How Fungicides Work

About Fungicides

• Fungicides are chemicals that prevent or slow down the growth of fungi. Fungal pathogens infect plants in a series of stages. Fungicides work by interrupting or stopping one or more of these stages. There are several different chemicals in a variety of formulations available to treat fungal infections in plants. They work either through contact or systemic action to stop the metabolism of fungal pathogens, either killing them or slowing their development. Some fungicides act specifically, blocking specific enzymes or groups of enzymes. Others inhibit a range of metabolic stages and are not as selective.

Contact Fungicides

• Contact fungicides are typically used as a spray that is applied directly to the plant as a barrier that prevents infection. They must remain on the surface of the plant and work only when applied before symptoms appear. Once the infection is present, it will continue to develop even after contact fungicide treatment. This is because the fungicide cannot penetrate the plant in order to act inside the tissues. Contact fungicides will, however, treat the initial developmental stages of infection, as long as the fungus has not penetrated the plant's tissues. To be effective, contact fungicides must cover the entire surface of the plant. New growth that develops after treatment will be unprotected so repeated applications are often necessary.

Systemic Fungicides

• Systemic fungicides move inside the plant tissue but typically move upward, not down, and some will only move into the part of the plant treated. There are systemic fungicides available that can stop development of the fungal pathogens, and most are selective, limited to a particular fungus or group of fungi. Systemic fungicides provide both protection and curative treatment. Systemic fungicides are applied after symptoms appear and help to treat the infection, but their effectiveness may be limited in some situations such as severe or widespread infections.

Resistance

• Repeated use of fungicides from the same class of chemicals, or active substance, can cause the fungi to develop a resistance to the fungicide. This resistance spreads through later fungal growth so that it becomes a fixed genetic trait, making the fungicide useless in treating the fungus. Over time, the resistant strain will dominate and any fungicide within the same class will be ineffective. To prevent resistance, alternate fungicides so that those with different modes of action are used. Unnecessary treatment should also be avoided. If no infection is present or suspected, or if there has been no previous infection, for example, don't treat plants with fungicide. Do not treat with reduced application rates because this will not prevent resistance. It usually increases it instead, and the treatment will have been wasted because it is not likely to do its job.