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Summer is coming with its heat and humidity…and fungus, which thrives given the right combination of these factors, threatens to affect our marijuana crop. So, although it’s more difficult to control temperature and humidity in our outdoor grow space, let’s review why we get fungus and how we can treat it.


Plaga de Hongos en tu cultivo cannábico

Fotograph from Cannabislandia

Fungus, the microscopic plague: Botrytis, Powdery Mildew, Mildew and Fusarium mold


There are many types of fungus, but we’re going to focus on those that we see most frequently in order to learn how to detect them and what to do in order to stop them and eliminate them from our marijuana crop. In contrast with other plagues, fungi are microscopic and we are only able to detect their presence once we see the first signs of damage that they are causing.




Botrytis is the most common fungal plague that you can find in a marijuana crop. It proliferates in temperatures between 15 and 25ºC, low ventilation, and concentrated humidity.

Drastic changes in temperature between night and day, for example, easily generate this type of humidity. The fungus spreads very rapidly, in days and even hours. Pay a great deal of attention to this one, because it’s a real shame to lose months of work on your crop in one single moment, above all at this time of year.



Bostrytis is very easy to detect quickly since it appears in the form of a gray mold. In fact, this fungus has also been commonly called just that.

You’ll see necrosis on stems and leaves. The tissue of the leaf changes color and texture. It oxidizes, dries out and degrades and you’ll also notice brown and grayish marks, as well as typical moldy hairiness. It is found, above all, on the buds, as it dries them out, makes them look grainy, and creates a cotton like substance inside the bud.


How to fight it

Humidity is the principal cause that fuels this plague, so we need to ventilate the grow area well in order to reduce humidity. If you have had caterpillars, you will surely have botrytis, since the tiny excrement that the caterpillars leave behind can give way to this type of fungus. After getting rid of the caterpillars, pay attention to the signs and try to avoid it at all costs.

If your cannabis plant is suffering from botrytis, the first thing that you have to to is cut all of the infected area off, as well as a little beyond in case there are microscopic infected particles that have already spread. After trimming the plant, disinfect the grow area well as well as the tools that may have contacted the plant in order to avoid traces.

We even recommend that you prune the plants a bit more, to help avoid accumulated humidity and so that air is able to blow through the plant. You may try to use fungicides though we can’t guarantee that it will eliminate the fungus, in addition to the fact that you will be leaving behind chemical traces. You can also try to use organic fungicides like propolis or horsetail in order to prevent it.


Botrytis en los cogollos de marihuana

Photograph from Greencultured





This fungus is also easy to detect. It creates a gray or whitish powdery layer, like ash. You will see gray or white marks that start to appear on the leaves and if you don’t stop it, it will continue towards the stems and buds. When you remove the dust, you can see that the part under it has been turned yellow, since it interrupts the plant’s photosynthesis process.


How to fight it

Powdery mildew appears at 20ºC, high humidity and drastic temperature changes, so we can play with these values in order to keep it at bay: lower the temperature, if you have an interior crop, reduce the humidity, ventilate the space, etc. One of the biological components that works for this pathogenic fungus is Trichoderma Harzianum, which is often used for planting specifically to prevent this type of fungus. On a preventative level, we have other biological products like horsetail or propolis.

Nevertheless, if we are already being plagued and there is no other way to eliminate it, as long as we are within the safe period in terms of which phase our plant is in, we can also decide to spray chemical fungicides that contain elements like Cyproconazole, Tetraconazole or Triadimenol, which tend to be very effective against Powdery Mildew.


Oídio en hoja de cannabis

Photograph from CultivandoCannabis





Mildew is much like Powdery Mildew and they can be easily confused. Instead of appearing on top of the leaf like Powdery Mildew, Mildew tends to come from the bottom of the leave, although you can surely also see its signs on top of the leaf where yellowish and translucent marks that resemble oil appear.



The symptoms are greasy marks that can be seen on the borders of the marijuana leaf, which change from yellow to brown, dry the leaf, and then rot it. Below the leave is where the white powder that is similar to that of Powdery Mildew can appear.


How to fight it

Just as with the previous fungus plagues that we’ve sited, this one is also caused by humidity, and moderate temperatures between 15 and 25ºC. Mildew can infiltrate into cuts in the plant caused through watering and appear after rain. Balancing these factors will help us to prevent it, as will the use of propolis, horsetail and Bordeaux mixture.

El hongo Mildiu en hojas de marihuana

Photograph from Fumatinga





Fusarium is a fungus that lives in the soil. It blocks the plants access to lifeblood and nutrients. This makes it such that the marijuana plant shrivels and rots.



You can see that the oldest leaves rot rapidly and stems rot from the base. You can also see that the inside of the stem takes on a brown and even reddish color.


How to fight it

Just like the other fungi, the 3 elements of temperature, humidity and ventilation are basic.

You can also try to have fewer hours of light. Beyond that, one of the most effective biological preventative measures is Trichoderma, which we should apply to the soil in the pot and then not avoid using any other chemicals to eliminate it.

Another thing that we should keep in mind is not to go overboard with fertilizers since excess nitrogen, for example, fuels Fusarium and makes it so hard to eliminate that we may even need to get rid of the whole plant if it becomes very infected. The worst thing about this fungus is that it rots the base of the stem which makes it very difficult to recuperate the plant.

As we mentioned before, it is also very important to clean the entire grow area and the tools that have been in contact with the fungus in order to keep from spreading it to the rest of the crop.

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