Fungicides are chemicals that are designed to kill fungi and their spores. Typically sprayed on plants, they destroy fungi by rupturing the cell walls or interfering with their metabolic functions.
There are around 148,000 known species of fungi, which range from microscopic single-cell yeasts to interconnected multicellular networks that can span several hectares. Almost unbelievably, the largest organism (that we know of) on earth is a fungus called the giant Armillaria ostoyae, which thrives in Oregon’s Blue Mountains.
While some types of fungi are edible or have medicinal properties, a few species are considered a threat to both crops and human health. As we explain in this post, these types of fungi can be eliminated with the help of fungicides.
In this post:
What is a fungicide?
Fungicides are either chemical compounds (both organic and inorganic) or biological organisms that kill parasitic fungi. They do this either by directly destroying the mature fungi colony or by eliminating the spores. 
It’s important to note that fungicides are different from fungistatic agents. The former actually kills the fungi while the latter merely inhibits the growth and spread of fungi by interfering with their reproduction. Fungicides are primarily used to protect crops by exterminating the parasitic fungi species that cling to them. Certain types of fungicides are also used to treat fungal infections in animals.
Fungicides can kill fungi in several different ways. Some products destroy fungi through direct contact, while others kill through translaminar means or by operating in a systematic manner that interferes with the organism’s biological functions.
Commercial fungicide products are most commonly sold in a liquid form. These types of treatments, which are designed to be sprayed on infected plant surfaces, typically contain 0.5% sulphur. However, the sulphur content in some powdered fungicides can be as high as 90%, making them extremely toxic – even to humans. Other active ingredients commonly used in fungicides include the following:
- Neem oil
- Rosemary oil
- Jojoba oil
- Bacillus subtilis (a bacterium)
- The beneficial fungus Ulocladium oudemansii
How do fungicides work?
The most common way fungicides kill fungi is by damaging the organism’s cell membranes, which causes the cytoplasm – together with the organelles and nutrients – to leak out. However, fungicides can also kill fungi by:
- Inactivating critical enzymes
- Inactivating protein production
- Interfering with respiration
- Inhibiting energy production
Other types of fungicides target specific metabolic pathways. For example, one class of fungicides known as phenylamides, inhibits the action of RNA polymerase in oomycetes (water moulds) and prevents the cells from dividing. Similarly, benzimidazole fungicides inhibit the formation of beta-tubulin polymers, which are essential in the process of cell nuclear division.
Although not technically fungicides, some newer products kill fungi indirectly by forcing the host plant to develop a systemic acquired resistance (SAR) against fungal infections. These SAR inducers trigger certain biochemical pathways that activate the plant’s natural defence mechanisms, such as the production of antifungal proteins.
Types of fungicides
Fungicides can be classified according to how they interact with a fungal infection and the plant itself. The three main categories are: contact fungicides, translaminar fungicides, and systemic fungicides.
1. Contact fungicides
As contact fungicides aren’t absorbed by plants, they only offer protection against fungus on the surface that’s been sprayed. They’re also known as protective or preventive fungicides. Contact fungicides contain dimethyl benzyl ammonium chlorides and dimethyl ethylbenzyl ammonium chlorides as active ingredients.
2. Translaminar fungicides
Translaminar fungicides can spread from the upper sprayed surface of a plant’s leaves to the lower, unsprayed surface. After being absorbed by the leaves, the translaminar flow enables the fungicide to move from one surface to another.
3. Systemic fungicides
Much like translaminar fungicides, systemic fungicides are absorbed by plants and then redistributed to other parts. However, this process isn’t limited by the thickness of the leaves. Instead, systemic fungicides are redistributed through the xylem vessels of the plants. Triadimefon, which has the chemical formula C14H16ClN3O2, is an example of an active ingredient in systemic fungicides.
How to use fungicide
Fungicides are generally sprayed on plants to either prevent or kill a fungal infection. However, before you use any type of fungicide, there are a few safety precautions and best practices to consider. 
For example, if you spot the early signs of an infection in one or more plants, take preventative action and apply a fungicide to the affected plants and those nearby. In most cases, the earlier you tackle the problem, the easier it will be to get the infection under control.
Another strategy is to apply fungicide during weather that’s conducive to fungi growth, such as during the rainy season. For optimum results, consider respraying the plants every seven or 14 days. It’s also a good idea to alternate the type of fungicide you’re using; otherwise there’s a risk the fungi may become resistant to the treatment.
And of course, when handling any kind of chemical, it’s essential to wear the appropriate protective gear and follow the instructions on the product label. 
Is fungicide safe?
Fungicides are generally safe to use and cases of acute toxicity to humans are very low. That being said, some fungicides can irritate your skin, eyes, and throat. There’s also evidence to suggest that long-term exposure to fungicides can lead to serious health problems. Some active ingredients, like pentachloronitrobenzene, can trigger allergic reactions, too.
Is fungicide harmful to humans?
As fungicides are designed to kill fungi, the harmful effects on humans are relatively mild. Although the biological differences between humans and fungi mean the risk of acute toxicity is generally low, fungicides aren’t risk-free.
Large amounts of fungicides can affect certain metabolic pathways in our bodies, especially if you’ve been exposed to fungicides over a prolonged period. As we’ve already discussed, chronic exposure can have adverse effects on human health.
Are fungicides bad for the environment?
Like any chemical that doesn’t occur naturally in an ecosystem, fungicides can be harmful to the environment. If fungicides accumulate in the ground, the soil can become toxic to many helpful microorganisms.
A build-up of fungicides can also disrupt the normal biological processes of the organisms that are responsible for maintaining the nutrients in the soil. Consequently, the soil may eventually become unsuitable for plant growth.
Summary
Fungicides are chemicals that are designed to protect plants by killing fungi or helping to prevent them growing. They’re broadly classified into three main types – contact, translaminar, and systemic fungicides. Although fungicides are generally safe to use, large amounts of these chemicals may pose a risk to humans and the environment.
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