Five types of fungicide with different modes of action that you should be able to identify from their label

Fungicides have been used in agriculture since ancient times. Greeks and Romans, for instance, used sulphur, an element that is still being used as an effective fungicide in many fungal disease control programmes.

But the starting point of chemotherapy applied to plant pathogens was in the early 19th century, when some important inorganic compounds like copper, mercury, antimony and arsenic started being used as fungicides. This marked the first round of the fight against fungi as we know it today.

Together with copper, lime sulphur has been a widely used fungicide since the first decades of the 19th century. From 1880, Bordeaux mixture, a potent fungicide resulting from a mix of copper sulphate, hydrated lime and water, became an effective treatment in the south of France for the control of mildew, a disease that seriously threatened the vineyards. Up until that time, only inorganic fungicides had been in use.

The chemical revolution in agriculture started with the introduction of the first organic fungicides, which were compounds in which carbon is the main component and plays a crucial role in their composition. From then on, the industry developed more and more effective fungicides for crop disease control.

The arrival of systemic fungicides, with a pathogen-eradicating action even after the onset of infection, represented a revolution in the world of fungicides that is still in use today. But these aren't the only fungicides that exist. Here we explain the differences between the most common types that you can currently find in most reliable grow shops:

Protectant fungicides

Also known as 'contact' fungicides, these are active on the surface of the plants, where they create a chemical barrier between the plant and the fungus, thereby preventing its spread. They're applied before the spores of the fungus appear and must be used before the onset of the infection, with repeated applications if conditions are favourable for the development of the disease. Rain can wash the anti-fungal spray away, which means that the plants can become unprotected again.

Penetrant fungicides

Also known as 'site-specific', these fungicides are absorbed by the plants after application and have both protective and eradicating functions: protective from the initial point of contact; and eradicating once they're taken up by the plants. These fungicides are usually regarded as systemic, but it must be noted that there are different systemic degrees depending on the distribution of the fungicide throughout the tissue of the plant: translaminar, xylem-mobile, and truly systemic.

Translaminar fungicides

Translaminar fungicides redistribute from the upper side of the leaves to their unsprayed underside. Spray coverage needs to be uniform for the fungicide to be highly effective. These fungicides, like the protectant type, are more likely to be washed away by the rain but are capable of simultaneously preventing spore germination and fungus development. Their healing action is normally limited to 24-72 hours after infection, and they're not effective once symptoms become visible.

Xylem-mobile fungicides

Once they've been absorbed by the plants, these fungicides move upward through the xylem (the plant tissue formed by lignified cells that transport the sap) towards the leaves. This movement goes from the base to the top of the plants.

Systemic fungicides

These fungicides are also known as 'eradicating' and can be used in plants that are already infected by fungi. They're a quick and coordinated response against the pathogens as the signal circulates through the plant. They move in both directions, i.e. upwards through the xylem and downwards through the phloem (the nutrient-conducting tissue). These fungicides are called 'truly systemic' and are one of the most commonly used types in marijuana cultivation as they're absorbed through both the leaves and the roots, and they move through the whole plant when the infection has been detected.