Fungicide Theory of Use and Mode of Action

By J. W. PSCHEIDT, Extension Plant Pathology Specialist, OSU.

Fungicides can be classified by chemical class, general mode of action, specific mode of action, or by physical properties once in the plant. Many fungicides within a family, such as the benzimidazoles, have the same mode of action against fungi. Often, it is recommended to tank-mix or to alternate fungicides with different modes of action to prevent or delay the buildup of resistant fungi. In general, fungi resistant to one chemical-family member, such as iprodione, also are resistant to all other chemicals in the family - in this example, to vinclozolin.

A fungicide's mode of action can be described in general or specific terms. A fungicide with broad-spectrum activity is effective against a large variety of pathogenic fungi. Examples of broad-spectrum fungicides include captan, sulfur, and mancozeb. Some fungicides have a very narrow spectrum of activity; for example, metalaxyl is effective only against oomycetes like Phytophthora. Alternatively, a fungicide may affect a broad range of fungi but by only a specific mode of action. For example, Topsin is useful to control many fungal diseases; it acts by binding to tubulin, thereby blocking mitosis.

Another way to describe a fungicide's mode of action is to say that it is used for protection, as a curative, or as an eradicant.

Protectants are applied to healthy plants to prevent fungal spores from germinating or penetrating host tissue. They must be applied before the fungal spore has a chance to infect the plant. New plant tissue that develops after application generally is unprotected. Protectants generally are not effective once the fungus grows into plant tissues. Examples of protectants include mancozeb, coppers, and chlorothalonil.

Note: Some formulations of chlorothalonil, such as Bravo, can protect newly developed plant tissues because rain action redistributes the fungicide to other plant parts.

Curative materials generally act within the plant and are effective against fungi shortly after penetration. "Kickback activity" and "curative" are interchangeable terms. These materials must be applied within a certain time after infection starts. Materials such as dodine, triflumizole, or myclobutanil have 36-, 72-, and 96-hour curative activity, respectively, against the apple scab fungus.

Eradicants can be of two types. Lime sulfur is an eradicant that acts by killing fungi on contact. For example, lime sulfur can be used during the dormant season to kill overwintering cleistothecia of grape powdery mildew. At dormant-season rates, it would be phytotoxic during the growing season. Recently, eradicants such as triadimefon and myclobutanil have been developed that not only kill powdery mildew colonies but prevent sporulation as well. Many of these compounds also are active against rusts and various leaf-spotting fungi.

Many newer fungicides have systemic properties, which means they are absorbed and translocated by certain plant parts. Most of these fungicides are locally systemic. Green plant tissues such as leaves or shoots absorb the materials (such as DMI type fungicides) and move them short distances within the transpiration stream (generally toward the leaf margin) or between plant cells. The new QoI or strobilurin compounds have a slightly different distribution. These compounds move into and through the leaf but do not move in the transpiration stream. This activity has been termed by the manufacturers as translaminar or quasi-systemic. Long drying times and warmer temperatures after application favor the uptake of all of these materials.

Metalaxyl is a fungicide that can be absorbed by plant roots and translocated throughout the plant. Translocation, however, is only acropetally (upward), with the transpiration stream. Aliette is the only truly systemic compound and is translocated both basipetally (downward) and acropetally whether applied to roots or leaves.

A fumigant is a vapor-active chemical used in the gaseous phase to kill microorganisms. Many nematicides are fumigants but kill fungal propagules as well as insects and weed seeds. Some fumigants such as sulfur are effective only in the gaseous phase. Both temperature and humidity can influence the activity of sulfur in the vapor phase.

Antibiotics are produced by one organism against another. Many of the antibiotics in use today are bactericidal, but some also are fungicidal.

Some fungicides only inhibit fungi (are fungistatic) rather than kill them (fungicidal). Fungistats must be applied continually over the life of the plant to suppress disease development. For example, metalaxyl will prevent zoospores from penetrating roots but only inhibits an established Phytophthora infection.

Terms used to describe fungicides are not mutually exclusive. For example, dodine is an excellent broad-spectrum, protectant fungicide but also has 36-hour curative activity against apple scab.

Most fungicides directly inhibit or kill the target fungus, but Aliette can act by stimulating host defense mechanisms. Details of its activity in host resistance are poorly understood.

The hazelnut branch on the left was treated with Orbit for eastern filbert blight control. Note the plant growth regulator effect with darker green, smaller leaves.

References: Books

1. Delp, C.J. 1988. Fungicide Resistance in North America. St. Paul, MN: APS Press. Soft cover.
2. Lyr, H. 1995. Modern Selective Fungicides: Properties, Applications, Mechanisms of Action, 2nd ed. Villengang, Germany, and New York: Gustav Fischer Verlag. Hard cover.
3. Torgeson, D.C. 1967. Fungicides: An Advanced Treatise, vols. 1 and 2. New York: Academic Press.

Periodicals

1. Crop Protection Reference. Revised and reissued annually. New York: Chemical and Pharmaceutical Press, Inc. (Sample label guides or catalogues from individual chemical companies.)
2. Meister, R.T. and C. Sine. 1996. Farm Chemicals Handbook '96, vol. 8. Willoughby, OH: Meister Publishing Co.
3. Thomson, W.T. 1995. Fungicides. In: Agricultural Chemicals, book IV. Fresno, CA: Thomson Publications. Revised regularly.
4. Turf and Ornamental Reference for Plant Protection Products. Revised and reissued annually. New York: Chemical and Pharmaceutical Press, Inc. (Sample label guides or catalogues from individual chemical companies.)

   Article last updated on January 1, 2009