the most widely used copper fungicide throughout the world, This fungicide is the product of reaction of copper sulphate (CuSO4) and calcium hydroxide (hydrated lime; Ca (OH)2). Copper ingredient of Bordeaux mixture proves sometimes to be toxic to the plant also but it is the lime which acts as “safener” and helps reducing this phytotoxicity. Bordeaux mixture should always be prepared fresh because, on standing, the precipitate forms small spherocrystals or crystallites and loses the fungicidal efficiency.
This fungicide should not be prepared in metallic containers as it can react with metals. However, the most commonly used formula for Bordeaux mixture is 8 : 8 : 100, i.e., 8 pounds (4 kg) of copper sulphate, 8 pounds (4 kg) of lime, and 100 gallons of water.
For spraying young actively growing plants, the amount of copper sulphate and lime are generally reduced to required degree, and the formulas used may be 2 : 2 : 100, 2 : 4 : 100 and so on. For plants sensitive to copper, greater concentration of lime should be used; 4: 12: 100, 8: 24: 100 may be some representative formulas for such cases.
Bordeaux mixture is an excellent fungicide for basically two reasons:
(i) Inherent fungi-toxicity, and
(ii) Tenacity.
On weathering, its fungicidal efficiency improves further instead of deteriorating. It is always to be used in high volume because of lime. Bordeaux mixture is sometimes phytotoxic to tender leaves and other parts. However, this fungicide is specific against downy mildews, blights, coffee rust, various leaf spot diseases, rot diseases, and so on. It has remained the standard fungicide against downy mildews of grapes and other hosts since its inception in 1885.
ii. Bordeaux Paste:
Bordeaux paste is the paste form of Bordeaux mixture, which is mainly used for tree wound dressings to prevent any fungal attack. It is commonly used in controlling stem bleeding disease of coconut. The paste is generally prepared using gradients in the ratio of 1: 1: 1, i.e., 1 lb of copper sulphate, 1 lb of hydrated lime, and 1 gallon of wafer.
iii. Burgundy Mixture:
Burgundy Mixture is a modified form of Bordeaux mixture. In it, lime is substituted by sodium carbonate (hence called ‘soda Bordeaux’). It is prepared using 10 lb (4.5 kg) of copper sulphate, 12 lb (5.7 kg) of sodium carbonate, and 50 gallons of water. Burgundy mixture is less phytotoxic, is often used for tender foliage and does not produce blemishes. However, burgundy mixture as well as any other substitute of Bordeaux mixture could not become as popular as is the Bordeaux mixture itself.
iv. Chestnut Compound:
Chestnut compound is a cuprammonium fungicide as it contains copper sulphate (2 parts) and ammonium carbonate (11 parts). The two substituents are well powdered and thoroughly mixed and the dry mixture is stored in an airtight receptacle for 24 hours before being used.
To prepare a solution of this mixture for spray in soil, one ounce of the mixture is first dissolved in a little hot water, which then is added in cold water to obtain 2 gallons of the solution. Chestnut compound was first suggested by Bewley in 1922 and is used specially to control damping off diseases of seedlings.
v. Chaubattia Paste:
Chaubattia paste was first developed by U.B. Singh in 1942-43 at the Government Fruit Research Station (GFRS), Chaubattia in the Almora district of Uttaranchal (then Uttar Pradesh) in India. The paste is prepared by mixing copper carbonate (800 g) and red lead (800 g) in one litre of lanolin or raw linseed oil. This fungicide is used in orchards of apple, pears, and peaches to control stem black, stem brown, stem canker, and pink diseases.
2. Sulphur Compounds:
There are varieties of inorganic and organic sulphur compounds which are excellent fungicides and are used to control a variety of plant diseases.
They are:
i. Inorganic Sulphur Compounds:
The elemental sulphur is used as dust, wettable powder, paste, or liquid to control powdery mildews. It is also effective against certain rusts, leaf blights, and fruit rots. The elemental sulphur, in its different forms, is sold in market under different trade names such as Kolodust, Kolofog, Micronized sulphur.
The earliest scientific record on use of element sulphur dates back to 1821 A.D. when a paper was read before the London Horticultural Society on the value of sulphur for the control of peach mildew. Since then, it gradually became recognized as a valuable fungicide for the control of powdery mildews and it remains so to date as one of the most effective and widely used chemical for the purpose.
Different preparations like lime-sulphur, self-boiled lime sulphur, and dry lime-sulphur are manufactured by boiling lime and sulphur together. These preparations are used as sprays to control many powdery mildew diseases such as scab disease of apple, peach leaf curl disease, and blight and anthracnose diseases of fruit trees.
The sulphur sensitive fungi convert elemental sulphur to hydrogen sulphide which is toxic. Horsfall (1956) suggested that the sulphur competes with oxygen for protons and electrons in fungus respiration. Tweedy (1964) suggested that sulphur takes electrons between cytochrome b & c of the electron transport system.
ii. Organic Sulphur Compounds (Thiocarbamates):
The organic sulphur compounds (thiocarbamates) are the derivatives of dithiocarbamic acid (dithiocarbamate), which is toxic to fungi. These are undoubtedly the most widely used fungicides of present time. The first dithiocarbamate fungicide (thiram) was discovered by Tisdale in 1934. This discovery led to the development of a series of effective and widely used fungicides in the subsequent 10 years.
Some most important ones are the following:
a. Thiram (Tetramethyl Thiuram Disulphide, C6H12N2S4):
Trade names-Thiram, Arasan, Tersan, Thylate; used mostly for seed and bulb treatment, and control of certain foliage diseases e.g., rust of fruits and vegetables. For seed treatment 50% powder is normally used. Better results are obtained with soak treatment in a suspension of thiram containing 250 gm of active ingredient in 100 litres of water.
b. Maneb (Manganese Ethylene Bisdithiocarbamate, C4H6N2MnS4):
Trade name – Manzate, Dithane M-22, contains manganese; used to control foliage and fruit diseases of many vegetables. It is used in the form of spray and is very effective against foliar diseases particularly blight diseases of potato and tomato. When zinc is added to maneb, it forms Dithane M-45 (Mancozeb).
c. Zineb (Zinc Ethylene Bisdithiocarbamate, C4H6N2ZnS4):
Trade name – Dithane Z-78, Parzate c, Lonacol; excellent foliar and soil fungicide; used to control leaf-spots, blights, and fruit rots of vegetable and fruit trees. Its use as soil fungicide helps controlling damping-off and other soil-borne diseases.
d. Ferbam (Ferric Dimethyl Dithiocarbamate, C9H18FeN3S6):
Trade name – Formate, Ferbam, Coromate, Carbamate; used to control many foliage diseases of fruit trees and ornamentals, downy mildew of chillies, anthracnose of citrus, bunt of rice, early blight of tomato, etc. It is used as spray or dust. The usual formulations being 80% active ingredient as dust and 50% wet powder as spray.
e. Zeram (Zinc Dimethyl Dithiocarbamate, C6H12N2S4Zn):
Trade name – Zerlate, Karbam white, Corozate, Cuman; used to control anthracnose of cucurbits, ripe rot of chillies, early blight of potato, early blight of tomato, etc. It is the most stable of dithiocarbamate fungicides and is used as a spray in 0.2-0.3% concentration of active ingredient.
f. Nabam (Sodium Ethylene Disdithiocarbamate, C4H6N2Na2S4):
Trade name – Dithane D -14, Parzate liquid, Dithane A-40; used to control mainly many leaf-blight and seedling blight diseases at the rate of 5-6 kg/hectare of active ingredient. It is available both as powder, containing at least 93% of anhydrous salt and a stock solution containing 20% anhydrous salt.
g. Vapam (Sodium N-Mathyl Dithiocarbamate, C2H4NNaS2):
Trade name – Vapam; used for soil treatment to control many fungal pathogens and nematode larvae, and used to control diseases like wilt of cotton, damping-off of papaya, root rot of beat, root nematode of potato, etc.
The exact mode of action of thiocarbamate fungicides is not known with certainty. It is considered that the theram disulphides and dimethyl dithiocarbamates disrupt the activity of cell by attachment to 1:1 complex metal and dithiocarbamate ions to enzymes. Bisdithiocarbamates produce isothiocyanate, which reacts with SH groups essential for functioning of SH bearing enzymes.
3. Mercury Compounds:
i. Inorganic Mercury Compounds:
Two inorganic compounds of mercury (Hg) called mercuric chloride (HgCl2; Cal-Chlor) and mercurous chloride (Hg2Cl2; Calomel, Calocure, Calogreen, Velsicol) are used at 1 : 1000 dilutions for soaking seeds, rhizomes, and corms. Sometimes, the solution is used for soil-drenching to eradicate soil-borne pathogens.
In fruit orchards, wounds on stems and branches are disinfected with mercuricmurcuric chloride solution. However, the use of inorganic mercury compounds is limited because they are highly toxic to animals and humans.
ii. Organic Mercury Compounds (Organomercurials):
Many organomercurial fungicides are now available in the market with different compositions under different trade names.
Following are some important organomercurials:
a. Ethyl mercury chloride (C2H5HgCl2) – Trade name – Ceresan, Granosan; used as foliar spray on rice for blast, seed treatment in groundnut, pea, cotton, and cereals.
b. Ethyl mercury phosphate (CH3CH2HgH2PO) – Trade name – New Improved Ceresan, Semesan; used in seed treatment of cereals, cotton, and linseed.
c. Methyl mercury dicyandiamide – Trade Name – Panogen; used in seed treatment of small grains and ornamentals.
d. Phenyl mercury acetate (PMA) – Trade Name – Ceresan dry, Mer-Sole; used as foliar spray against rice blast, apple scab; in seed treatment of cereals, maize, cotton, groundnut, vegetables.
In addition, methoxy ethyl mercury chloride (Ceresan wet, Aretan, Agallol), phenyl mercury urea (Agrox), hydroxymercuric chlorophenol (Semesan Bel), Chloromethoxypropyl mercuric aretate (Quicksan), phenyl mercury acetate and ethyl mercury chloride mixture (Agrosan GN), etc. are the other organomercurials used to control diseases.
Organomercurials were primarily intended for seed treatment, some of them have proved effective as foliage and fruit protectants. Organomercurials may injure seeds when applied in high concentrations or when treated seeds stored under inadequate moisture conditions. However, these fungicides are specific in mode of their action. Mercury present in them is absorbed by the spores or mycelium either as vapour or ion and acts upon SH groups of susceptible enzymes.
The organomercurials used for dry seed treatment usually contain 1% mercury. They are used at a rate of 250 g/quintal seed. In wet seed treatment, the compounds contain 3-6% mercury and 0.25-0.5% solution is used for seed, tubes, and sugarcane sett treatment. Organomercurials are extremely toxic therefore specific precautions are obligatory.
Treated seeds must not be used as such for food and livestock. Organomercurials are persistent hence enter into the food chain resulting in hazards, which are too well known. Several countries including India have now banned the use of these fungicides.
4. Quinones:
Quinones usually occur naturally in many plants and are also produced upon oxidation of phytophenolic compounds. They are believed to act through their interference in the oxidation-reduction mechanism of the pathogens. Only two quinone compounds, namely, chloranil and dichlone are used commercially as fungicides.
Chloranil (2, 3, 6, 5-tetrachloro-1, 4-benzoquinone) = Trade name – Spergon; used mainly in seed and bulb treatment; useful against smut of barley and sorghum, bunt of wheat, many damping-off diseases. Foliar application of this fungicide is a waste as it decomposes in light. That is why it is mainly used as seed treating fungicide. Dosage for seed treatment is 2-3 gram/kg of seed depending upon the size and weight of the seeds.
Dichlone (2, 3-dichloro-1, 4-naphthoquinone) = Trade name – Phygon; used in seed treatment and as foliar sprays; useful against apple scab, peach leaf curl, grain smut of jowar, and many damping off and blight diseases. Unlike chloranil, it is stable to heat and light. It is degraded by alkali.
5. Benzene Compounds:
Many benzene compounds have significant antimicrobial property and have been developed into fungicides.
Some important ones are:
i. Diazoben (Dimethyl amino benzene-diazo-sodium sulphonate) = Trade name – Dexon; used for seed and soil treatment for the control of damping off and root rot caused by species of Pythium and Phytophthora. Zentmyer and Gilpatrick found in 1960 that Dexon shows curative effect on avocado seedlings afflicted with Phytophthora cinnamomi when applied in the root region.
ii. Chlorothalonil (Tetrachloro-isophthalonitril) = Trade name – Daconil, Bravo, Termil; used against leaf spots, late and early blights, downy mildews, rusts, anthracnoses, and scab as spray materials. Termil is available in tablet form and is used in greenhouses to control Botrytis on ornamentals and to control blights of tomato.
iii. Dichloran (2, 6-dichloro-4-nitroaniline) = Trade names – Botran, DCNA; used as spray or in soil treatment to control fruit, vegetable, and flower diseases.
iv. Dinocap (2-(1-methylheptyl) -4-6-dinitrophenyl crotonate) = Trade name – Karathane, Mildex; used especially against powdery mildews. It has proved to be an excellent substitute of sulphur for control of powdery mildew on sulphur-sensitive plants. Usual formulations are 25% wettable powder or 50% emulsified concentrate for sprays.
v. PCNB (Pentachloronitrobenzene) = Trade names – Terrachlor, Brassicol, Quintozone; used to control soil-borne diseases such as those caused by Rhizoctonia, Sclerotium and Sclerotinia. It is a long lasting soil-fungicide and is very successful against club-root of cabbage caused by Plasmodiophora brassicae. Usual formulation is 20% dust. It mainly acts as antisporulant and fungistatic rather than fungicidal.
6. Heterocyclic Nitrogen Compounds:
This group of heterogenous but some of the best fungicides, also known as dicarboximide fungicides, includes Captan, Folpet, Glyodin, Captafol, Iprodione, Vinclozolin, and Dyrene.
Captan (N-trichloromethyl thio-4-cyclohexane-1, 2 dicarboximide) = is of common use in India under the trade names – Captan, Orthrocide, Vancide; proves to be an excellent safe fungicide in controlling leaf spots, blights, fruit rots, etc. It’s most common use is as seed protectant against Pythium and in reducing postharvest losses in storage caused by a number of fungal pathogens.
Folpet (N-trichloromethylthiophthalamide) = Trade names – Folpet, Phaltan, Orthrophaltan; is closely related to Captan in physical and biological properties and proves better for certain purposes. In addition, it controls many powdery mildews.
Glyodin (2-heptadecylglyoxalidine) = Trade names – Glyodin, Crag Glyodin; used to control apple scab and foliar diseases of fruits and vegetables. It is marketed as 70% wettable powder or 34% solution in isopropanol. Fungitoxicity of glyodin is thought to be due to interference with biosynthesis of purines, which are constituents of nucleic acids.
Captafol (N-1, 1, 2, 2-tetrachloroethylsulfenyI) -cis-4-cyclohexene 1, 2-dicarboximide) = Trade names – Difolatan, Difosan, Sanspor, Orthrodifolatan; is used primarily to control early and late blight of potatoes. It is marketed as 80% wettable powder or emulsified concentrate containing 80% active ingredient.
The mechanism of action of Captan, Folpet and Difolatan is due to — SCCl3 group, which interferes with sulphahydril enzymes and oxidative processes particularly activity of co-carboxylase.
7. Organo-Tin Compounds:
There are three fungicides, namely, Du-Ter, Brestan, and Brestanol based on organic tin compounds. Du-Ter (Triphenyltin hydroxide) is very effective against Cercospora, Bipolaris (Helminthosporium), Alternaria, Pythium, Phytophthora, and Rhizoctonia. Brestan (Triphenyltin acetate; TPTA) is effective against Cercospora, Alternaria, Septoria, etc, Brestanol (Triphenyltin chloride) is similar to Du-Ter and Brestan in activity but it also possesses some systemic activity, Organo-tin compounds are bactericidal also. They are phytotoxic to plants hence recommended for restricted use.
Type # 2. Systemic Fungicides:
A systemic fungicide is one which is taken in and translocated within the plant. The systemic fungicides, when absorbed by roots, are translocated through apoplast (xylem, cell walls) to the aerial parts of the plants. Once they reach the leaves they fail to be translocated through phloem downward, and get spread towards the margin of the leaves and stay concentrated there.
The successful use of a systemic fungicide, oxathiin, was first demonstrated by von Schmeling and Kulka in 1966, this work proved to be a major landmark in the history of fungicidal management of plant diseases. The discovery of oxanthin was soon followed by confirmation of systemic activity of pyrimidines and benzimidazoles. Later on, many more systemic fungicides such as metalaxyl, triazoles, organic phosphates, etc. were discovered and launched.
A systemic fungicide differs from an antibiotic in-spite of the fact that both are systemic in nature. This difference is based mainly on two properties, the antibiotics are microbial in origin while systemic fungicides are non-microbial, and the antibiotics can be translocated upward and downward both (i.e. form roots to leaves and vice versa) while the systemic fungicides are translocated only upwards (i.e. from roots to leaves). Thus, the systemic fungicides are translocated uni-directionally while the antibiotics bi-directionally within the plant body.
An ideal systemic fungicide should be equipped with the following features:
i. The chemical must be either originally fungi-toxic or soon becomes such after entering into the host plant.
ii. The chemical or its derivative should have property to alter the host metabolism so that biochemical or physiological resistance to pathogen may be induced or enhanced.
iii. The chemical or its derivative must not adversely affect the host plant to such an extent that the quantity and quality of its products is reduced.
iv. The chemical should be quickly absorbed efficiently at the point of application and translocated to the site of the pathogen for quick action.
v. The chemical must show considerable degree of stability within the host plant.
vi. The chemical should not be toxic to mammals (humans and animals). If not so, the toxicity should be low enough to avoid residue problems at the consumer stage.
Although there is hardly any systemic fungicide that fulfills all the criteria stated above, the commonly used systemic fungicides of present day are considered safe enough. In the last thirty years, several systemic fungicides have become available, and their number, ease of application, duration of effectiveness, and even the number of diseases they control are increasing steadily.
Systemic fungicides are gradually replacing many of the protectant fungicides because of their effectiveness and long-lasting activity, which result in the need for only a limited number of applications to protect a crop from one or many diseases.
Several systemic fungicides are currently available, and many more are in experimental stage. Systemic fungicides belong to many different groups of compounds.
Following are, however, some important systemic fungicides, which are widely used:
i. Oxathiins:
Oxathiins were the first systemic fungicides to be discovered by Von Schmeling and Kulka (1966). They developed two oxathiin compounds, namely, carboxin (vitavax) and oxycarboxin (plantvax) for complete control of internally seed-borne infection of loose smut (Ustilago nuda) in barley. Oxathiins are selectively concentrated in cells of fungal pathogens and inhibit enzyme succinic hydrogenase, which plays an important role in mitochondrial respiration. They also interfere with synthesis of protein, RNA, and DNA in rapidly metabolising fungal cells.
Two most important oxathiins are:
a. Carboxin (5, 6-dihydro-2-methyl-1 4-oxanthiin-3 carboxyanilide) = Trade name – DMOC, Vitavax; used in seed treatment; effective against damping off diseases caused by Rhizoctonia, smuts of grain crops like loose smut of wheat (Ustilago tritici) and barley (U. nuda), bunt (Tilletia), and flag smut (Urocystis) of wheat.
Carboxin has become the most popular systemic fungicide for seed treatment to control loose smut in wheat and barley (250 g/100 kg seed). It has replaced the earlier recommendation of hot water or solar treatment against this disease. In combination with theram or copper oxyquinolinate the efficacy of carboxin is increased and the cost of treatment is decreased.
b. Oxycarboxin (2, 3-dihydro-5-carboxyanilido-6-methyl-1, 4-oxanthiin-4-dioxide) = Trade name – DCMOD, Plantvax; toxic to basidiomycetous fungi and hence is effective in controlling a variety of rust diseases. In addition to basidiomycetes, Oxycarboxin (Plantvax) is reported toxic to certain deuteromycetous fungi, such as Curvularia, Botrytis, Bipolaris (Helminthosporium), Aspergillus, Monilia, Cladosporium, and Cunninghamella.
Carboxin (Vitavax) and Oxycarboxin (Plantvax) both are derivatives of salicylanilide but show quite opposite specificities with respect to their fungi-toxic spectrum, Carboxyl shows water solubility of 170 ppm at 20°C, while the water solubility of Oxy-carboxyl on the same temperature is 1000 ppm.
ii. Benzimidazoles:
Benzimidazoles are effective systemic fungicides against numerous types of diseases caused by fungi. Mostly they are converted to methyl benzimidazol carbamate (MBC, carbendazim) on the plant surface, which interferes with nuclear division of fungal pathogens. Benzimidazoles include some important broad spectrum compounds, such as benomyl, thiabendazole, thiophonate, and carbendazim.
a. Benomyl [Methyl 1-(butylcarbamamoyl)-2- benzimidazole cabamate] = Trade name – Benlate, Tersan 1991, TZB; is the best known benzimidazole fungicide and is generally used in seed treatment, foliar spray, root dip, and fruit dip; this safe and broad spectrum fungicide is the most striking fungicide at present and is effective against powdery mildews (Erysiphe, Sphaerotheca, Podosphaera, Uncinula), scab of apple (Venturia inaequalis), brown rot of stone fruits, blast of rice Magnaporthe grisea (Pyricularia oryzae), etc. Also, it is highly effective against and suppresses infection by Rhizoctonia, Thielaviopsis, and Ceratocystis. Benomyl, through its soil application, is reported to control many vascular wilt diseases, such as Fusarium wilt of tomato and melons, Verticillium wilt of potato, cotton and tobacco, and cereal rusts.
b. Thiabendazole (2, 4-thiazolyl benzimidazole or TBZ) = Trade name – Mertect; used commonly to control storage rots of many fruits. It is a broad spectrum fungicide and is especially effective against any fungal pathogens belonging to class Deuteromycetes.
c. Carbendazim = Trade name – Bavistin; is a breakdown product of Benomyl. Benomyl, in contact with moisture, readily breaks down into methyl benzimidazole carbamate (MBC) and butyl carbamate.
The latter is converted into butyl isothiocyanate, which is volatile. MBC, however, is stable and possesses almost same fungi-toxic properties as benomyl. Bavistin is fairly stable in soil and ensures continuous supply of the fungicide to the above ground parts of the plant.
d. Thiophanate = Trade name – Topsin, Cercobin; used to control several root and foliar fungal pathogens including powdery mildews. Thiophanates are often classified under benzimidazoles although they are different in that they are based on thiourea. Chemically, thiophanate is 1, 2-bis (3-ethoxycarbonyl-2-thio ureido) benzene. Thiophanate methoxy (methoxy carbonyl thioureido benzene) is more effective than the ethoxy compound and is more commonly used now. The trade name of thiophanate is Topsin M or Cercobin M.
iii. Sterol Biosynthesis Inhibitors:
Sterol biosynthesis inhibitors (SI) represent a new group of systemic fungicides, which have been developed during the last few years. These fungicides are widely recognised and recommended to control plant diseases, and are broadly divided into two groups on the basis of their mode of action.
They are:
Group I:
This group includes the inhibitors of C-14 demethylation (DMIs) that affects cytochrome P-450 enzymes. Pyrimidines (fenarimol, naurimol, etc.), thiazoles (bitertanol, triadimafon, propiconazole, penconazole, cyproconazol, fluisilazole, flutriazole, difenoconazole), imidazoles (prochloraz), and piparazines (triforine) fall into this category.
Group II:
This group consists of the inhibitors of C-14 reductase. Morpholines (fenpropimorph and tridemorph) are the fungicides belonging to this group.
a. Pyrimidines:
Diamethirimol = Trade name – Milcurb; used especially to control powdery mildew diseases of crops caused by Sphaerotheca fuliginea.
Ethirimol = Trade name – Milstem; effective in soil or seed treatment than as foliar sprays; effectively controls powdery mildew of barley (Erysiphe)
Other pyrimidines like Fenarimol (trade name – Rubigan) and Nuarimol (trade name – Trimidol) are effective against powdery mildews and also various other leaf-spot, rust, and smut diseases. Bupirimate (trade name – Nimrod) is used as spray to control powdery mildew of apples. Triarimol has systemic and curative effect on scab of apple. It has also been found effective against diseases caused by Ustilago striiformis and Urosystis agropyri.
Exact mode of action of pyrimidines, especially diamethirimol and ethirimol, is not known. But, evidences suggest that they interfere with tetrahydrofolic-acid-directed C-1 wide range of cellular reactions and inhibition of some pyridoxal-requiring enzymes.
b. Imidazoles:
Imidazoles (Pochloraz) is an effective SI fungicide against fungal pathogens belonging to Ascomycetes and Deuteromycetes causing powdery mildews, leaf spots, blights, and fruit rots.
c. Triforine:
Triforine (a piparazine SI fungicide) is sold under the trade names Cella, Funginex or Saprol. It is a foliar spray fungicide and is effectively used against powdery mildews, leaf and fruit spots, fruit rots, anthracnoses, and some rusts.
d. Triazoles:
Triazoles include some of the best sterol inhibiting (SI) fungicides such as triadimefor (trade name – Bayleton), triadimenol (trade name – Bayton), propiconazol (trade name – Tilt), boutrizole (trade name – Indar or RH-124), biloxazol (trade name – Baycor), diclobutrazol (trade name – Vigil), etaconazol (trade name – Vangard), defenoconazole (ciba Geigy), and several others. These fungicides persist for longer duration and facilitate lesser number of applications and longer intervals between sprays.
Triazoles are applied as foliar sprays and as seed and soil treatments, and are effective against powdery mildews, rusts, smuts, leaf spots and blights. Triadimefor (Beyleton) is used to treat powdery mildew and cereal rusts in India. Propiconazole (Tilt) proves effective against cereal rusts even with one spray. Boutrizole (Indar or RH-124) completely controls leaf rust of wheat (Puccinia recondita) and yellow or stripe rust of wheat (Puccinia stiiformis).
e. Morpholines:
Morpholines are heterocyclic ring compounds. They include two compounds, namely, tridemorph (trade name – Calixin) and dodemorph (trade name – Meltatox). Tridemorph (Calixin) is used in our country in the form of emulsifiable concentrate and is effective against-all fungal pathogens except those belonging to peronosporales.
However, this fungicide is used to control powdery mildew of wheat, pea and cucurbits, stipe or yellow rust of wheat and barley, sigatoka leaf spot of banana, and various diseases of tea, coffee, cereals, etc. Dodemorph (Meltatox) is preventive and eradicant fungicide used to control powdery mildews and leaf spots of cereals, ornamentals, etc.
Type # 3. Acylalanines:
Acylalanines, discovered by Ciba Geigy in Basle (Switzerland) in 1973, are acylalanides. This group includes matalaxyl, furalaxyl, and banalaxyl, which showed curative and systemic activity against Oomycetous pathogens.
Out of these, metalaxyl is the most important one and is discussed here:
Metalaxyl, the first systemic fungicide developed against Oomycetous fungi in 1973 and introduced for commercial use in 1977 under the names Ridomil (for foliar application) and Apron (for seed treatment), is highly effective against Pythium, Phytophthora, and many downy mildews. This fungicide is also effective as postharvest fruit dip to control brown rot of citrus. It is quite water soluble and readily translocated. It inhibits the germination of zoospores in Plasmopara viticola (downy mildew of grapevine) and sporangium formation in Phytophthora.
It also inhibits the formation of oospores and chlamydospores, and the germination of chlamydospores. It is worth notable that metalaxyl is fungistatic in action as it does not kill rather inhibits the target fungal pathogen. Resistance against this fungicide is commonly shown by the pathogens; maximum cases have been registered in Phytophthora infestans, many in downy mildews, and one in Pythium.
In the light of this, a combination of metalaxyl with dithiocarbametes is recommended for-use against late blight of potato. Staub et al. reported in 1980 that metalaxyl interferes with RNA synthesis. It is also found that this fungicide elicites the production of phytoalexin that imports resistance in the host plant.
Type # 4. Organophosphates:
Organophosphorus fungicides include kitazin (IBP), fosetyl-Al (Aliette), edifenphos (Hinosan), pyrazophos (Afugan, Euramil), and triamiphos (Wepsin).
i. Kitazin (S-benzyl diethyl phosphorothioate) was developed by Kumiai Chemical Industry, Tokyo (Japan). Later, its modified form, kitazim P, was developed, which is now preferably used to control blast disease of rice. This fungicide inhibits C-glucosamine incorporation into the cell wall of rice blast pathogen resulting in interference in the biosynthesis of chitin.
ii. Fosetyl-Al (Aliette) was developed against Oomycetous fungi after matalaxyl. It has a narrower spectrum of biological activity than matalaxyl but is more effective against foliar, root and stem diseases caused by Pythium and Phytophthora, and the downy mildews of variety of crops.
This fungicide is applied as foliar spray, soil drench, root and postharvest dip, as well as soil mix. Treatments remain effective for two to six months depending on the crop. Fosetyl-Al breaks down in soil or in plant tissues into CO2 and phosphoric acid, the latter being the fungi-toxic principle in the fungicide. Phosphoric acid very effectively inhibits sporangia and chlamydospore formation in Phytophthora. Fosetyl-Al has also been used against some bacterial disease of ornamentals.
iii. Edifenphos (Hinosan), Pyrazophos (Afugan), and Triamiphos (Wepsin) are used against specific diseases. Hinosan shows high efficiency against blast disease of rice (Magnaporthe grisea), like Kitazin, and has been extensively used in India. Afugan is used against powdery mildews and Bipolaris (Helminthosporium) diseases. Triamiphos (Wepsin) is credited being the first practically applied organophosphorus fungicide and is still used.
Type # 5. Miscellaneous Systemic Fungicides:
There are many excellent systemic fungicides.
Some important ones are the following:
a. Dithane R-24 = probably this is world’s first fungicide to ever be employed for strategic control of leaf rust of wheat (Puccinia recondita); it completely controls the disease.
b. Imazalil = Trade name – Fungaflor; applied as foliar spray, seed treatment, postharvest treatment; effective against many ascomycetous and deuteromycetous fungal pathogens that cause powdery mildews, leaf spots, and vascular wilts.
c. Ethazol = Trade name – Turbon, Terrazole, Koban; is a seed and soil fungicide and is used effectively to control damping off and root and stem rots (Pythium and Phytophthora). A mixture of ethazol with PCNB or with Topsin M shows efficiency against Fusarium and Rhizoctonia diseases.
d. Pyracarbolid = Trade name – Sicarol; closely relates to oxanthiins and is used to control rusts, smuts, and Rhizoctonia diseases. It is considered to be more effective than oxanthiins.
e. Chloroneb = Trade name – Demosan; used as seed and soil fungicide and effectively controls seedling blight of cotton, bean, beet, etc.
f. Propamocarb = Trade name – Banol, Previcur; first released commercially in 1978 exhibits good systemic activity when used as soil drench against downy mildews of cucurbits, crucifers, onion, lettuce, etc. This fungicide creates leakage in the cells of the pathogen.
Type # 6. Antibiotics:
Antibiotics are those substances which are produced by one microorganism and become toxic to another one. Most of the antibiotics are produced by actinomycetous bacteria and some fungi. The chemical nature and formulas of antibiotics are quite complex and not fully known in most of the cases. However, antibiotics were first used to control plant diseases in 1950.
Antibiotics used to control plant diseases are generally absorbed and then translocated systemically inside by the plants. They control plant diseases usually either by acting on the pathogen or on the host, directly or after undergoing transformation.
Their effect remains stable for some time, and they act not only as eradicants but also as protectants and provide resistance against invasion of pathogens. Since the antibiotics are taken up by the plants and are then translocated therein systemically, they are systemic compounds but not systemic fungicides as they are of microbial origin and can be translocated upward and downward both.
Following are the most important antibiotics used to control plant diseases:
a. Streptomycin = Produced by Streptomyces griseus; Trade names – Agrimycin, Phytomycin, Orthrostreptomycin; effective against both Gram-positive and Gram-negative bacterial plant pathogens and also against several fungal pathogens belonging to lower group of fungi, e.g., Pseudoperonospora humili. Chemical structure of streptomycin is that of a glycoside in which aglycone streptidine is linked to N-methylglucosamine through an unusual sugar streptose. It is strongly basic in nature and is marketed either as sulphate or hydrochloride.
b. Cycloheximide = Produced by Streptomyces griseus; Trade names – Actidione, Actispray, Actidione PM, Actidione RZ; used to control diseases such as cedar rust of apple, powdery mildew of beans and rose, rust of wheat and bunt of wheat. This antibiotic is costly and is obtained as a byproduct in streptomycin manufacture. Its fungi-toxicity is reported to persist for a period of five weeks after spraying.
c. Aureofungin = Produced by Streptomyces cinnamomeus var. terricola, this is a broad spectrum systemic compound used to control citrus gummosis (Phytophthora citrophthora), Diplodia rot of mango, Alternaria rot of tomato, etc.
d. Griseofulvin = Produced by Penicillium griseofulvum; used against fungal pathogens causing powdery mildews and certain rusts, etc. Griseofulvin shows remarkable effect on the development of hyphae. Even at a very low concentration of 10 Mg/ml it causes curling, stunting, abnormal branching with distortion, and loss of apical dominance in hyphae or germ tubes. This antibiotic interferes with nucleic acid.
e. Blasticidin = Produced by Streptomyces griseochromogenes; used to control blast disease of rice caused by Magnaporthe grisea (Pyricularia oryzae); also effective in reducing multiplication of viruses in some viral diseases of plants. Blasticidin is used as a wet powder. Its activity is attributed to interference with protein synthesis in ribosomes of blast fungus.
f. Kasugamycin = Produced by Streptomyces kasugaensis – selectively effective against Magnaporthe grisea (P. oryzae) and Pseudomonas spp. This antibiotic helps preventing secondary spread and sporulation and has little effect until after penetration. Like Blasticidin, it also interferes with protein synthesis acting on ribosomes.
In addition, there are some other antibiotics such as pimaricin, nystatin, rimocidin, filipin, phytoactin, polymyxin, erythromycin, and puromycin, which exhibit activity against many plant pathogens.
Type # 7. Soil Fumigants:
There are many volatile organic compounds which are generally used as soil fumigants as they give off gases after being applied to the soil. Common examples of such compounds are – chloropicrin, methamsodium (vapam) DD mixture, methyl bromide, dazomet, formalin, carbon disulphide, etc., which are effective against nematodes, many fungi, insects, and weeds. Most of the soil fumigants are available in liquid form and are used through injection into the soil, or through irrigation water.
a. Chloropicrin = This compound is useful both as a fungicide and a larvicide. It is injected into the soil at a depth of 3-6 inches in holes each made 9-12 inches apart. The soil is covered with impervious plastic sheets at least for 48 hours.
b. Metamsodium = (Sodium N-methyldithiocarbamate) is sold as Vapam, which is a soil fumigant produced in recent years. It is a colourless liquid that decomposed rapidly in the wet soil releasing methyl isothiocyanate (CH3—N = C = S), carbon disulphide, hydrogen sulphide, and methylamine. In addition to being a nematicide and selective weedicide, vapam has been found to be very effective against wilt of cotton, damping off of papaya, and root rot of beet (Sclerotium rolfsii).
c. DD Mixture = (1, 3-dichloropropene and 1, 2-dichloropropane) is sold as DD, Nemafume or Nematox. It was commercialized after the Second World War and is extensively used as nematicide. It also kills insects, wireworms, and some fungi at high dosages, which is 400-500 litre/ha at 10- 27°C to obtain best results.
d. Methyl Bromide = (CH3Br) is sold as Dowfume MC2, Metho-O-gas, and Terabol. This soil fumigant is recommended to be used in soil a few weeks before the crop is planted because it is too phytotoxic to standing crop. This sterilizing gas is compressed and marketed in liquid form filled in metal containers. The use of methyl bromide has been restricted to the treatment of nursery beds, vegetables and fruit production, and ornamentals. Methyl bromide affects ozone layer in the atmosphere hence it is scheduled for withdrawal from use.
e. Dazomet = (Tetrahydro 3, 5-dimethyl-2H-1, 3, 5-thiazene-2-thione) is effective against nematodes, wireworms, and certain soil-borne fungi (e.g., Pythium, Colletotrichum, Fusarium, Verticillium). It is a wettable powder and is used at the rate of 440 kg/ha with fertilizer spreader. It is also applied by spraying a suspension in water followed by thorough mixing and ‘sealing in’ with water. Dazomet breaks down in soil and releases methyl isothiocyanate.
f. Formalin = Use of formaldehyde drench is one of the oldest chemical treatments of soil. 37- 40% formalin solution prepared in water controls damping off and seedling blights. Formalin solution (1 : 128) used at seedling stage at the rate of 200 gallons/acre provided excellent control of onion smut in USA. This chemical corrodes the hands and is somewhat erratic in effectiveness therefore should be used carefully.
g. Carbon disulphide = (CS2) is perhaps the earliest known chemical used as soil fumigant to treat nematodes. It is used as formalin and is recommended to be used against Phylloxera vitifoliae of vine and Armillarea mellea in citrus orchards. CS2 is volatile and cheap and also acts as an insecticide.
Telone, ethyl dibromide or EDB, and nemagon are some other soil fumigants. Telone (a mixture of cis and trans-isomers of 1, 3-dichloropropane) has been introduced as nematicide by Dow chemical company in 1956. When mixed with ethyl dibromide, a mixture called dorolone is obtained the dosage of which is 200 litre/ha. Ethyl dibromide or EDB (1, 2-dibromoethane) is similar to DD mixture in its biological activity and shows excellent nematicidal properties.
The rate of application of EDB is 80-120 litre/ha depending upon the soil type and the crop. Its disadvantages are that it reduces nitrification in soil and slows phytotoxicity in bromine sensitive plants. Nemagon or DBCP (1, 2-dibromo 3-chloropropane) is low in phytotoxicity hence proves very useful as pre-plant and post-plant treatment of established plantation crops. However, the use of this soil fumigant has been discontinued due to its residual problem.
Type # 8. Oils:
Oils, mainly the mineral oils of petroleum origin, have been found capable of controlling several plant diseases. Mineral oils of petroleum origin have been commercially and extensively used for the control of Sigatoka disease of banana (Mycosphaerella musicola); this treatment has been so effective that it almost completely replaced conventional fungicides. Oils have also been used successfully in controlling greasy spot of citrus (Cercospora citrigrisae) and angular leaf spot of tung tree (Mycosphaerella aleuritides).
Puccinia graminis tritici on wheat, Sphaerotheca pannosa (powdery mildew) on rose, and Septoria apii on celery are effectively controlled by oil sprays. However, oils obtained from seeds of plants like sunflower, olive, corn, and soybean gave excellent control of powdery mildew of apple, when applied from 1 day before to 1 day after inoculation of the plants with the fungus. So far, none of them are used commercially.
The therapeutic action of oils is perhaps due to the formation of the oily physical barrier on the host surface. This barrier interferes with gas exchanges thus alters host physiology so as to stop fungus development. It is therefore assumed, but not with certainty that oils exert their therapeutic action not directly on the pathogen but rather on the physiology of the host.
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