Fungicide

Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores.[1] A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, and profit. Fungicides are used both in agriculture and to fight fungal infections in animals. Chemicals used to control oomycetes, which are not fungi, are also referred to as fungicides, as oomycetes use the same mechanisms as fungi to infect plants.[2]

Fungicides can either be contact, translaminar or systemic. Contact fungicides are not taken up into the plant tissue and protect only the plant where the spray is deposited. Translaminar fungicides redistribute the fungicide from the upper, sprayed leaf surface to the lower, unsprayed surface. Systemic fungicides are taken up and redistributed through the xylem vessels. Few fungicides move to all parts of a plant. Some are locally systemic, and some move upwardly.[3]

Most fungicides that can be bought retail are sold in a liquid form. A very common active ingredient is sulfur,[4] present at 0.08% in weaker concentrates, and as high as 0.5% for more potent fungicides. Fungicides in powdered form are usually around 90% sulfur and are very toxic. Other active ingredients in fungicides include neem oil, rosemary oil, jojoba oil, the bacterium Bacillus subtilis, and the beneficial fungus Ulocladium oudemansii.

Fungicide residues have been found on food for human consumption, mostly from post-harvest treatments.[5] Some fungicides are dangerous to human health, such as vinclozolin, which has now been removed from use.[6] Ziram is also a fungicide that is toxic to humans with long-term exposure, and fatal if ingested.[7] A number of fungicides are also used in human health care.

Natural fungicides

Plants and other organisms have chemical defenses that give them an advantage against microorganisms such as fungi. Some of these compounds can be used as fungicides:

Whole live or dead organisms that are efficient at killing or inhibiting fungi can sometimes be used as fungicides:

Resistance

Pathogens respond to the use of fungicides by evolving resistance. In the field several mechanisms of resistance have been identified. The evolution of fungicide resistance can be gradual or sudden. In qualitative or discrete resistance, a mutation (normally to a single gene) produces a race of a fungus with a high degree of resistance. Such resistant varieties also tend to show stability, persisting after the fungicide has been removed from the market. For example, sugar beet leaf blotch remains resistant to azoles years after they were no longer used for control of the disease. This is because such mutations have a high selection pressure when the fungicide is used, but there is low selection pressure to remove them in the absence of the fungicide.

In instances where resistance occurs more gradually, a shift in sensitivity in the pathogen to the fungicide can be seen. Such resistance is polygenic – an accumulation of many mutations in different genes, each having a small additive effect. This type of resistance is known as quantitative or continuous resistance. In this kind of resistance, the pathogen population will revert to a sensitive state if the fungicide is no longer applied.

Little is known about how variations in fungicide treatment affect the selection pressure to evolve resistance to that fungicide. Evidence shows that the doses that provide the most control of the disease also provide the largest selection pressure to acquire resistance, and that lower doses decrease the selection pressure.[15]

In some cases when a pathogen evolves resistance to one fungicide, it automatically obtains resistance to others – a phenomenon known as cross resistance. These additional fungicides are normally of the same chemical family or have the same mode of action, or can be detoxified by the same mechanism. Sometimes negative cross resistance occurs, where resistance to one chemical class of fungicides leads to an increase in sensitivity to a different chemical class of fungicides. This has been seen with carbendazim and diethofencarb.

There are also recorded incidences of the evolution of multiple drug resistance by pathogens – resistance to two chemically different fungicides by separate mutation events. For example, Botrytis cinerea is resistant to both azoles and dicarboximide fungicides.

There are several routes by which pathogens can evolve fungicide resistance. The most common mechanism appears to be alteration of the target site, in particular as a defence against single site of action fungicides. For example, Black Sigatoka, an economically important pathogen of banana, is resistant to the QoI fungicides, due to a single nucleotide change resulting in the replacement of one amino acid (glycine) by another (alanine) in the target protein of the QoI fungicides, cytochrome b.[16] It is presumed that this disrupts the binding of the fungicide to the protein, rendering the fungicide ineffective. Upregulation of target genes can also render the fungicide ineffective. This is seen in DMI-resistant strains of Venturia inaequalis.[17]

Resistance to fungicides can also be developed by efficient efflux of the fungicide out of the cell. Septoria tritici has developed multiple drug resistance using this mechanism. The pathogen had five ABC-type transporters with overlapping substrate specificities that together work to pump toxic chemicals out of the cell.[18]

In addition to the mechanisms outlined above, fungi may also develop metabolic pathways that circumvent the target protein, or acquire enzymes that enable metabolism of the fungicide to a harmless substance.

Fungicide resistance management

The fungicide resistance action committee (FRAC) has several recommended practices to try to avoid the development of fungicide resistance, especially in at-risk fungicides including Strobilurins such as azoxystrobin.

Products should not be used in isolation, but rather as mixture, or alternate sprays, with another fungicide with a different mechanism of action. The likelihood of the pathogen's developing resistance is greatly decreased by the fact that any resistant isolates to one fungicide will be killed by the other; in other words, two mutations would be required rather than just one. The effectiveness of this technique can be demonstrated by Metalaxyl, a phenylamide fungicide. When used as the sole product in Ireland to control potato blight (Phytophthora infestans), resistance developed within one growing season. However, in countries like the UK where it was marketed only as a mixture, resistance problems developed more slowly.

Fungicides should be applied only when absolutely necessary, especially if they are in an at-risk group. Lowering the amount of fungicide in the environment lowers the selection pressure for resistance to develop.

Manufacturers’ doses should always be followed. These doses are normally designed to give the right balance between controlling the disease and limiting the risk of resistance development. Higher doses increase the selection pressure for single-site mutations that confer resistance, as all strains but those that carry the mutation will be eliminated, and thus the resistant strain will propagate. Lower doses greatly increase the risk of polygenic resistance, as strains that are slightly less sensitive to the fungicide may survive.

It is better to use an integrative pest management approach to disease control rather than relying on fungicides alone. This involves the use of resistant varieties and hygienic practices, such as the removal of potato discard piles and stubble on which the pathogen can overwinter, greatly reducing the titre of the pathogen and thus the risk of fungicide resistance development.

See also

References

  1. ^ Interaction of 2,4,5-trich|orophenylsulphonylmethyl thiocyanate with fungal spores
  2. ^ Latijnhouwers M, de Wit PJ, Govers F. Oomycetes and fungi: similar weaponry to attack plants. Trends in Microbiology Volume 11 462-469
  3. ^ Mueller, Daren. "Fungicides:Terminology". Iowa State University. Retrieved June 1, 2013.
  4. ^ C.Michael Hogan. 2011. Sulfur. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC Archived October 28, 2012, at the Wayback Machine
  5. ^ Pesticide Chemistry and Bioscience edited by G.T Brooks and T.R Roberts. 1999. Published by the Royal Society of Chemistry
  6. ^ Hrelia et al. 1996 - The genetic and non-genetic toxicity of the fungicide Vinclozolin. Mutagenesis Volume 11 445-453
  7. ^ National Center for Biotechnology Information. PubChem Compound Database; CID=8722, https://pubchem.ncbi.nlm.nih.gov/compound/8722(accessed Jan. 13, 2019)
  8. ^ "TEA TREE OIL Uses & Effectiveness". WebMD. WebMD, LLC.
  9. ^ Nakahara, Kazuhiko; Alzoreky, Najeeb S.; Yoshihashi, Tadashi; Nguyen, Huong T. T.; Trakoontivakorn, Gassinee (October 2003). "Chemical Composition and Antifungal Activity of Essential Oil from Cymbopogon nardus (Citronella Grass)". Japan International Research Center for Agricultural Sciences. 37 (4): 249–52. INIST:15524982.
  10. ^ Pattnaik, S; Subramanyam, VR; Kole, C (1996). "Antibacterial and antifungal activity of ten essential oils in vitro". Microbios. 86 (349): 237–46. PMID 8893526. INIST:3245986.
  11. ^ Prabuseenivasan, Seenivasan; Jayakumar, Manickkam; Ignacimuthu, Savarimuthu (2006). "In vitro antibacterial activity of some plant essential oils". BMC Complementary and Alternative Medicine. 6: 39. doi:10.1186/1472-6882-6-39. PMC 1693916. PMID 17134518.
  12. ^ US 6174920 Method of controlling powdery mildew infections of plants using jojoba wax
  13. ^ "Drop of white the right stuff for vines". Science Daily. 2002-09-12. Retrieved 2009-04-01.
  14. ^ Campbell, Malcolm (2003-09-19). "Fact Sheet: Milk Fungicide". Australian Broadcasting Corporation. Retrieved 2009-04-01.
  15. ^ Metcalfe, R.J. et al. (2000) The effect of dose and mobility on the strength of selection for DMI (sterol demethylation inhibitors) fungicide resistance in inoculated field experiments. Plant Pathology 49: 546–557
  16. ^ Sierotzki, Helge (2000) Mode of resistance to respiration inhibitors at the cytochrome bc1 enzyme complex of Mycosphaerella fijiensis field isolates Pest Management Science 56:833–841
  17. ^ Schnabel, G., and Jones, A. L. 2001. The 14a-demethylase (CYP51A1) gene is overexpressed in V. inaequalis strains resistant to myclobutanil. Phytopathology 91:102–110.
  18. ^ Zwiers, L. H. et al. (2003) ABC transporters of the wheat pathogen Mycosphaerella graminicola function as protectants against biotic and xenobiotic toxic compounds. Molecular Genetics and Genomics 269:499–507

External links

Antifungal

An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as cryptococcal meningitis, and others. Such drugs are usually obtained by a doctor's prescription, but a few are available OTC (over-the-counter).

Bacillus subtilis

Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants and humans. A member of the genus Bacillus, B. subtilis is rod-shaped, and can form a tough, protective endospore, allowing it to tolerate extreme environmental conditions. B. subtilis has historically been classified as an obligate aerobe, though evidence exists that it is a facultative anaerobe. B. subtilis is considered the best studied Gram-positive bacterium and a model organism to study bacterial chromosome replication and cell differentiation. It is one of the bacterial champions in secreted enzyme production and used on an industrial scale by biotechnology companies.

Benzimidazole fungicide

Benzimidazole fungicides are a class of fungicides including benomyl, carbendazim (MBC), thiophanate-methyl, thiabendazole and fuberidazole. They can control many ascomycetes and basidiomycetes, but not oomycetes. They are applied to cereals, fruits, vegetables and vines, and are also used in postharvest handling of crops.The solubility of benzimidazole fungicides is low at physiological pH and becomes high at low pH. In plants, carbendazim, thiabendazole and fuberidazole are mobile, i.e. systemic, and benomyl and thiophanate-methyl are converted to carbendazim. This conversion also occurs in soils and animals. In soil and water, carbendazim is mainly degraded by microbes. They are metabolized through hydrolysis and photolysis in plants. These fungicides kill cells during mitosis by distorting the mitotic spindle; β-tubulin, a protein important in forming the cytoskeleton, is targeted. They mostly inhibit polymerization of β-tubulin by interacting with it directly, but other interactions also exist.Starting in the late 1960s, they were widely used to control fungal pathogens such as Botrytis cinerea, Cercospora, powdery mildew and eyespot. These systemic fungicides were very effective at first. Because there is only one target site, benzimidazole resistance quickly became a serious problem. When they were the only fungicides used, pathogens became resistant after two to four seasons; when mixed with other fungicides, resistance developed more slowly. Resistant genotypes with certain point mutations were selected. Mutant pathogens resistant to one benzimidazole fungicide are usually resistant to all of them. The F200Y and E198A,G,K mutations are the most common. Because of resistance problems, use of benzimidazole fungicides has declined. They are suspected to be toxic to animals, including humans. The Fungicide Resistance Action Committee lists them as having a high risk of resistance evolution.

Carbendazim

Carbendazim is a widely used, broad-spectrum benzimidazole fungicide and a metabolite of benomyl. It is also employed as a casting worm control agent in amenity turf situations such as golf greens, tennis courts etc. and in some countries is licensed for that use only.The fungicide is used to control plant diseases in cereals and fruits, including citrus, bananas, strawberries, pineapples, and pomes. It is also controversially used in Queensland, Australia on macadamia plantations. A 4.7% solution of carbendazim hydrochloride, sold as Eertavas, is marketed as a treatment for Dutch elm disease.

Studies have found high doses of carbendazim cause infertility and destroy the testicles of laboratory animals.Maximum pesticide residue limits (MRLs) have reduced since discovering its harmful effects. The MRLs for fresh produce in the EU are now between 0.1 and 0.7 mg/kg with the exception of loquat, which is 2 mg/kg. The limits for more commonly consumed citrus and pomme fruits are between 0.1 and 0.2 mg/kg.

Chlorothalonil

Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) is an organic compound mainly used as a broad spectrum, nonsystemic fungicide, with other uses as a wood protectant, pesticide, acaricide, and to control mold, mildew, bacteria, algae. Chlorothalonil-containing products are sold under the names Bravo, Echo, and Daconil. It was first registered for use in the US in 1966. In 1997, the most recent year for which data are available, it was the third most used fungicide in the US, behind only sulfur and copper, with 12 million pounds (5.4 million kilograms) used in agriculture that year. Including nonagricultural uses, the United States Environmental Protection Agency (EPA) estimates, on average, almost 15 million lb (6.8 million kg) were used annually from 1990 to 1996.

Dichlorophen

Dichlorophen is an anticestodal agent, fungicide, germicide, and antimicrobial agent. It is used in combination with toluene for the removal of parasites such as ascarids, hookworms, and tapeworms from dogs and cats.

Enilconazole

Enilconazole (synonyms imazalil, chloramizole) is a fungicide widely used in agriculture, particularly in the growing of citrus fruits. Trade names include Freshgard, Fungaflor, and Nuzone.

Enilconazole is also used in veterinary medicine as a topical antimycotic.

Federal Insecticide, Fungicide, and Rodenticide Act

The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) is a United States federal law that set up the basic U.S. system of pesticide regulation to protect applicators, consumers, and the environment. It is administered and regulated by the United States Environmental Protection Agency (EPA) and the appropriate environmental agencies of the respective states. FIFRA has undergone several important amendments since its inception. A significant revision in 1972 by the Federal Environmental Pesticide Control Act (FEPCA) and several others have expanded EPA's present authority to oversee the sales and use of pesticides with emphasis on the preservation of human health and protection of the environment by "(1) strengthening the registration process by shifting the burden of proof to the chemical manufacturer, (2) enforcing compliance against banned and unregistered products, and (3) promulgating the regulatory framework missing from the original law".

Hexaconazole

Hexaconazole is a Broad-spectrum systemic triazole fungicide used for the control of many fungi particularly Ascomycetes and Basidiomycetes. Major consumption is in Asian countries and it is used mainly for the control of rice sheath blight in China, India, Vietnam and parts of East Asia. It is also used for control of diseases in various fruits & vegetables.

Index of pesticide articles

This is an index of articles relating to pesticides.

Mancozeb

Mancozeb is a dithiocarbamate non-systemic agricultural fungicide with multi-site, protective action on contact. It is a combination of two other dithiocarbamates: maneb and zineb. The mixture controls many fungal diseases in a wide range of field crops, fruits, nuts, vegetables, and ornamentals. It is marketed as Penncozeb, Trimanoc, Vondozeb, Dithane, Manzeb, Nemispot, and Manzane. In Canada, a mixture of zoxamide and mancozeb was registered for control of the mildew named Gavel as early as 2008.

Metalaxyl

Metalaxyl is an acylalanine fungicide with systemic function. Its chemical name is methyl N-(methoxyacetyl)-N-(2,6-xylyl)-DL-alaninate. It can be used to control Pythium in a number of vegetable crops, and Phytophthora in peas. Metalaxyl-M or Ridomil Gold are trade names for the optically pure (-) / D / R active stereoisomer, which is also known as Mefenoxam.It is the active ingredient in the seed treatment agent Apron XL LS.The fungicide has suffered severe resistance problems. The fungicide was marketed for use against Phytophthora infestans. However, in the summer of 1980, in the Republic of Ireland, the crop was devastated by a potato blight epidemic after a resistant race of the oomycete appeared. Irish farmers later successfully sued the company for their losses.

Maximum pesticide residue limits for the EU/UK are set at 0.5 mg/kg for oranges and 1.0 mg/kg for apples

Prochloraz

Prochloraz, brand name Sportak, is an imidazole fungicide that was introduced in 1978 and is widely used in Europe, Australia, Asia, and South America within gardening and agriculture to control the growth of fungi. It is not registered for use in the United States. Similarly to other azole fungicides, prochloraz is an inhibitor of the enzyme lanosterol 14α-demethylase (CYP51A1), which is necessary for the production of ergosterol – an essential component of the fungal cell membrane – from lanosterol. The agent is a broad-spectrum, protective and curative fungicide, effective against Alternaria spp., Botrytis spp., Erysiphe spp., Helminthosporium spp., Fusarium spp., Pseudocerosporella spp., Pyrenophora spp., Rhynchosporium spp., and Septoria spp.Like many imidazole and triazole fungicides and antifungal medications, prochloraz is not particularly selective in its actions. In addition to inhibition of lanosterol 14α-demethylase, prochloraz has also been found to act as an antagonist of the androgen and estrogen receptors, as an agonist of the aryl hydrocarbon receptor, and as an inhibitor of enzymes in the steroidogenesis pathway such as CYP17A1 and aromatase. In accordance, it has been shown to produce reproductive malformations in mice. As such, prochloraz is considered to be an endocrine disruptor.

Procymidone

Procymidone is a pesticide. It is often used for killing unwanted ferns and nettles, and as a dicarboximide fungicide for killing fungi, for example as seed dressing, pre-harvest spray or post-harvest dip of lupins, grapes, stone fruit, strawberries. It is a known endocrine disruptor (androgen receptor antagonist) and to interfere with the sexual differention of male rats. It is considered to be a poison.

Propamocarb

Propamocarb is a systemic fungicide used for control of soil, root and leaf disease caused by oomycetes. It is used by watering or spraying. Propamocarb is absorbed and distributed through the plant's tissue.

Propiconazole

Propiconazole is a triazole fungicide, also known as a DMI, or demethylation inhibiting fungicide due to its binding with and inhibiting the 14-alpha demethylase enzyme from demethylating a precursor to ergosterol. Without this demethylation step, the ergosterols are not incorporated into the growing fungal cell membranes, and cellular growth is stopped.

Stratification (seeds)

In horticulture, stratification is a process of treating seeds to simulate natural conditions that the seeds must experience before germination can occur. Many seed species have an embryonic dormancy phase, and generally will not sprout until this dormancy is broken.The term stratification can be traced back to at least 1664 in Sylva, or A Discourse of Forest-Trees and the Propagation of Timber, where seeds were layered (stratified) between layers of moist soil and exposing these strata to winter conditions. Thus, stratification became the process by which seeds were artificially exposed to conditions to encourage subsequent germination.

The Conversion (Seinfeld)

"The Conversion" is the 75th episode of the NBC sitcom Seinfeld. It is the 11th episode of the fifth season, and first aired on December 16, 1993.

Triclopyr

Triclopyr (3,5,6-Trichloro-2-pyridinyloxyacetic acid) is an organic compound in the pyridine group that is used as a systemic foliar herbicide and fungicide.

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