Fungicides are categorized in several ways based on different characteristics. The most common characteristics used and the categories are described below. Table 1 (Adobe Acrobat PDF) is a list of selected fungicides currently registered in the United States that represent the major fungicide groups and chemistry within these groups.
Mobility in a plant: contacts or systemics: Contact fungicides (also called protectants) remain on the surface of plants. Many contacts are potentially phytotoxic (toxic to plants) and can damage the plant if absorbed. Systemics (also called penetrants and mobile fungicides) are absorbed into plants. Most systemics move very short distances from the site of application, such as across a leaf blade from one surface to the other (local systemic or translaminar). Some fungicides are weakly systemic and can move further from the application site than local systemics, reaching all parts of the leaf the fungicide is deposited on. A few systemics move more extensively because they are mobile in xylem tissue. When applied to the root zone, these are absorbed by roots and then move upward through the plant with the transpiration stream (xylem-mobile systemic). Xylem-mobile systemics applied to leaves move throughout the leaf where deposited, but cannot be redistributed out of that leaf; however, any material deposited on stems can move upwards into leaves. Phloem-mobile systemics (also known as “true” or amphimobile systemics) have bi-directional mobility, some material moves in phloem out of the leaf where deposited upwards to other leaves and downwards to roots. Systemics cannot move again after translocation.
Animated examples of fungicide mobility:
Contact fungicide - applied to foliage.
Local systemic or translaminar- applied to foliage.
Xylem mobile systemic - applied to soil.
Xylem mobile systemic - applied to foliage.
Amphimobile systemic - applied to foliage.
Role in protection: preventive or curative. Contacts are products suited for preventive (prophylactic) use as they work by contact action on the surface of the plant to which they have been applied. Repeated applications are needed to protect new growth of the plant and to replace material that has been washed off by rain or irrigation, or degraded by environmental factors such as sunlight. Sometimes contacts are referred to as “residual” products because the deposited fungicide remains on the plant surface, occasionally as a visible residue, for several days. Due to their ability to penetrate plants, some systemics possess both preventive and curative (eradicant or kick-back) activity, thus affecting the pathogen after infection.
Breadth of activity: single-site or multi-site. Single-site fungicides are active against only one point in one metabolic pathway in a pathogen or against a single critical enzyme or protein needed by the fungus. Since single-site fungicides are highly specific in their toxicity, having little effect on most organisms, they can safely be absorbed into plants, thus these fungicides tend to have systemic properties. As a result of this specific activity, fungi are more likely to become resistant to the fungicide because a single mutation in the pathogen usually allows it to overcome the action of the fungicide, such as by preventing it from binding to the active site in the fungus.
Typically, older contact fungicides have multi-site activity and thus usually affect many fungi in different classes. Through the development of in vivo screens, and due to the increase in the stringency and number of regulatory tests required to register a new active ingredient, fungicide manufacturers have found it easier to develop single-site systemics recently. As a result, fungicide resistance has become a more important concern in disease management.
Mode of action. Fungicides kill fungi by damaging their cell membranes, inactivating critical enzymes or proteins, or by interfering with key processes such as energy production or respiration. Others impact specific metabolic pathways such as the production of sterols or chitin. For example, phenylamide fungicides bind to and inhibit the function of RNA polymerase in oomycetes, while the benzimidazole fungicides inhibit the formation of beta tubulin polymers used by cells during nuclear division.
Some recently developed products are unique in that they do not directly affect the pathogen itself. Many of these elicit a response from the host plant known as "systemic acquired resistance" (SAR). These SAR inducers basically mimic chemical signals in plants that activate plant defense mechanisms such as the production of thicker cell walls and anti-fungal proteins. The utility of SAR inducers, however, has been limited so far since many pathogens are capable of over-powering such defenses.
Knowledge of exactly how a fungicide affects a fungus is helpful for selecting products. First, mode of action determines which fungi will be affected by a fungicide and thus which diseases can be controlled by using the fungicide. Second, fungicides with different modes of action are needed in a disease management program to delay fungicide resistance development.
Biochemical mode of action is the primary basis used to classify fungicides into chemical groups. A list of fungicides organized by chemical group is maintained by the Fungicide Resistance Action Committee (FRAC) (http://www.frac.info/frac/index.htm) and is in Appendix II of a US Environmental Protection Agency (EPA) document (www.epa.gov/opppmsd1/PR_Notices/pr2001-5.pdf). Fungicides in Table 1 (Adobe Acrobat PDF) are listed according to the FRAC/EPA classification scheme plus an additional class for biofungicides. Basic chemical information for fungicides is at the PAN Pesticide Database (www.pesticideinfo.org). All fungicides within a chemical group share a common mode of action and resistance mechanism, even though their chemical structures may be different. Two exceptions in the FRAC/EPA classification scheme are the “multi-site activity” fungicide class and the “unknown” class of fungicides with unknown modes of action.
Breadth of activity: narrow-spectrum or broad-spectrum. Narrow-spectrum fungicides are effective against only a few usually closely related pathogens. These usually have single-site activity and are often systemic. Broad-spectrum fungicides can often control a wide range of unrelated pathogens. These usually are contacts with multi-site activity, but some have single-site activity. Several fungicides have activity that places them on a continuum between narrow- and broad-spectrum fungicides.
Type of chemical: inorganic or organic. Fungicides also can be classified based upon their chemical composition. Chemically, organic molecules are those that contain carbon atoms in their structure whereas inorganic molecules do not. Many of the first fungicides developed were inorganic compounds based on sulfur or metal ions such as copper, tin, cadmium and mercury that are toxic to fungi. Copper and sulfur are still widely used. Most other fungicides used today are organic compounds and thus contain carbon. The term "organic" as used here is based on chemistry terminology and differs from "organic" used to describe a system of agriculture that strives to be holistic and to enhance agroecosystem health.
Table 1 (Adobe Acrobat PDF) provides a list of fungicides and their characteristics that are representative of the major chemical groups registered in the USA.
Active ingredient (a.i.) is the active component of a pesticide, and is that moiety which is patented, synthesized, and registered by the manufacturer.
Biofungicide is a naturally based microbial or biochemical product. There are three types of biopesticides. (a) Microbial biopesticides have an active ingredient that is a biological control agent (organism capable of attacking or competing with a pathogen or pest). (b) Plant biopesticides or plant-incorporated protectants are defined by the EPA as "pesticidal substances that plants produce from genetic material that has been added to the plant". (c) Biochemical biopesticides contain naturally-occurring substances. Some biochemicals may also be products of fermentation. Biochemicals can directly affect disease-causing organisms or may stimulate SAR. Biopesticides generally are narrow-spectrum, have low toxicity, decompose quickly, and thus are considered to have low potential for negative impact on the environment (www.epa.gov/pesticides/biopesticides/). Many biofungicide products are being approved for organic crop production (www.omri.org). While many have low toxicity, biopesticides are not necessarily safer than pesticides containing synthetic ingredients.
Chemical group or class is the name given to a group of chemicals that share a common biochemical mode of action and may or may not have similar chemical structure.
Chemical name is denoted by a nomenclature system designed by the International Union of Pure and Applied Chemistry (IUPAC); this set of rules is used to name organic, carbon-based compounds.
Common name is the name proposed by the basic manufacturer for the active ingredient. This name is ratified by a committee either at IUPAC or the ACS (American Chemical Society).
Reduced risk is a special classification approved by the EPA for specific uses of pesticides that have low risk to human health, low toxicity to non-target organisms, low potential to contaminate water or other environmental resources, and/or that broaden the adoption and effectiveness of integrated pest management strategies. Registration is expedited for uses designated as reduced risk.
Formulation is the pesticide product. It is a prepared mix of active ingredient plus ingredients that improve pesticidal performance, such as carriers, solvents, wetting agents, spreaders, and stickers. Types of formulations include wettable powders, dry flowables, emulsifiable concentrates, and granules.
Trade name/Trademark is the patented name under which a product is sold to the end user.
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