Surface fungicides, which keep harmful fungi from penetrating the tissues of a plant, include inorganic and organic compounds. Sulfur compounds, long used on plants, have been supplemented for some time by other chemicals, especially by compounds of copper, such as Bordeaux mixture. After 1945, organic salts of iron, zinc, and mercury were synthesized as fungicides. Most post-1965 fungicides are systemic, acting directly on fungal cells. Antifungal drugs, such as miconazole and terbinafine, are used for human fungal infections.
Plant fungicides are usually applied by spraying or dusting, but some types are applied to seeds and soil for the destruction of vegetative spores. Fungicides used on wood, including creosote, prevent dry rot, and certain compounds are used to make fabrics resistant to mildews. Most agricultural fungicides are preventive; those applied after infection are called eradicant, or contact, fungicides.
In the United States, fungicides are governed by the 1972 federal Environmental Protection and Control Act. They must be registered with the Environmental Protection Agency and must conform to specifications. They must control the disease without injuring the plant and must leave no poisonous residue on edible crops. Antifungal drugs are approved by the Food and Drug Administration.
See also pesticide.
Fungicides can either be contact or systemic. A contact fungicide kills fungi when sprayed on its surface; a systemic fungicide has to be absorbed by the plant.
The majority of fungicides that can be bought retail are sold in a liquid form. The most common active ingredient is sulfur, running at 0.08% for the weaker concentrates, and has high as 0.5% for the more potent fungicides. In powdered form, the concentration is usually around 90%, and the product is very toxic.
Fungicide residues have been found on food for human consumption, mostly from post-harvest treatments. Some fungicides are dangerous to human health, such as vinclozolin, which has now been removed from use.
Whole live or dead organisms that are efficient at killing or inhibiting fungi can sometimes be used as fungicides:
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 often 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 mutation 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 back 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 decreased the selection pressure.
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 pathogens evolving multiple drug resistance – 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 alternation of the target site, 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 one amino acid (glycine) being replaced by another (alanine) in the target protein of the QoI fungicides, cytochrome b. This presumably 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.
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 5 ABC type transporters with overlapping substrate specificities that together work to effectively pump toxic chemicals out of the cell.
In addiction 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.
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 developing resistance is greatly decreased by the fact that any resistant isolates to one fungicide will hopefully 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. 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 only ever marketed as a mixture resistance problems were not seen.
Fungicides should only be applied 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 also recommended that where possible fungicides are only used in a protective manner, rather than to try to cure already infected crops. Far fewer fungicides have curative/eradicative ability than protectant. Thus fungicide preparations advertised as having curative action may only have one active chemical; a single fungicide acting in isolation increases the risk of fungicide resistance.
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 practises, such as the removal of potato discard piles and stubble on which the pathogen can overwinter, greatly reduce the titre of the pathogen and thus the risk of fungicide resistance development.