What is Botrytis or Gray Mold?

What is Botrytis?

Botrytis or Gray Mold is a pathogenic fungus (necrotrophic) that affects more than 220 plant species, including marijuana. Actually, it is probably the most common fungus in cannabis crops, either indoors, outdoors, or in greenhouses, and it usually affects plants during their latest stage of flowering, without forgetting the drying process.

The most important factor regarding the appearance of Botrytis is the humidity level, which is essential for its growth. The higher the environmental humidity level, the more chances for our plants to be attacked by fungal diseases and pests.

The ideal temperature for botrytis growth is between 17ºC and 25ºC, though it can grow with higher temperatures. A great difference between day and night temperatures can also stimulate the appearance of Gray Mold, for humidity levels raise when temperatures drastically drop at the end of the day.

Botrytis Cinerea
Botrytis Cinerea

Botrytis Cinerea is a fungus that colonizes healthy plant tissues (parasitism), infected tissues (opportunism), and also dead ones (saprotrophic nutrition).

Outdoors, Botrytis attacks can be devastating, especially in crops that may take a little longer to flower, as well as in those plants that develop hard and dense buds. During autumn most nights are humid and cold, which greatly favors the appearance of fungal attacks.

Botrytis spores, dispersed by the wind, can remain latent for years in the environment until climate conditions are favorable for their growth.

 What are the symptoms of Gray Mold?

Marijuana bud attacked by Botrytis

Any part of the plant can be attacked by Botrytis: roots, stems, leaves, etc. The first visible symptom is a change in the color and texture of the plant. Leaves develop necrosis and dry out very fast. Stems become fragile, brown, and ulcerated.

Infection of botrytis in cannabis bud
Infection of botrytis in cannabis bud

But the most common place to find this fungus is in the buds. They become pale grey, dry out, and develop a cotton-like substance in the interior part.

How to combat Botrytis attacks?

First, we must cut off any affected part of the plant so that healthy parts don't get infected. Then, we should ventilate the cannabis growing space so the humidity level decreases to the desired value. Carefully checking the plants ensures that we'll notice any other attack until harvest time.

Infected parts should never be consumed since they can easily cause a lung infection.

After harvesting our plants, we thoroughly clean the growing space, pots, etc. with bleach to eliminate any traces of Botrytis.

Any infected part should be carefully removed
Any infected part should be carefully removed

There are several fungicides to treat Botrytis on the market; one of the best ones is, doubtless, the Botryprot fungicide, 100% natural and highly efficient.

We can also use - sprayed on the plants - beneficial bacteria to combat Gray Mold, such as Trichoderma Harzianum, Ulocladium Atrum, or Gliocladium Roseum.

How to reduce fungal attacks?

Most fungi, and of course Gray Mold, grow better with high humidity levels, so keeping the humidity below 50% during the whole flowering period highly reduces the chances for our plants of being attacked by Botrytis.

To do so, we can either improve our ventilation system (air extractor, interior fans, etc.) and/or use a dehumidifier.

Cannabis flower infected by botrytis
Cannabis flower infected by botrytis

Enough space should be left between the plants so that air can circulate easily and humidity does not remain stagnant in the grow tent. Pruning low branches and shoots will greatly facilitate air circulation between the plants.

Lack of hygiene will cause plants to become infected by fungi. It is very important to frequently clean the crop , and not leave organic or decomposing plant matter. Furthermore, it is essential not to touch the buds!

If the plant has wounds, the fungus will attack it more easily. Keeping plants in perfect health will help reduce the risk of contracting botrytis. Wilted leaves can be removed to prevent the possible appearance of the fungus on the plant. It is very important not to leave dead matter such as pieces of branches or parts of leaves, because Bortytis will take advantage of this entrance door to attack the plant.


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At the end of flowering we will take into account irrigation. Excessive watering would be very dangerous and could affect the buds, causing them to rot. We will wait until the soil is dry and the pot is very light before watering again. It is preferable to water in the morning; Doing so in the afternoon would increase the humidity too much during the night.

Excess Nitrogen at the end of the flowering period can also favor the appearance of Botrytis, although it is a rare case because we usually don't give nitrogen to our plants at this stage.

Life cycle of botrytis cinerea
Life cycle of botrytis cinerea

Since Botrytis usually attacks the biggest buds, pruning the tops of the plants can help, for a plant with multiple tops usually develops slightly smaller buds (where Botrytis doesn't grow so easily).

Pruning the lower part of the plants is also advisable since it is where we find the higher humidity level. Outdoors, avoid placing your plants in humid zones, it is always better to place them in the most sunny space.

Harvest the plants when they need water to reduce rot and fungal issues. Dry the plants in a well-ventilated space, but do not place the fans directly on the plants, or the drying process will be too fast, reducing the organoleptic properties of the flowers.

Which are the most resistant strains to Botrytis?

From a general point of view, mostly Indica strains develop harder and more compact buds, thus being more sensitive to fungal attacks than Sativa flowers, which develop less compact and dense, allowing better air circulation and retaining less moisture.

Remove any infected part before drying the plants
Remove any infected part before drying the plants

However, we can find a few exceptions among Indica plants, like the White Domina marijuana or Pakistan Chitral Kush.

Here you have a list of marijuana strains resistant to fungus , like Green Poison, High Level, or Tropicanna Auto XXL from Philosopher Seeds.

In a future article, we'll teach you a way of making a completely clean resin extraction using buds infected by Botrytis.

Botrytis and noble wine

It is also interesting to point out that Botrytis Cinerea is used by wine producers for their noble late-harvest wines. Sauternes, Barsac, or Montbazillac are only a few examples of wines produced thanks to the action of this fungus on the bunches of grapes. It increases the levels of sugar, developing rich and complex aromas highly appreciated by connoisseurs. The low yields of this type of crop explain the high price of these wines.

Rotten grape for noble wine
Rotten grape for noble wine

Happy harvest!

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

Comments in “What is Botrytis or Gray Mold?” (5)


Samuel Is an Alchimia client 2024-01-30
Does the lighting I use make a difference? I've always used HPS for growing, and would get the odd bit of mold, I'm now using Spiderfarm LEDS with photoperiod strains, but I think I'm getting more mold? My yields keep getting higher & higher, but during trimming the last batch I had to bin 4-5 dry ounces because the main colas where black in places. I also live in a very cold & wet Ireland in winter, and crazy humidity during summer months. I've been growing for 10+ years and its starting to become a problem. I've read that bud rot is the curse of a successful grower? Or I s that nonsense? Love the site. Thank you all.


Sir Dankamuss 2021-08-24
How does the Botryprot fungicide work? How exactly is it applied and when? Thank you.

Alchimia Staff

Tim Alchimia 2021-08-24
Hi and thanks for your comment. The following information has been adapted from the Botryprot manufacturer's website: Foliar application: depending on the intensity of the attack, from 2 to 2.5 ml per L every 10 to 15 days. The application will need to be repeated if it rains the day after spraying. Use water with pH of 5.5 – 6.5. ph to mix. Wet the entire plant thoroughly, including flowers (soak them), when applying BOTRYPROT, ensuring that the product penetrates inside. Apply during the hours of low light, evening or morning. I hope that helps! Best wishes and happy growing!


Mel 2020-10-17
1. Is it safe to smoke moldy marijuana? 2. Once the mold appears, is there any way to get rid of the mold? I live in western Washington near the coast and my plants are outside...so humidity control is impossible. Thanks in advance

Alchimia Staff

Tim Alchimia 2020-10-19
Hi Mel, thanks for your comment and question. As we state in the article: "Infected parts should never be consumed since they can easily cause a lung infection". If you can remove the affected parts without infecting the rest of the flower, then there shouldn't be a problem. As for preventing and treating the fungal attack, aside from the measures mentioned in the article, there's not a great deal else you can do apart from keeping the plants healthy and not over-feeding them as this is often a contributing factor. Of course, growing outdoors means that you can't adjust humidity levels, but you can ensure that your grow area has good air movement so that humidity won't gather there, don't plant too close together and also choose genetics that are proven in high-humidity areas and that resist Botrytis well. I hope that helps. Best wishes and happy growing!


diver99 2017-09-04
Thanx Tim....I'm a novice grower...I need all the advice I can get, thanx for the reply...diver99


diver99 2017-08-20
Is there an odor with the appearance of gray mold?

Alchimia Staff

Tim Alchimia 2017-08-29
Hi diver99, In my experience botrytis, budrot or grey mould doesn't have a particularly noticeable smell, not strong enough to notice over the aromas of the flowers themselves unless the problem is really bad and you've got your nose right on the mouldy spot. There can be a faint mustiness or a barely-noticeable sour odour, like damp wood but in general it's not a particularly smelly fungi.

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