Any chemical compound, natural or synthetic, designed or developed to kill or control the growth of fungus is known as a fungicide. Fungicides occupy a large part of the pesticide world, their use extending from industries to agriculture to one's home, for a number of purposes, including: (i) protection of seed grain during storage and/or shipment; (ii) protection of mature crops, seedlings and flowers in fields as well as in storage; (iii) suppression of mildews that attack painted surfaces; (iv) control of slime in many water-based industries (e.g. paper pulp); and (v) protection of wood and fabric in homes. Much of the supply of fungicides goes to prevent or minimize crop losses caused by phytopathogenic fungi. Most infections develop in humid conditions at moderate temperatures, making the consumption of all pesticide types increase at the onset of the period of rains.

Fungicides must be sufficiently toxic to the target fungi at dose concentrations that the plant can tolerate. Fungicidal activity can also be fungistatic in nature. Fungistatic agents do not kill the fungi but inhibit fungal growth temporarily. Some fungistatic chemicals like phenanthrene derivatives and Bordeaux mixture inhibit spore production without affecting the growth of vegetative hyphae. These chemicals are anti-sporulants. They cannot stop the disease but reduce the available inoculum that can spread the infection.

Fungicides can be classified in many ways, for example: (i) mobility in the plant; (ii) type of protection, to the plant; (iii) breadth of activity; (iv) mode of action (MOA); and (v) chemical group.

Fungicidal mobility can be further grouped into contact fungicides and residual fungicides (non-systemic fungicides), and systemic fungicides.

A contact fungicide is one that remains on the surface where it is applied but does not penetrate into the plant system. They form a protective layer over the surface and prevent the fungus from entering the host, thereby inhibiting spore germination or mycelial growth, thus displaying a protective function. The activity on the plant surface prevents infection from occurring (e.g. such fungicides include zineb, sulfur, 1,4-oxathiin, imidazole and pyridines). These fungicides react with the thiol groups present in the enzymes of the fungus and inhibit metabolic processes. They are multi-site inhibitors (i.e. they act on different sites in a metabolic process). Repeated applications are needed to protect new growth of the plant and to replace material that has been washed off by water, or degraded by environmental factors like sunlight. Some examples of contact fungicides are sulfur, copper oxychloride and dithiocarbamates.

Systemic fungicides are absorbed into plant tissue and may offer some post-infection activity. Very few fungicides are truly systemic, that is very few move freely throughout the plant. Some move only upwards in the plant through the xylem tissue, and some move locally into treated leaves and around the treated area, while still others move downwards in the plant from the site of application.

Systemic compounds have curative properties and can arrest the pathogen post-infection. They are commonly site specific and act only on select species. They interfere with biosynthetic processes such as respiration, protein synthesis and membrane functions. They inhibit fungal growth and spore production more effectively than spore germination. Systemic fungicides are needed in significantly smaller amounts per hectare. Examples of systemics include carbendazim and benomyl (both benzimidazoles), ofurace, metalaxyl, benalaxyl (all phenylamides), iprobenfos and edifenphos (both organophosphates) and penconazole, triadimenol, bitertanol (all triazoles). Chemical classes, for example pyridines, pyrimidines and imidazoles, also include active ingredients (a.i.) that are systemic in nature. Fungicides that are capable of eradicating a fungus after it has caused infection are called therapeutants. The a.i. in this group can penetrate the plant and stop the growth of the pathogen in the plant tissue; they are usually most effective 24–72 h after the occurrence of infection, depending on the fungicide. The fungicides that not only stop disease development after symptoms have developed but provide protection to the host are called eradicants. For example, organic mercurials, lime sulfur, phenylamides and strobilurins are eradicants. Anti-sporulant fungicides are those that contain a.i.s that can prevent sporulation or spore production.

Based on the breadth of activity, fungicides can act on a single site (acting only on one point in one metabolic pathway, or against a single critical enzyme or protein needed by the fungus), or have multiple sites (different metabolic pathways). Single-site fungicides are less toxic to plants.

Fungicides act by interfering with key processes, or by inactivating critical enzymes or proteins. These are known as the compound's MOA. Interfering with energy production or respiration are some examples of MOA. Fungicides can be classified under particular chemical groups, indicating a common biochemical MOA. These groups may or may not have similar chemical structures.

On the basis of protection type, fungicides can be: (i) seed protectants (e.g. captan, thiram, organic mercurials); (ii) soil fumigants (e.g. chloropicrin, formaldehyde, vapam); or (iii) tree-wound dressing fungicides (e.g. Bordeaux paste or spray).