Eliminate Botrytis (bud rot, gray mold) on cannabis buds: The best ways to stop gray mold fast and prevent it from spreading.
What is gray mold (Botrytis cinerea)?
Botrytis blight, also known as bud rot or gray mold, is a fungal disease that affects more than 200 plant species. In cannabis, it causes a soft rot that damages buds both during growth and after harvest.
The causative agent of gray mold is the asexual fungus Botrytis cinerea, whose name roughly translates to “grape disease” in Greek.
Botrytis cinerea is an airborne plant pathogen with a necrotrophic lifestyle—germinating on damp cannabis tissue, secreting cell wall-degrading enzymes, and then feeding on the dead material. Its ability to induce programmed cell death plays a key role in the fungus’s success.[1]
What does gray mold look like?
Gray mold appears as a thick carpet of velvety spore mass. Affected areas of the plant develop a fuzzy web of gray-blue mycelium, visible to the naked eye. When disturbed, a cloud of dusty conidia is released into the air in search of its next host.
As bud rot progresses, the damaged, water-soaked plant tissue collapses into a black mush.
Where is gray mold found on cannabis?
Gray mold occurs in both growing plants and harvested marijuana buds. The disease often starts inside the bud before spreading outward—usually deep within the densest flowers, where airflow stalls and dew lingers.
The risk of gray mold infection increases as cannabis matures and its buds enlarge. It can also colonize pruning wounds, shaded popcorn buds, and even freshly harvested nugs if they’re jarred while still too wet.
Early signs of gray mold
The earliest symptoms of gray mold on cannabis typically appear as small bruises or discoloration on the buds. These bruises collapse into rot, followed by the development of mycelium—gray mold.
Other symptoms include yellowing and wilting of leaves near the infection site. Regular inspection for gray mold can be done by carefully opening buds and checking inside, where the mold often hides.
How fast does bud rot spread?
Once established, gray mold spreads rapidly throughout the plant. The rate of infection increases under favorable conditions, allowing Botrytis to migrate from a single floret to an entire top cola within 48–72 hours.
In many cases, cannabis plants are nearly destroyed just a couple of days after infection.
How to Stop Gray Mold Fast
Botrytis hates dryness, light, and fresh air. The disease can be managed by removing moldy buds, cleaning plants, and adjusting the environment. Combine ruthless sanitation with climate control and you cripple its life cycle.
Discard Infected Buds
Tools: A knife or scissors dipped in a jar of high purity alcohol (USP grade isopropyl or Everclear), Q-tips, and a garbage bag.
1. Cut out around the damaged area using scissors or a knife dipped in alcohol.
2. Place the moldy bud in the garbage bag.
3. Use a Q-tip soaked in alcohol to clean any freshly exposed damage on the plant.
4. Repeat as needed. Be sure to dip the blades in alcohol between each cut.
It’s good practice to solarize garbage bags filled with moldy buds (by placing them in direct, hot sunlight) to kill any fungi before disposal.
Remove Dead/Dying Leaves
Botrytis can survive as sclerotia (a mass of hyphal threads) or mycelium on dead plant material, such as leaf litter.[3] By remaining dormant, the fungus can wait for an opportunity to attack a living host plant.
Removing dead and dying leaves from the plant is helpful, as Botrytis may otherwise reside there. Eliminating this habitat is key to preventing another mold infection.
Pluck Fan Leaves
Stripping a plant of its leaves is a highly debated topic among growers, but one point of agreement is that during the late bloom cycle, removing fan leaves can help prevent mold problems.
By cutting off large fan and sucker leaves, more light and air can penetrate the plant canopy.
Fan leaves can be plucked by hand, though you may need to use a blade if they don’t come off cleanly. Be sure to remove all petioles (the stem part of the leaves), which often remain attached to the plant’s stalk.
Tidy Up
Botrytis is frustratingly difficult to eliminate due to the many ways it attacks plants. Inoculum sources are not species-specific, and the fungus can survive for long periods as mycelia and conidia (a type of spore), hiding in debris or around the grow room.[1]
🌞 Outdoors: Remove any brush or mulch around plants.
🏠 Indoors: Quarantine plants and thoroughly scrub the grow room from top to bottom with a diluted bleach solution.
Gray mold is prevalent in high-humidity environments with moderately warm temperatures.
Lower Humidity
Moisture is often a more limiting factor for gray mold than temperature, which is why lowering the relative humidity (RH) in the grow room works well to stop mold fast.[4]
🌞 Gray mold thrives in humid, dark areas—hence the benefit of opening up plants to allow more sunlight and airflow. Outdoors, consider cutting back anything blocking the sun.
🏠 Indoors, aim to maintain low RH (40–50%) toward the end of the bloom cycle. You can lower RH by increasing air exchange (cool air in, strong exhaust out), using dehumidifiers, and running circulation fans.
Protect Buds
Protecting buds from getting wet is an important step in preventing gray mold. Botrytis fungi germinate on damp or wet plant tissue before spreading throughout the plant.
🌞 Dew formation and unexpected rainfall can be avoided by placing a tent or greenhouse cover over the plants. You can also move them inside.
💦 Maximizing the time between waterings helps reduce moisture around the plant. Water the soil early in the day so the ground dries out by nightfall. Avoid foliar sprays.
Increase Airflow
🌞 Moving potted plants to a hot, windy hillside is an ideal solution for outdoor growing. If the plants are rooted in the ground, surrounding vegetation may need to be cut back to improve airflow.
🏠 Increasing horizontal airflow helps create a more uniform climate in the grow room and reduces the likelihood of cool spots that may turn into condensation problems.[5] Oscillating fans can be mounted on the sides of the grow room to better direct air into the plant canopy.
For hoop house-style greenhouses, consider rolling up the sides of the covering to increase horizontal airflow to the plants during the late bloom cycle.
Finish Flowering
An early harvest is better than a moldy harvest. Keep a close eye on buds in late bloom, and be ready to pull early if you need to.
Bud Rot After Harvest
Equally important in preventing mold is the room used for drying buds once they are harvested. Think about this: In 2015, the HHE program visited an outdoor cannabis farm in Washington State during harvest to evaluate potential hazards. They found that Botrytis cinerea was the main fungal species present in the air.[6]
Causes
More often than not, the problem is related to humidity.
Drying room issues
Buds drying in a room that’s too humid or in polluted airspace are an easy target for mold.
Buds were put away wet
Large buds take longer to dry than smaller “popcorn” nuggets. Ensuring the batch dries evenly helps prevent moisture release during storage, which can lead to mold growth.
Bad storage containers
The usual suspects include zip baggies and pickle jars, both of which are not airtight.
Quick Drying
Traditional slow drying and curing methods are great for bud quality. That all goes out the window once mold appears. It’s better to take a hit in quality by quick drying than to lose it all to mold.
The fastest way to dry buds is to break them down from the branches and place them on drying racks or screens in a warm, dry location. However, you’ll lose the natural shape of the buds that hang-drying would preserve.
💧 Dry out the air to dry out the buds. Relative humidity should be kept below 45%
Dehumidifiers can reduce room humidity quickly. However, overuse may cause buds to burn unevenly.
🌡️ Turning up the heat will speed up the drying process, but excessive heat degrades cannabinoids and terpenes. Keep temperatures below 80°F to minimize degradation.
Most hygrometers for sale have built-in thermometers like this one on Amazon so that you can measure the temperature and humidity of the drying room on the same device.
💨 Fans can be used to create a gentle breeze for drying buds. Be sure to remove any mold from buds before using fans, as airflow can spread spores.
Salvage Buds: Desperation Methods
If the steps above are still not enough to control your gray mold problem, here are some last-resort, desperation methods for salvaging buds—to be used with caution.
- Drying buds in the sun
- Drying buds with high temps
- Cooked in edibles, oils
- Ice water extraction followed by an alcohol extraction
- Make quick-wash iso hash (QWISO)
Exposing cannabis to solar radiation is known to cause THC loss and degradation of other cannabinoids.[12] Still, it remains a common practice among growers in tropical regions without access to electricity. One DIY workaround to minimize quality loss is building a wood stove that doesn’t need to burn hot to create a draft, which can drastically reduce room humidity.
Is smoking gray mold harmful to humans?
Although no mycotoxins have been reported from Botrytis, it is still not recommended to smoke affected marijuana or concentrates. Precautions should be taken when handling moldy cannabis buds, such as wearing face protection. When a bag of moldy buds is opened up, a cloud filled with innumerable mold spores is released.
A medical condition known as Winegrower’s lung has been linked to the inhalation of Botrytis fungi.[2]
On the other hand, Botrytis cinerea is regularly consumed in wines. “Noble rot” is what winemakers call Botrytis, and there are highly-valued dessert wines made with the fungus.[7]
How do buds get ruined by mold?
Bud density
Cannabis buds bulk up in density during the end of their life cycle. The loss of intra-floral ventilation due to buds increasing in size combined with high humidity makes them prime candidates for Botrytis cinerea spores searching for a host.
The hypersensitive response
Plants have evolved natural responses to combat pathogen attacks. One of the primary responses to pathogen attack is an oxidative burst that triggers hypersensitive cell death in and around the attacked area.[7] This is referred to as the hypersensitive response and is done to cut off the food supply and keep pathogens at bay confined to that part of the plant. However, Botrytis works as a necrotrophic pathogen to utilize the dead tissue created and multiply on it.[8]
In other words, the fungi can exploit a cannabis plant’s defense mechanism for its pathogenic growth.
Chemical attack
Botrytis cinerea facilitates the formation of powerful chemicals that destroy buds and cause necrosis in plants. This includes various low molecular weight metabolites such as botrydial, oxalic acid, and HSTs.[1]
The fungi produce small RNA (sRNA) molecules that cause gene silencing to suppress host immunity.[9]
Spread
The fungi spread rapidly. When disturbed, they release a cloud of infectious spores, further spreading the disease. Damage occurs not just on buds but also on stems at pruning wounds where it can rot through the entire stem.
The stigmatic fluid present in buds serves as a nutrient medium for airborne conidia to germinate on.[10]
How does Botrytis infection start?
The infection starts with a fungal spore landing on plant tissue. Plants may be attacked at any stage, but freshly injured areas and aging or dead tissue are preferred. The spores germinate in suitable conditions of temperatures approximately 60–77°F with high relative humidity.[11]
The fungus can maintain growth in a range of temperatures from 28–90°F, although growth is halted at the extremes.[4]
❄️ Cool temperatures may slow the disease down, but the disease continues to infect and spread as long as it is moist.
🏜️ Hot, dry weather with plenty of direct sunlight is perhaps the best solution to shut the disease down.
Bud Rot Sprays
Due to the nature of cannabis use (smoking), we do not recommend foliar spraying maturing buds with fungicides or similar solutions. However, if the outbreak is caught early on, an organic fungicide such as Trifecta Crop Control may prove effective.
Weekly foliar sprays during the vegetative stage using biological fungicides like like Bacillus subtilis (Cease Biological Fungicide) or Bacillus amyloliquefaciens (Southern Ag Garden Friendly Fungicide) out‑compete Botrytis and prime plant immunity. Discontinue use by week 4 of flowering to avoid residue.
Cannabis Strains Resistant to Mold
Tropical sativa strains are highly resistant to gray mold. These mold resistant strains have often developed the resistance naturally, through many generations of growing in wet, humid climates. Their airy buds allow air to flow within, and damaged buds tend only to turn brown and rot instead of harboring gray mold.
Landrace strains from Thailand, Laos, Cambodia, Vietnam are very resistant to gray mold. These long flowering sativa strains are fun to try if you’re growing in a tropical area but are difficult to manage indoors or outdoors in temperate climates.
Read our list—8 Resilient Mold Resistant Strains for sativas and hybrids that perform well-grown outdoors on the Big Island of Hawaii.
References
You may enjoy further learning about Botrytis cinerea through these links.
Disclaimer: We do not promote or undertake in illegal activity.
I have found that removing the effected bud and spraying with 3% H202 and saturating the infected area goes a long way towards stoping the spread of the fungus.
Hi Barrie, I heard this recently. My plants are in flowering stage, a week or two to harvest, and I’m concerned about getting h2o2 on the buds. Should I be?
Thanks,