Infectious fungi sometimes develop resistance to particular fungicides, especially when a product is used repeatedly without alternating with chemically unrelated fungicides. When fungicide resistance develops, there is no value in increasing rates, shortening intervals between sprays, or using other fungicides with similar modes of action. Since the 1970s, fungicide resistance has been a real issue with certain fungicide chemistries used for disease management. Since fungicides registered for management of Asian soybean rust are limited, development of fungal resistance to one or more of these compounds is of great concern.
Fungicide resistance is "the stable, inheritable adjustment by a fungus to a fungicide, resulting in a less than normal sensitivity to that fungicide" (Dekker, 1995). In other words, fungicides don't work as well, or at all, on populations of fungi that have become less sensitive to them. Since fungicide resistance develops on a genetic level, off-spring of such fungi are also resistant.
To understand how fungicide resistance develops, let's first examine how fungicides work. Fungicides are compounds that destroy or prevent the growth of fungi. In general, fungicides interfere with the fungal germination and penetration process or can inhibit fungal growth and reproduction within plant tissues. More specifically, fungicides may interfere with cell division, may inhibit the activity of certain enzymes, or may alter the function of cell membranes. The manner in which a fungicide impacts fungal biology is called its mode of action.
All fungicides are referred to by a chemical name, a common name, and a trade name. The chemical name is the scientific name of the active ingredient of the fungicide, and the common name is a shortened version that everyone can recognize and pronounce. The trade name is used by companies to market the active ingredient; a single active ingredient can be marketed under more than one trade name by different companies or for different crops.
As an example, the common, chemical, and trade names for one active ingredient that is registered for Asian soybean rust is:
Equus 720 SST
Fungicides are also grouped in classes along with other compounds with similar chemistry. For the example above, chlorothalonil (common name) is in the nitrile class. All compounds in this class have similar modes of action. In other words, all fungicides in this class affect fungi by affecting cell wall synthesis. The class to which a particular fungicide belongs is important; once a fungus develops resistance to one compound in a class, it becomes resistant to all other fungicides in that class. This is called cross resistance and has been well documented in the literature.
Fungicides can also be classified by how they move in a plant (Table 1). Contact fungicides are protectant chemicals; they don't penetrate plant tissues once applied and remain active only on the surface. Penetrants do move into plant tissues. Penetrants that remain in the area of initial entry are called localized penetrants; those that are translocated upward from the point of entry (through the xylem) are acropetal penetrants (or xylem systemics); and those that are translocated throughout the plant (through both xylem and phloem) are called true systemic penetrants. Fungicides that spread to the opposite leaf surface have translaminar activity.
Since contact fungicides are active only at the site of placement and do not penetrate plant tissue, they may be washed off, and most do not protect new tissues that grow after the fungicide is applied. In addition, they do not prevent infections as long (called residual activity) as other chemistries and must be applied more frequently. Contact fungicides are useful materials in fungicide rotation schemes, however, because they affect fungi activity at multiple sites, which reduces the likelihood that fungicide resistance will develop (more on this subject later).
Penetrant fungicides are active within the plant but can also function as contact fungicides at the site of placement. They are not subject to wash-off and have a longer residual activity than contact fungicides. Acropetal penetrants and true systemics will protect new growth, and some of these compounds have curative properties. A disadvantage of penetrant fungicides is that they affect fungi at one or very few specific metabolic sites, thus the probability of fungicide resistance is much greater than with contact fungicides.
|Table 1. Classes of different fungicides by type|
|Type of fungicide||Fungicide class|
inorganics (copper and sulfur)
sterol inhibitors (demethylation inhibitors)
Decisions on which fungicide to use are often based on whether the compound has preventive (protectant) or curative (eradicant) properties. Preventive fungicides place a barrier between the host tissue and the fungal spore, inhibiting the infection process by preventing spore germination or penetration. Curative fungicides work after the infection process has begun but before extensive visible symptoms of disease develop. It is important to remember that curative compounds cannot "erase" the damage that has already occurred in the infection process (e.g., necrotic (or dead) leaf tissue will always remain necrotic).
There are several reasons why fungicides "fail" in a given situation. Most commonly, the disease is simply misdiagnosed: products applied for one disease are often ineffective against other diseases. Second, fungicides are not properly applied: timing, technique, or the rate may not be adequate. Third, environmental factors or cultural practices may so favor disease development that fungicides are not effective. Finally, the fungus is or has become resistant to the active ingredient of the fungicide. In some cases, populations of fungi may be naturally resistant to a given compound; in others, fungi may mutate to become tolerant or resistant.
When a fungus becomes resistant to a fungicide, this resistance may be caused by one of several potential mechanisms: the fungus does not absorb the fungicide to lethal levels; the fungicide, once absorbed, is not converted to the active compound; the active ingredient is detoxified; or the fungus uses an alternate metabolic pathway to circumvent the action of the fungicide.
Development of resistance is tied to the fungicide mode of action depending on whether the fungicide acts on a single site within the fungus or at multiple sites. Earlier in this discussion, we described how fungi are less prone to develop resistance to contact fungicides, which inhibit several vital functions in the fungal cell, than to many penetrant fungicides, which may inhibit only one vital function in the fungal cell. A single fungus needs to mutate only once to overcome the action of a single-site fungicide but must mutate more often to overcome the action of a multi-site fungicide, and the likelihood of this is rare.
Development of resistance is also tied to whether factors for resistance in the pathogen are controlled by one gene (monogenic) or by more than one gene (polygenic). A single fungus with monogenic resistance requires only one mutation to circumvent the toxic properties of a given fungicide. Alternatively, a single fungus with polygenic resistance requires mutations in more than one gene, and this, too, is less likely.
In general, several general strategies are recommended to minimize the risk of fungicide resistance. First, don't rely on fungicides alone for disease control. Follow good management practices and use resistant host plant material as it becomes available. Second, avoid repeated use of the same fungicide or materials with closely related modes of action. Third, alternate or tank mix fungicides with different modes of action. For Asian soybean rust, several premix fungicides (with Section 18 labels) containing both triazole and strobilurin chemistries are available. Finally, use recommended rates and proper management techniques. Although these general principles can help to reduce risk of fungicide resistance, they do not eliminate it. Pathogen populations resistant to a particular fungicide can still develop, even when good management practices are employed. Refer to product labels before tank-mixing products to ensure compatibility and to avoid phytotoxicity.
Dekker, J. 1995. Development of resistance to modern fungicides and strategies for its avoidance. P. 22-38 in: Modern Selective Fungicides. Properties, Applications, Mechanisms of Action, 2nd ed. Gustav Fischer Verlag Publ., New York.
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