🔄 Systemic Fungicides

Systemic fungicides protect crops by entering plant tissues and suppressing fungal pathogens from within. This lesson introduces major systemic fungicide groups, their characteristics, and field use.

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Metalaxyl, Carbendazim,- characteristics and use

The idea is earlier but 1960 only commercial systemic fungicides

have come to market. A systemic fungicide is a compound that is taken

up by a plant and is then translocated with in the plant, thus protecting the

plant from attack by pathogenic fungi or limiting an established fungal

infection.

If a candidate chemical is to be an effective systemic fungicide the

following criteria must be satisfied. It must be fungicidal or to be converted

in to an active fungitoxicant with in the host plant.It must possess very low

phytotoxicity. It must be capable of being absorbed by the roots, seeds or

leaves of the plant and then translocated, at least locally, within the plant.

The earlier protectant fungicides applied as foliar sprays formed dried

deposits on the leaves of the host plant, protecting it from fungal attack.

However the deposits are of course gradually removed by the effects of

weathering and cannot protect new plant growth formed after spraying or

any part of the plant not covered by spraying. These disadvantages can

be overcome by the use of systemic fungicides which since penetrates

the plant cuticle. They also offer the possibility of controlling an

established fungal infection. There fore systemic fungicides should exhibit

both protectant and eradicant activity.

Benzimidazoles

These represent a new era in fungicide use when they were introduced in

late 1960s.

Benomyl Carbendazim

The most important members of this group are benomyl [methyl –1

(butyl carbamoyl) benzimidazole-2- carbamates] TN: Benlate and

thiobendazole . Both are wide spectrum systemic fungicides effective

against many pathogenic fungi including powdery mildews and soil borne

pathogens. These fungicides may persist in plants for several months.

Benomyl was introduced in 1967. It was synthesized from cyanide and

methylchloroformate. Benomyl and thiabendazole are both wide spectrum systemic

fungicides active against many pathogenic fingi including powdery mildews and soil

borne pathogens, Verticillium alboatrum on cotton and black spot on roses.

Du-pont; Benlate LD50: - 9590

Foliar fungicide in wine grape fruits, vegetables, citrus, cereal seed dressing.

Benomyl is the more active compound and is widely applied as a foliar spray,

seed dressing or to the soil for control of grey mould ( Botrytis cinerea ), apple scab

( Venturia inequalis ) canker and powdery mildew ( Podosphaera leucotricha ), leaf spot

( Cercospora beticola ), major fungal diseases of soft fruits and some pathogens of

tomato and cucumber.In aqueous solution benomyl is rapidly hydrolysed to

methyl benzimidazole – 2 – carbamate and this is probably the active

fungitoxicant carbendazim which is used as a wide spectrum systemic

fungicide formulated as 50 per cent WP.

Metalaxyl

Metalaxyl is included in the phenylamides group of systemic fungicides.

The compounds of this group show protective and systemic activity against

Oomyceles causing foliar, root and crown diseases in wide range of crops eg. downy

mildews and late blight. The first members of this group Metalaxyl and Furalaxyl

were introduction by Ciba-Geigy in 1977 (Metalaxyl = Ridomil). With a high activity at

low rates of foliar or soil application metalaxyl controls diseases caused by air or soil

borne comycetes in crops like potatoes, grapes, tobacco, cereals, hops and vegetables.

A wettable powder formulation with mancozeb, (a complex of Zn and Mn salts

fungicides) is widely used as a foliar spray against hlight on potatoes. Metalaxyl has the

broadest spectrum of fungicidal activity of this group of fungicides; it is good against

downy mildew on vines, lettuce, maize and Pythium diseases and can be formulated as

a seed dressing.

E) Carbendazim (Bavistin, Derosol)

Foliar fungicide in grapes, fruits, vegetables, cereals, cereal seed dressing

LD50: 15.000 Carbenda in methylbenzimidazole-2-carbamate is used as a wide

spectrum systemic fungicide and may be formulated as a 50% w.p. for control of

Botrytis, Gloeosporium rots, powdery mildews and apple scab. Carbendazim is

absorbed by the roots and foliage of plants and is quicker acting than Benomyl.

The activity of the benzimidazole fungicides (Benomyl, thiabendazole and

carbendazim) is due to the inhibition of nuclear division due to their action on the

microtubule assembly and the resistance developed in fungi is the result of mutant

strains possessing an altered microtubule assembly.

Carboxin and related compounds (Oxathiins)

Oxathiins are another group of heterocyclic compounds with interesting

systemic fungicidal properties. Carboxin and the sulphone analogue

known as oxycarboxin are primarily effective against basidomycetes

class of fungi which includes such economically important group of fungal

pathogens rusts, smuts and bunts of cereals and the soil fungus

Rhizactonia solani.

The fungitoxicity is due to inhibition of glucose and acetate oxidative

metabolism and RNA and DNA synthesis.

Carboxin and Oxycarboxin

Oxathins are another group of heterocyclic compounds with systemic fungicidal

properties. Examples are Carboxin (5, 6 dihydro-2-methyl-1, 4 oxathin-3-carboxanilisde)

(Vitavax) and the sulphone analogue known as Oxycarboxin (Plantvax).

LD50: 3820

LD50: 2000

Seed dressing: cereals, cotton Sol and foliar fungicide

smuts & Rests Rusts

Carboxin is prepared by reaction of α-chloroacetoacetanilide and 2-thiothanol

followed by cyclization. Oxycarboxin is obtained by subsequent oxidation of

carboxin with hydrogen peroxide. Both are fairly water soluble and are not phytotoxic.

They are active against Basicdiomycetes class of fungi causing rusts, smuts and bunts

of cereals and soil fungi Rhizoctonia solani . Carboxin can be formulated with other

fungicides like thiram, copper oxine.

C) Oxycarboxin

Oxycarboxin has systemic action against rusts of cerals, and vegetables and

seed treatment or soil application can be done. Carboxin is absorbed and

translocated by plant roots. In water, soil and plants; the compound is oxidiseds to

sulphozide but further oxidation to sulphone was not observed. The sulphoxide is much

less fungicidal and so oxidation causes loss of activity. The primary mode of action

of carboxin and related compounds probably involves the blocking of succinate oxidation

in the mitochondria of sensitive fungi.

Antibiotics

Antibiotics are chemicals produced by living organisms that are

selectively toxic to other organisms. The first successful antibiotic against

human diseases was penicillin discovered by Fleming (1929) but it has

never achieved commercial significance as a systemic fungicide.

Glitoxin, an antifungal antibiotic produced by the soil fungus Tricoderma

viride inhibited the growth of Botrytis and Fusarium spores at 2-4 ppm

concentration but the compound was too unstable for use as a soil

fungicide.

Streptomycin and cycloheximide are antibiotics obtained from the

culture filtrates of Streptomyces griseus is used for the control of bacterial

pathogens of plants.

Griseofulvin isolated in 1939 from Penicillium griseofulvum is an

important antifungal antibiotic showing a wide spectrum of activity

especially against Botrytis in lettuce and Alternaria solani on tomato.

Blasticidin, a pyrimidine derivative isolated from Streptomyces

griseochromogenes give excellent control of rice blast and also inhibits

certain bacteria. The antibiotic polyoxin D is another pyrimidine

derivative which is toxic towards several fungi including rice blast. The

fungi toxicity is due to interference with chitin synthesis.

Melanin Biosynthesis Inhibitors (MBIs)

prevent it penetrating the plant epidermis; these compounds block

melanin synthesis in a variety of Ascomycetes and fungi imperfecti. They

provide practical control of rice blast and experimental control of some

Colleotricum species. Tricyclozole prevents the rigidity of penetration by

Pyricularia oryzae . Validamycin is an aminoglucoside antibiotic active

mainly against Rhizactonia diseases and has been widely used to control

rice sheath blight. Probenazole related to saccharin is effective by root

application against rice blast and bacterial leaf disease Xanthomonas

oryzae . It is not fungi toxic Invitro and probably acts indirectly by

enhancing the resistance response of the host plant.

Aminopyrimidines

Some 30 years ago there is a series of 2-amino-4-hydroxypyrimidines had

specific systemic activity against powdery mildews. These structure

activity studies led to development of dimethrinol and ethrinol .

Dimethrinol discovered in 1965, showed outstanding systemic activity by

root application against certain powdery mildews in vegetables and some

ornamentals.

Piperazine, pyridine, pyrimidine, imidazole and triazole fungicides

These fungicides are considered together because they all show a

common biochemical target, namely synthesis of ergosterol; they are

methylation inhibitors.

Triforine is the only piperazine derivative showing systemic activity

against powdery mildews on cereals and vegetables.

Buthiobate and pyrifenox are pyridine fungicides. Buthiobate is used

mainly in Japan against powdery mildews while pyrifenox controls a wide

range of leaf spot pathogens of fruits and vegetables.

Triarimol, fenarimol and nunarimol are pyrimidine derivatives

introduced by Eli Lilly in the late 1960s. Triarimol has been withdrawn due

to its undesirable toxicological properties. Fenarimol, a systemic and

protective fungicide is used as a foliar spray to control a broad spectrum

of powdery mildews, scabs, rusts and leaf spots. Nuarimol is used against

powdery mildews in cereals.

Fenarimol Nunarimol

Imazalil, the first imidazole agricultural fungicide (1960) is now used as

seed dressing in cereals.

Prochloraz Imazalil

Prochloraz (Boots, 1973) is a broad spectrum fungicide with good activity

against ascomycetes and fungi imperfecti but rather less activity against

basidamycetes.

Triflumizole controls a wide range of pathogens Viz., powdery mildews

and scabs.

Triadimefon [(1-(4-chlorophenoxy) 3,3-dimethyl –1-(1,2-triazol –1yl)

butan-1-one)] TN: Bayleton has systemic activity against a broad range of

plant pathogens effective at rates of 0.1 – 2.0 ppm. This discovery led to

the introduction of several 1,2,4- triazole fungicides.

Triadimefon

These fungicides owe their fungitoxicity due to their ability to inhibit

ergosterol biosynthesis. Ergosterol is a major sterol in many fungi where it

plays a major role in membrane structure and function.

Morpholine fungicides

Dodemorph (roses) and tridemorph (cereals) are systemic foliar

fungicides effective against powdery mildews at 0.75 litres ha [-1] .

Dedemorph Tridemorph

Organo phosphorus fungicides

Today more than 100 OP compounds show fungicidal action. However

relatively few compounds are of practical use as fungicides, Many are

phytotoxic and very specific against fungal species.

One of the first Op fungicides was triamiphos claimed to be the first

systemic commercial fungicide. Since then many compounds were

derived. Pyrazophos TN: Afugan is a foliar systemic fungicide effective

against apple powdery mildew.

Triclophos – methyl is effective against Rhizactonia and other soil borne

diseases as a drench in vegetables and against black scarf and canker in

seed potatoes.

Iprofenfos TN: Kitazin P introduced in 1968, is a systemic rice fungicide

applied as granules in paddy water to control rice blast and it inhibits

mycelial growth in tissues.

Edifenphos is also very effective against rice blast.

OP fungicides have shown to block the synthesis of phospholipids. The

reduction in phospholipids alters the membrane structure, increasing the

permeability and consequent loss of vital cellular components and

eventually killing the fungus.

Phenyamides and related compounds

 Metaloxyl - broad spectrum

 Furalaxyl - soil drenching

 Benalaxyl - potato blight

 Oxadixyl –with mancozeb to control potato blight

Carbamates

 Prothiocarb - ornamentals

 Propamocarb - Fruits and vegetables

 Cymaxanil - Potatoes and vines

Miscellaneous compounds

Isoprothiolane - systemic fungicide against rice blast

Dithiolane

Formaldehyde (Formalin 40 %) is used as seed dressing and soil

sterilant

Substituted

azepenes - systemic fungicidal activity against leaf spot, powdery mildew

and rust diseases.

Recent potent fungicides for future use

Chlorooximes

Very effective broad spectrum fungicide. Substitutients in the oxime moiety plays a key

role in the biological activity.

Cyano-oximes : The most active compound in this group is Cymoxanil which controls

grape vine downy mildew disease. The analogous compound a propargyll derivative, is

equally effective. After isosteric replacement of the acetylenic triple bond by the cyanide

triple bond the resulting compound showed enhanced activity against downy mildew

Aryl sulfonylallyl trichloromethyl sulfoxides : A series of 2-aryl sulfonylallyl

trichloromethyl sulfoxides, have been found effective as broad spectrum fungicides with

residual activity against grape downy mildew

CH2 0

II

ArSO2C - CH2 - S – CCl3

β methoxyacrylates with oxime ether side chain:

Azoxystrobin, is a well known broad spectrum fungicide which facilitates the control

of a wide range of major plant pathogens. If the central pyrimidine ring is replaced with

an oxime ether moiety, it yields a highly effective fungicide.Analogous compounds

containing a heterocyclic moiety instead of phenyl ring were also prepared to ascertain

the fungicidal activity.

Pyrimidine derivatives: A series of novel 2-anilinopyrimidine compounds, based on

lead compound were synthesized and introduced to the market in 1994. The synthetic

compound mepanipyrim, exhibited excellent activity against grey mould of vine and veg

etables, scab of apple and pear and brown rot of peach.

Biofungicides

These comprise of antibiotics and a few microbes such as Pseudomonas cepacia,

Peniophora gigantea and Trichoderma viride which control a number of fungi associated

with major crops

Mode of action of fungicides

Non systemic

The toxic action of sulphur in the cell is still not clear, however, several theories have

been proposed from time to time. The theory accepted at present is that sulphur acts as

hydrogen acceptor in metabolic systems to form H2S, and in doing so disrupts the

normal hydrogenation and dehydrogenation reactions in the cell. But in case of Cu

fungicides, the Cu ions precipitate or inactivate the proteins (enzymes of sulphydryl

group) and thus kill the spores.

The mercury fungicides also act either as vapour or in ionic form and destroy

sulphydryl group of (.SH) enzymes. Organomercurials are more toxic than the inorganic

mercuric ones due to enhanced lipid solubility facilitating diffusion through the spore

membrane to the site of action.

The mode of action of quinone derivatives may be due to binding of the quinone

nucleus to .SH and -NH2 groups in the cell leading to disturbance in the electronic

transport systems. The activity of captan and related analogues may thus involve the

role of CI and S atoms of the molecule leading to inactivation of sulphydryl group of

enzymes.

b) Systemic

The general mode of action of systemic fungicides is associated with a) interference

with the electron transport chain influencing the energy budget of the cell, b)

reduction in the biosynthesis of new cell material required for growth and development of

the organism, and c) disruption of cell structure and permeability of cell membrane.

Benomyl and its related compounds interfere with mitosis in cell division in

angiosperms and fungi. Benzimidazoles, thiophanates, oxathins, phenylamides

(metalaxyl derivatives) influence DNA synthesis and are also mitosis inhibitors. The

triazole group of fungicides interfere with the biosynthesis of fungal steroids and

ergosterol which are important constitutents of the cell wall. Pyrimidine derivatives

inhibit purine biosynthesis and several pyridoxal dependent enzymes. The mode of

R4Sn - R3SnX - R2SnX2 - RSnX3 - SnX.

action of morpho lines is still not well understood but appears to be inhibition of sterol

biosynthesis. The mode of action of organ phosphorus fungicides is different from

insecticides due to the absence of cholinesterase enzyme in fungi. The widely

accepted theory is that it inhibits permeation through cytoplasmic membrane of the

substrates for chitin synthesis.

The thiono compounds appear to be inactive against fungi and this may be due to

fungus being unable to activate the thiono group to the ox on form by oxidation. The

effect of penetration into the fungal hyphae depends on the polarity of the P=O group

and needs to be balanced by a larger liphophilic group such as, the second thiophenyl

group in case of edifenphos, cyclohexyl group in case of cerezin, benzyl mercaptan in

case of kitazin or kitazin-P and the phenyl radical in the case of inezin.

Metabolism

The stability of fungicides in soil depends on chemical structure, nature of soil and

climatic conditions. In general, the fungicides are not as stable as organochlorine

insecticides. The most versatile dithiocarbamate group of compounds, decompose in

acidic soils to give non-toxic amines and carbon disulfide. Metabolism of alkyltin

compounds in liver microsomal monooxygenase system and in mammals leads to the

following sequence of detannylation (carbon-tin cleavage) reaction

The first step reaction product possessed increased toxicity and potency as inhibitors

to mitochondrial respiration whereas in the subsequent steps the reaction product

possesses less potency and has altered nature of biocidal activity.

The carboximides such as captan, folpet, captafol are hydrolysed under neutral and

alkaline conditions. Chloroneb degrades to the phenolic derivative but reconversion to

parent molecule is a microbial process and this might be the probable reason for long

term effectiveness of chloroneb in soil.

Benzimidazole systemic fungicides like benomyl, thiabendazole and thiophanate

methyl are first converted to carbendazim, an active ingredient at the site of action.

These are finally degraded to non toxic compounds such as aniline, phenyl diamine and

cyanoaniline. The breakdown of benomyl into MBC occurs by intramolecular process in

slightly acidic or neutral media. A hydrogen bond is formed between the free electron

pair of the N atom of the benzimidazole ring and hydrogen on the nitrogen of the

butylcarbamoyl side chain, forming an unstable four membered ring which opens up to

yield MBC and butylisocyanate. The cyanate rapidly forms butylcarbamic acid with water

which in turn decomposes into CO2 and butylamine. The major metabolites of

dimethirimol are ethirimol and 2-amino derivative.

Benomyl and thiophanate-methyl, in plants decomposes first to MBC which then

gives photoproducts) like carbomethoxyguanidine (i), carbomethoxyurea (ii) and

guanidine (iii)other minor compounds depending on the nature of the solvents used

Piperazine, a metabolite of the fungicide triforine in barley degraded to non toxic

products such as iminodiacetic acid, glycine, and oxalic acid on the surface of the plants

by photodecomposition Chlorthalonil, 2,4,5,6-tetrachloroisophthalonitrile, in benzene

solvent was photodegraded to amonophenyl adduct 3,5,6-trichlorobiphenyl-2, 4

dicarbonitrile, (i) as major photoproduct and disulfides. chlorophenyl methyl carbonate

and an unknown product.

R

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Summary Cheat Sheet

Topic	Key exam point
Main class	Systemic fungicides
Main feature	Absorbed and translocated within plant tissues
Common examples	Carbendazim, metalaxyl, triazoles, carboxin
Benzimidazole group	Includes carbendazim-type fungicides
Metalaxyl note	Important systemic fungicide against oomycete-related diseases
Oxathiin group	Includes carboxin and oxycarboxin
Antibiotics and MBIs	Also appear under specialized systemic or translaminar disease-control themes
Agronomic advantage	Curative/protective action with internal movement in plant
Exam distinction	Systemic fungicides differ from contact fungicides by translocation behavior
Trap	Do not call all fungicides systemic; copper and sulphur fungicides are classic contact types

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