Botrytis in cannabis is a fungal disease capable of damaging flowers, leaves and stems within a short period of time. It can also continue developing during drying when humidity is high. Learning to recognise the first symptoms and maintaining a stable environment helps prevent a small affected area from eventually compromising a large part of the crop.
In this Hydroponics Blanes guide, we explain how to identify Botrytis in cannabis, which conditions encourage the fungus to appear, how to reduce the risks and which measures should be taken when an infection occurs. The blog content is for informational purposes only. At Hydroponics Blanes, we DO NOT sell marijuana containing THC. We sell fertilisers, nutrients, indoor growing lighting systems, fans, extractors, climate controllers, dehumidifiers and other equipment intended for home growing permitted by current regulations.
What is Botrytis in cannabis?
Botrytis is a disease caused mainly by the fungus Botrytis cinerea. It is commonly known as grey rot or grey mould in cannabis because of the greyish, velvety layer that appears on affected tissues when the infection reaches an advanced stage.
It is a necrotrophic fungus, which means that it can destroy plant cells in order to feed on damaged tissues. It finds favourable conditions in dry leaves, damaged flowers, pruning residues, wounds on branches and plant areas exposed to prolonged moisture accumulation.
Spores can spread through the air, hands, clothing, tools, insects or contact between plants. They can also remain in plant debris from previous crops. For this reason, cleaning the growing area is just as important as controlling temperature and humidity.
Why does Botrytis appear in indoor cultivation?
Botrytis in indoor cultivation appears when several conditions occur together: available moisture, vulnerable tissues, moderate temperatures and poor air circulation. The problem usually increases during flowering because dense flowers retain moisture inside and take longer to dry.
The most common causes are:
- High relative humidity for several hours.
- Insufficient extraction during the dark period.
- Limited ventilation between branches and flowers.
- Plants positioned too close together inside the grow tent.
- Rapid temperature drops when the lighting switches off.
- Condensation on leaves, walls or flowers.
- Excessive watering or watering carried out at the end of the lighting period.
- Dry leaf residues inside the flowers.
- Damage caused by pruning, broken branches or insects.
- Tools not disinfected between plants.
The relationship between humidity and Botrytis during flowering requires particular attention. The hygrometer may show an apparently correct value while the inside of a compact flower retains much higher humidity. Areas where the air becomes stagnant may also show different values from the rest of the room.
How to identify Botrytis in cannabis
Knowing how to identify Botrytis in cannabis makes it possible to act before the spores reach other branches. The infection does not always begin with a visible grey layer. It often starts inside a flower and silently progresses outwards.
Early symptoms of Botrytis
One of the first signs may be a small leaf that suddenly dries out inside a flower. The leaf may turn brown, curl or detach too easily. When the internal area is inspected, dark, soft or decomposing tissues may be visible.
Common symptoms include:
- Small brown or greyish areas.
- Dry inner leaves without an apparent cause.
- Soft, damp or brittle tissues.
- Flowers that lose firmness when touched.
- White, grey or brown fuzz on the damaged area.
- Stems with dark marks around wounds.
- Branches that suddenly begin to wilt.
- A damp smell or an odour similar to decomposing plant matter.
As the fungus progresses, the tissues darken and may soften due to the death of plant cells. A grey sporulation layer then appears, creating the characteristic appearance of grey mould in cannabis.
How to recognise Botrytis on buds
Botrytis on buds usually begins inside the most compact flowers. From the outside, the bud may look healthy, but a dry leaf or a small brown area may indicate that the fungus is already developing internally.
The inspection should be carried out carefully, without shaking the plant or blowing on the suspicious area. Rough handling can release spores and carry them towards healthy flowers.
Particular attention should be paid to the following:
- Very dense central buds.
- Flowers close to walls or corners.
- Areas with poor air circulation.
- Branches damaged by weight.
- Flowers previously affected by caterpillars.
- Plants exposed to rain, fog or dew.
Differences between Botrytis and powdery mildew
Powdery mildew usually appears as a superficial white powder on leaves and stems. Botrytis causes rot, loss of firmness and darkening of the tissues. At an advanced stage, it forms a greyish or brown mass with a cotton-like appearance.
A flower affected by Botrytis should not be consumed, vaporised or used for extractions. Removing only the visible part does not guarantee that the remaining material is free from mycelium or spores.
Development cycle of Botrytis cinerea
The fungus can remain present in leaf debris, stems, dried flowers and abandoned plant material. When temperature and humidity increase, the mycelium begins to develop and forms structures capable of producing spores.
Spores mainly travel through the air. A fan directed straight at an infected area can disperse them throughout the space. They can also be carried by scissors, gloves, clothing, insects or water droplets.
When a spore reaches a damp plant surface, it may germinate and penetrate through a wound, an ageing leaf or a weakened area. The risk increases when tissues remain wet for several hours.
Damage caused by Botrytis
Botrytis can appear at different stages of cultivation. In young plants, it may contribute to collar rot or damping off. The stem becomes thinner close to the substrate, weakens and eventually bends over.
In adult plants, it can establish itself in pruning wounds, broken branches or damaged stems. The affected area develops brown shades and may eventually interrupt water flow to an entire branch.
During flowering, the damage is generally concentrated on the flowers. The inner sections retain moisture, contain a large amount of plant matter and receive less ventilation. As the infection progresses, the flowers become soft, dark and brittle.
The problem can continue after harvesting. During drying, an infected flower may release spores and reach nearby flowers. An overcrowded, poorly ventilated room with high humidity creates a favourable environment for the infection to spread.
How to prevent Botrytis
The best strategy is to prevent the fungus from finding favourable conditions. Knowing how to prevent Botrytis involves controlling the climate, maintaining consistent hygiene and inspecting plants regularly.
Controlling humidity during flowering
Humidity should be measured both when the lighting is on and during the dark period. A thermo-hygrometer with maximum and minimum value memory helps detect night-time peaks that might otherwise go unnoticed.
During the final stage of flowering, it is generally advisable to maintain relative humidity between 45% and 55%, while always avoiding condensation and prolonged periods above this range. Each space requires specific adjustment depending on temperature, plant density, extraction power and flower size.
When extraction is not enough to control the environment, a dehumidifier can help remove excess moisture from the air. The appliance should be selected according to the volume of the room and the amount of humidity generated by the plants.
Improving extraction and ventilation
Extraction renews the air and removes some of the accumulated moisture. The extractor should also remain active during the dark period, especially during the advanced flowering stage.
Inside the tent or growing room, there should be a light and evenly distributed breeze. Air should circulate below and above the plant canopy without being constantly directed towards the same flower or causing the branches to move violently.
A good setup can combine:
- An extractor suitable for the volume of the space.
- Sufficient air intake.
- Oscillating fans positioned at different heights.
- Adequate spacing between pots.
- Free space between plants and walls.
Avoiding sudden temperature changes
When the lighting switches off, the temperature can drop rapidly. As the air cools, relative humidity increases and condensation may form on leaves and flowers.
To reduce this problem, it is advisable to keep extraction active, avoid watering immediately before the lights switch off and monitor the difference between daytime and night-time temperatures.
Maintaining consistent hygiene
Dry leaves, pruning residues and fallen flowers should be removed immediately. Botrytis can develop on dead plant matter and use it as a starting point to reach nearby tissues.
Scissors should be cleaned before and after each use. When a tool comes into contact with an affected area, it should not be used on another plant without prior disinfection.
It is also advisable to clean:
- Pots and trays.
- Floors and walls.
- Ventilation ducts.
- Supports and nets.
- Reservoirs and watering accessories.
- Filters and air intake grilles.
Inspecting plants regularly
A quick daily inspection helps identify dry inner leaves, small marks and damaged branches. During rainy or humid periods, checks should be more frequent.
Outdoors, it is advisable to inspect plants after storms, periods of heavy dew or several cloudy days. Compact flowers, shaded areas and damaged branches deserve particular attention.
Products for preventing Botrytis
Products for preventing Botrytis can support a preventive strategy, but they do not replace humidity control, ventilation or cleaning. A biological product will have little effect if the space continues to experience condensation and stagnant air.
Quimera L by Mycoterra
Quimera L by Mycoterra is a liquid inoculant formulated with Trichoderma harzianum. It supports microbiological balance around the roots and helps create a biological barrier against certain harmful fungi.
On growshopweb.com, it is available in 75 ml and 300 ml sizes. It should be applied preventively, always following the dosage indicated by Mycoterra.
33PRO Trichoderma Mycorrhizae
33PRO Trichoderma Mycorrhizae combines mycorrhizae and Trichoderma harzianum. The mixture supports root development, improves the absorption of water and nutrients and helps maintain a more balanced biological environment against fungi such as Botrytis, Fusarium and Alternaria.
Hydroponics Blanes offers different sizes adapted to the number of plants and the volume of the growing area. It is advisable to apply the product at the beginning of the cycle or during transplanting, following the instructions provided by 33PRO.
Other options include:
Dehumidifiers, extractors and fans
In many growing spaces, climate control equipment represents the main products for preventing Botrytis. A properly sized dehumidifier, a suitable extractor and several well-positioned fans reduce the conditions required for the fungus to develop.
At Hydroponics Blanes, you will find:
- Dehumidifiers for grow tents and growing rooms.
- Can-Fan extractors with different airflow capacities.
- Cyclone oscillating fans.
- CLI-MATE humidity controllers.
- Thermo-hygrometers with memory.
- Ventilation ducts and accessories.
Before purchasing equipment, it is important to calculate the volume of the space and monitor humidity levels for several days. Technical advice helps select suitable power and avoid equipment that is either insufficient or oversized.
How to eliminate Botrytis from cannabis
When an infection appears, many people want to know how to eliminate Botrytis from cannabis. Rotten tissues cannot be recovered. The intervention should focus on removing the affected section, limiting the spread and correcting the environmental conditions.
Steps for removing an infected area
- Switch off the fans before touching the plant.
- Prepare a sealable bag close to the affected area.
- Use clean gloves and disinfected scissors.
- Cut several centimetres below the visibly damaged section.
- Place the removed material directly inside the bag.
- Seal the bag before moving it through the room.
- Disinfect the scissors, gloves and surfaces used.
- Inspect nearby plants.
- Reduce humidity and increase air exchange.
Do not shake, blow on or suddenly open an infected flower. These actions can release a cloud of spores. The removed material should not be left in an open waste bin either.
What should be done during flowering?
During advanced flowering, applications that increase humidity inside the flowers should be avoided. Any treatment must be authorised for the intended use and applied in accordance with the label, dosage and corresponding safety interval.
The safest measure is to remove diseased sections, improve environmental conditions and inspect the remaining flowers daily. When several areas of the same plant show signs of rot, isolation should be considered to protect the rest of the crop.
What should be done during drying?
If Botrytis appears during drying, the affected flowers should be removed immediately. It is also advisable to inspect the parts that were in contact and increase the spacing between branches.
The drying room requires air exchange, darkness, a stable temperature and indirect air movement. The fan should not be pointed directly towards the flowers. Excessive airflow can quickly dry the outside while the inside remains damp.
Errors that encourage grey mould
Many cases of fungi in cannabis cultivation appear due to a combination of several small mistakes. The most common include an overcrowded grow tent, switching extraction off at night, excessive watering and relying too heavily on a single preventive fungicide treatment.
The following factors also increase the risk:
- Ignoring a dry leaf inside a flower.
- Reusing tools without cleaning them.
- Leaving plant debris on the floor.
- Measuring humidity at only one point.
- Positioning plants against the walls.
- Spraying dense flowers during the final stage.
- Delaying the removal of a brown or soft area.
Preventing Botrytis throughout the cultivation cycle
Prevention should begin before introducing the plants. Cleaning the room, checking the extractors, inspecting the fans, calibrating the sensors and preparing a dehumidifier when necessary helps reduce many later problems.
During vegetative growth, it is advisable to maintain an open plant structure and remove damaged leaves. During flowering, monitoring should be intensified, especially on compact flowers and in areas where air circulation is limited.
The most effective strategy against Botrytis in cannabis combines cleaning, climate control, extraction, ventilation, frequent inspections and biological prevention. Acting at the first symptom can limit a localised infection, whereas waiting several days makes it easier for the fungus to spread to other flowers.
At Hydroponics Blanes, you will find products for preventing Botrytis, beneficial microorganisms, dehumidifiers, extractors, fans, climate controllers, fertilisers, nutrients and indoor growing lighting systems. All products must be used in accordance with the manufacturer’s label and applicable regulations.