What is Botrytis or Gray Mold?

What is Botrytis?

Botrytis Cinerea
Botrytis Cinerea

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 or Gray Mold
Botrytis or Gray Mold

Botrytis Cinerea is a fungus that colonises 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 Autumm most nights are humid and cold, what greatly favours the appearance of fungal attacks.

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

 Which are the symptoms of Gray Mold?

Marijuana bud attacked by Botrytis
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 colour and texture of the plant. Leaves develop necrosis and dry out very fast. Stems become fragile, brown and ulcerated.

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?

Gray Mold is clearly visible
Gray Mold is clearly visible

First, we must cut off any affected part of the plant so that healty 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 Botritys.

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 - benefitial bacteria to combat Gray Mold, such as Trichoderma Harzianum, Ulocladium Atrum or Gliocladium Roseum.

How to reduce fungal attacks?

Cannabis plant infected by Botrytis
Cannabis plant infected by Botrytis

Most fungus, 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.

We should also leave enough space between plants to promote better air circulation, so that humidity doesn't remain inside our growing space.

Keeping the space clean is also important to prevent any kind of pest or disease. We should clean it often and remove any decaying plant debris.

If the plant has cuts (from pruning, for example) it will be more sensitive to fungal attacks, so keeping our plants clean and healty is very important for Botrytis prevention.

Botrytis Cinerea life cycle
Botrytis Cinerea life cycle

It is also preferable watering our plants during the first hours of light, since watering them at night raises too much the humidity level.

At the end of the floweirng phase, reduce the amount of water in each irrigation, since too much water can affect the flowers, which could rot. During this stage, wait until the substrate is completely dried before watering again.

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

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 placing them on 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?

The top part of this bud has been attacked by gray mold
The top part of this bud has been attacked by gray mold

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.

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 Philo Skunk / Gokunk from Philosopher Seeds.

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

Botrytis and noble wine

It is also interesting pointing 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 connoiseurs. The low yields of this type of crop explain the high price of these wines.

Noble late harvest wine
Noble late harvest wine

Rotten grape for noble wine
Rotten grape for noble wine

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?” (4)


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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