Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi are capable of causing serious damage in agriculture, resulting in critical losses of yield, quality and profit. Although similar, oomycetes are not fungi. However, they use the same mechanisms to infect plants. Consequently, in the study of plant disease (phytopathology), chemicals used to control oomycetes are also referred to as fungicides. As well as in agriculture, fungicides are used to fight fungal infections in animal tissue.

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.

Other active ingredients in different brands include neem oil, rosemary oil, jojoba oil, and the bacterium Bacillus subtilis.

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.

Natural fungicides

Plants and other organisms over time have developed chemical defenses, (via natural selection), which give them an advantage against microorganisms such as fungi. Some of these compounds can be used as fungicides.

• Cinnamaldehyde
• Cinnamon essential oil
• Jojoba oil is fungicide, and can be used for controlling mildew.
• Neem oil
• Rosemary oil

Whole live or dead organisms that are efficient at killing or inhibiting fungi can sometimes be used as fungicides:

• The bacterium Bacillus subtilis
• Kelp (powdered dried kelp is fed to cattle to protect them from fungi in grass)

Fungicide resistance

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.