Botrytis in Cannabis

Why gray mold is so dangerous for cannabis flowers, how bud rot develops, and why climate, airflow, hygiene, and strain selection are the most important preventive measures.

Botrytis cinerea, also known as gray mold, is one of the most important pathogens of bud rot in cannabis. Official plant disease fact sheets from Canada explicitly describe Botrytis as the primary cause of flower rot in cannabis crops in British Columbia and other regions of Canada and the USA.

The fungus is particularly problematic during flowering because it can render entire inflorescences unusable and spreads rapidly under favorable conditions. A particularly insidious aspect is that the infection often begins inside dense buds before becoming clearly visible from the outside.

What is Botrytis anyway?

Botrytis is a widespread fungal pathogen that can infect a vast range of cultivated plants. In cannabis, it primarily causes bud rot—rot in the flower clusters—but can also damage stems and nodes.

The fungus is especially dangerous because it finds ideal conditions in places growers often struggle to check: inside dense flowers. There, humidity is higher, airflow is lower, and moisture can remain for longer periods.

From this internal area, the infection can work its way outward. The flower may then look relatively inconspicuous on the outside, even though the inside is already soft, discolored, or damaged.

Why Botrytis is so hard to detect early

Especially with dense flowers, Botrytis is often only noticed late. Official descriptions of cannabis bud rot list soft and discolored buds as early signs, followed by brown, dry, or crispy flower parts and, under high humidity, gray to grayish-white fungal structures.

The problem: These signs often only become clear once the infection has already progressed. Because Botrytis can start inside compact flowers, a superficial look at the plant is often insufficient.

Individual dead sugar leaves or brown spots in the middle of otherwise healthy-looking flowers are particularly tell-tale signs. Such small warning signals should always be taken seriously.

Typical signs of Botrytis in cannabis

Botrytis can look different depending on the stage. Early symptoms are often non-specific; later symptoms are clearly more noticeable.

These symptoms match the official descriptions for bud rot on cannabis. It is important not to wait until there is widespread visible mold, but to check more closely at the first signs of unusual discoloration and dead flower parts.

Which conditions favor Botrytis

Botrytis loves humidity, surface moisture, and poor airflow. Long phases where plant tissue remains damp or where there is very high relative humidity in the grow space are particularly critical.

For gray mold, it is generally described that surface moisture on plant tissue lasting several hours, very high humidity in the grow area, and cooler conditions can promote infection. Cannabis-related literature also emphasizes that Botrytis can rapidly destroy inflorescences under relative humidity conditions exceeding 70 percent.

For the grower, this means: It's not just the ambient room reading that counts, but also the microclimate directly inside and between the flowers.

Why dense buds are particularly at risk

A central problem with cannabis is the microclimate in compact flowers. Recent work on the epidemiology of bud rot shows that temperature and humidity inside older inflorescences can be higher than in the ambient air.

This explains why compact, dense flowers are particularly vulnerable when airflow is poor: the climate might seem acceptable on the outside, but fungus-friendly conditions are already prevailing inside.

This is precisely where strain selection becomes relevant. Flower structure, density, stretch, and overall growth habit contribute to how quickly damp zones form in the canopy and how demanding the climate management must be during late flowering.

At Cannaseuse, you can filter genetics not just by aroma or type, but also by practical profile characteristics such as growth structure, stretch, flowering time, and grower level.

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Further risk factors

Besides high humidity, injuries and unclean work processes also play a role. Botrytis can more easily colonize injured spots on stems, nodes, and flowers.

Spores can also be spread via airflow, plant debris, clothing, tools, or equipment. For the grower, this means: Climate alone is not enough. Hygiene and clean handling are also critical.

Prevention – the most important protection against bud rot

With Botrytis, prevention is significantly more effective than late reaction. The most important lever is a dry, well-ventilated flowering climate.

Humidity should remain low enough during the flowering stage to prevent condensation or persistently damp areas within the crop. Additionally, good air circulation helps, as it removes moisture from the canopy and reduces the formation of fungal-friendly microclimates.

Especially in the late flowering stage, it is not just exhaust air that counts, but also air movement within the crop: between the plants, below the canopies, and around dense flower areas.

These points follow directly from the described epidemiology and climate factors of Botrytis in cannabis and other greenhouse crops.

What to do if Botrytis is already present?

When Botrytis becomes visible, quick and consistent action is important. Affected flowers must be removed generously, because the visible infestation often shows only part of the actual damage.

Subsequently, the climate should be corrected immediately: lower humidity, increase air movement, and eliminate damp zones. Furthermore, tools should be cleaned and disinfected after cutting to prevent spreading.

This approach follows directly from the described spread via infected plant material, air humidity, and tools.

Is affected material still usable?

One must be clear here: moldy cannabis should not be consumed.

Current reviews on fungi and mycotoxins on cannabis and hemp flowers describe how fungi impair product quality, can form potentially toxic secondary metabolites, and that spores after inhalation can pose health risks, including lung infections, especially in immunocompromised individuals.

Botrytis cinerea is listed in such contexts as a relevant cannabis flower fungus. Therefore, if mold is visible, there is no sensible perspective for recovery; consistent sorting out is required.

Why Botrytis remains an issue even after harvest

Botrytis is not just a field or flowering problem, but also a post-harvest risk. If infected or overly damp material enters the drying or storage process, the problem is merely deferred.

Therefore, clean drying, controlled humidity, and careful visual inspection after harvest are important. Poorly controlled post-harvest conditions can severely degrade product quality and increase microbiological risks.

For practical grow logic, this often means: more compact, predictable, or easier-to-manage crops are usually easier to control in risk management than very dense, heavily shading canopies.

FAQ – Frequently asked questions about Botrytis in cannabis

Conclusion

Botrytis is one of the most serious opponents in cannabis flowering because the fungus can begin hidden inside the buds, spread quickly under damp conditions, and make entire inflorescences unusable.

The best strategy is therefore prevention: stable flowering climate, good air circulation, hygiene, regular inspection, and genetics that fit the space and the risk profile.

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