Systemic and contact fungicide: what they are and their differences

Fungi are threats that can pose serious risks to any crop, from grains such as soybeans to perennial crops such as coffee, citrus and other fruit trees. To combat this type of pest, it is important to know the difference between systemic and contact fungicides to understand when each needs to be used and the criteria for application.

Like any other type of pesticide, a fungicide must be applied responsibly and always following the guidelines of the responsible agronomist. It is up to the farmer to be well informed about the type of fungus they are facing and what are the most appropriate techniques and products for handling it.

This article will explain the differences between contact and systemic fungicide and when it is most advisable to apply each. Check it out!

What are contact fungicides?

Also called preventive or protective fungicides, contact fungicides are pesticides used to prevent crop contamination and combat species of fungi that act superficially.

In general, contact fungicide is used when the farmer already knows the real risk of infection due to the simultaneous occurrence of the three factors that make up the disease triangle: the presence of viable fungus inoculum, the culture being susceptible and the environmental conditions suitable for infection. As for the inoculum, it may be sufficient to observe the history of the presence of the fungus in the area, the risks of introduction from other areas or the presence of the disease in neighbouring regions. As for the favourable environmental conditions for most fungi, these are rainy weather and mild temperatures.

Contact fungicides remain on the plant surface, and it is common for the product to leave visible residues where they are applied. In many cases, it is appropriate to reapply the protective fungicide after a certain period of time, especially after rains that can wash away the product or when the plant develops, as the former application will not protect new leaves.

It is important to pay attention to the recommended doses: like other pests, fungi can develop resistance to the pesticide, compromising the product’s efficiency and making its use in future applications unfeasible. Therefore, it is necessary to follow the guidelines established by the responsible agronomist.

What are systemic fungicides?

While contact fungicides are deposited and remain on the outside of plant tissues, systemic fungicides act deeper, infiltrating leaves, stems and seeds and fighting fungi that are already attacking the inside of a plant. For this reason, systemic fungicides are also known as infiltration or absorption fungicides.

There are several categories of systemic fungicides, which are usually associated with the type of mobility of the product and the way it spreads through the plant — in addition to the fungi they target. The substance is expected to be rapidly assimilated and only affects the invader, leaving the host tissues intact.

In addition to eliminating the fungi that attack the inside of plants, systemic fungicides generate residues that are toxic to invaders when they degrade, which prevents new infections. Although generally used curatively, these fungicides also have a preventive property.

Systemic and contact fungicide: which one to choose?

Actually, the choice between a systemic and a contact fungicide is not exactly made by the farmer. To define which kind of product will be applied, it is essential to clearly understand the characteristics of the culture, climate, terrain and, of course, the fungi that will be targeted.

Despite being an entirely different category of living being, the Fungi Kingdom — the scientific classification of fungi, mushrooms and moulds — has some similarities with the Plantae Kingdom — to which all plants belong.

Fungi spread by spores, which are like seeds: once implanted in a place with favourable conditions, they develop and spread through the environment — ​​which can be another living being, such as a plant, an animal, or even other types of fungi. Even fungi growers, who grow edible mushrooms, need to be careful to avoid this kind of threat.

Regardless of whether they are contact or systemic, fungicides can operate in different ways. Some damage the cell membranes of fungi, deactivating enzymes and proteins essential for their development. Others inhibit the production of sterol or chitin, impairing the invader’s metabolism. There are still those that inhibit cell formation in cell division.

New types of fungicide act indirectly, inducing endangered plants to produce natural pesticides. However, this kind of product still has a limited application, so it is not used very often.

To define which fungicide to apply, it is necessary to understand the threat and select the most appropriate pesticide, which will be more effective in fighting the fungus. This does not mean that the producer has to wait for the pest to emerge to start acting. With knowledge of its culture, region and climate, it is possible to anticipate which species of fungus is more likely to proliferate in the field and already invest in protection.

At first, contact fungicides are the most suitable choice, as they form a chemical barrier that prevents plant contamination. However, as previously mentioned, the farmer must be careful not to use the wrong product, as many pesticides can be aggressive for certain crops.

This preventive use helps avoid many pests, but not all. When a plant has already been deeply contaminated, it is necessary to use a systemic fungicide with curative properties.

The product will act not only on the surface of the plant but also on its entire internal structure, fighting the fungus and often preventing its return — called retroactive capacity.

In some cases, it may be interesting to use both types of fungicide. In soybean crops, for example, it is common to apply contact fungicides throughout the plant’s development to ensure its healthy growth. When the time of harvest approaches and there are ideal climatic conditions for fungi, such as high humidity and high temperature, it is recommended to treat the crop with systemic fungicides, eliminating threats that are already deeply implanted.

As in any type of phytosanitary management with agricultural pesticides, it is necessary to have in-depth knowledge of the land, the culture, the fungi and the products available to deal with fungicides. A good tip is to get information from specialist agronomists and product suppliers.

How do you use systemic and contact fungicide in your crop? Leave a comment and join the conversation!