Botrytis blight, aka bud rot or gray mold, is a common fungal disease that affects cannabis buds during growth and after harvest.

We’ll go over how to identify gray mold in cannabis, removal of the infected buds, and methods used to stop and prevent any further mold outbreaks in the garden and drying room.

Identifying Gray Mold (Bud Rot)

Gray mold appears on infected buds as a thick carpet of velvety spore mass. The fuzzy web of light-gray-blue mycelium is visible to the human eye.

Gray mold can occur in both growing plants and drying/dried buds. The fungus often shows up on the inside of buds before being visible on the outside, especially within dense buds.

The agent of grey mold in buds is the asexual fungus Botrytis cinerea (roughly translates to “grape disease” in Greek). Botrytis cinerea is an airborne plant pathogen with a necrotrophic lifestyle. The ability to induce programmed cell death plays a key role in the success of the fungi. ^[1]

Early symptoms of gray mold on cannabis first appear as small bruises or discoloring of the bud. The bruises collapse into rot, followed by formation of the mycelium, gray mold.

Other symptoms include yellowing and damping off of leaves where it has infected plants. Regular inspection for gray mold can be done by carefully opening buds and checking inside, where gray mold tends to hide.

How to Stop Gray Mold Fast

By removing moldy buds, cleaning plants and adjusting the environment, further gray mold infections can be prevented.

-Discard infected buds

Tools: A knife or scissors dipped in a jar of rubbing alcohol (99% isopropyl), Q-tips and a garbage bag to start.

1.) Cut out around the damaged area using scissors or a knife dipped in rubbing alcohol.

2.) Put the moldy bud in the garbage bag.

3.) Clean any freshly exposed damage on the plant with alcohol using the Q-tip.

4.) Repeat as necessary. Dip blades in alcohol between cuts.

It’s good practice to solarize the garbage bags of moldy buds in the hot sun to kill the fungi before being disposed of.

Once the Botrytis infections have been removed from the plant, it’s time to alter the environment to prevent further outbreaks of grey mold.

-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 fungi can wait until has the chance to attack a living host plant.

Removing dead and dying leaves in the plant is helpful because the spores quickly develop there. Eliminating the sources of spores is a crucial part of preventing another attack.

-Pluck Fan Leaves

✂️ By cutting off the large “sucker leaves, light and air are able to penetrate though the canopy.

Stripping a plant of it’s fan leaves is a highly debated topic among growers but one thing that’s agreed on is that during late bloom cycle, removing some large leaves helps to prevent a mold infection.

Fan leaves can be plucked off with your hands, though you can use a blade if they don’t pop-off clean. Make sure to remove all of the petioles (the stem part of leaves) that tend to remain by the stalk of the plant.

-Tidy Up

Botrytis can be frustratingly difficult to stop due to the numerous ways it attacks plants. Inoculum sources are not species-specific, and the fungi can survive long times as mycelia and conidia (a type of spore) hiding in debris or around the grow room .^[1]

🌞 Outdoors, clear out the brush and mulches around plants.

🏠 Indoors, remove plants and scrub the grow room top to bottom with a bleach solution.

Gray mold is prevalent in high humidity environments, particularly wet/humid conditions mixed with moderately warm temperatures. 

-Lower Humidity

Moisture often is more of a limiting factor to gray mold than temperature^[4], which is why lowering the relative humidity (RH) of the grow room works well for fighting back a mold problem.

🌞 Gray mold thrives in high humidity and reduced light. Hence the benefit in opening plants up to allow for more sunshine and wind to come through. Outdoors, cut back any invasive foliage surrounding the garden.

🏠 Indoors, aim to maintain RH levels under 45% towards the end of bloom cycle. RH can be lowered with techniques like increasing the air exchange in the room (cool air in + strong exhaust), using dehumidifiers and circulation fans.

-Protect Buds

Protecting buds from getting wet is an important step in gray mold prevention. Botrytis germinates on damp/wet plant tissue before can spread throughout the plant.

🌞 Dew formation and unexpected rainfall can be thwarted by moving plants to a covered area at night or by building a plastic-covered PVC greenhouse over plants.

💦 Maximizing the period between waterings decreases the moisture in proximity of the plant. Soil can be watered early in the day so that the ground can be dry for the night. Avoid foliar sprays.

-Increase Airflow

🌞 Relocating plants to sunny, hot and windy hillsides is an ideal solution for the outdoor cannabis grower. If the plants are rooted in the ground, surrounding vegetation may need to be knocked down in order to give the plant more airflow.

🏠 Increasing horizontal airflow helps to create a more uniform climate in the grow room, and reduces the likelihood of cool spots that can turn into condensation problems.^[5] Oscillating fans can be mounted to the sides of the grow room for better air displacement into the plant canopy.

For hoop-house style greenhouses, try rolling up the sides of the cover to get more horizontal air flow to plants during 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 any potential hazards. What they found that Botrytis cinerea was the main fungal species in the air.^[6]

Causes

More often than not, the problem is humidity-related.

-Drying room issues

Buds that are drying in a room that’s too humid, or in a polluted airspace are an easy target for mold.

-Buds were put away wet

Big buds take longer to dry than popcorn nuggets. Getting the batch evenly dried will help to prevent moisture release in storage, which leads to mold infection.

-Bad storage containers

The usual suspects are 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 quickest way to dry buds is to break them all down off of the plant and put them on drying racks/screens in a warm dry spot. You’ll lose the shape of the bud you would have had if they were hang-drying.

💧 Dry out the air to dry out the buds. Relative humidity should be less than 45%

Dehumidifiers can dry weed quick, however their use has a bad reputation for making bud burn unevenly.

🌡️ Turning up the heat will speed up the drying process as well. Around 75-80°F won’t be too hot that it releases terpenes and cannabinoids from the buds.

Most hygrometers for sale have built in thermometers like this on Amazon, so 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. Mold should be picked out of buds before using fans to stop it from spreading.

Avoid cross contamination from air pollutants such as rotting food in kitchens, ect.

Desperation Methods

If the steps taken above are still not enough to control your gray mold problem, here are some last-resort desperation methods to be used with caution.

• Drying buds in the sun
• Drying buds with high temps
• Cook 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 other cannabinoid degradation^[12], still it is a common practice for many growers in tropical areas around the world who do not have access to power. A DIY method to circumvent the loss in bud quality is to make a wood stove that doesn’t need to burn hot to create a draft and drastically reduce the humidity of the room.

The Dangers of Smoking Gray Mold

It’s not advised to smoke any cannabis buds that have had a gray mold infection. Precautions should be taken when handling mold infected 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 Wine Grower’s lung is known to be caused by inhalation of botrytis fungi. ^[2]

On the other hand, botrytis cinerea is consumed in wines. “Noble rot” is what botrytis is called by winemakers, 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 growth 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 in search of a host.

-The hypersensitive response

Plants have evolved natural responses to combat pathogen attacks. One of the primary responses to pathogen attack is the process of 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 in order to cut off the food supply and keep pathogens at bay confined to that part of the plant. Botrytis however, 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 their 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 fungus produces small RNA (sRNA) molecules that cause gene silencing to suppress host immunity.^[9]

-Spread

The fungus spreads rapidly, and when disturbed releases a cloud of infectious spores, further spreading the disease. Damage can occur on not just buds but also on stems at pruning wounds where the fungus can rot through the entire stem.

The stigmatic fluid (present on buds) serve 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 a plant tissue. Plants may be attacked at any stage but freshly injured tissues and aging or dead tissues are preferred. The spores germinate in suitable conditions of temperatures approximately 60-77°F and high relative humidity or moisture.^[11]

The fungus is capable of maintaining growth in a range of temperatures from 28° to 90°F, although growth is halted at the extremes.^[4]

❄️ Cool temperatures may slow the disease down but as long as it is moist the disease continues to infect and spread.

🏜️ 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 matured buds with fungicides or organic solutions.

Cannabis Strains Resistant to Mold

Tropical sativa strains are highly resistant to gray mold. These mold resistant strains have often developed the resistance naturally, though many generations of growing in wet, humid climates. Their airy buds allow air to flow though, and damaged buds tend to only turn brown and rot instead of harboring gray mold.

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 of sativas and hybrids that do well grown outdoors for us on the Big Island of Hawaii.

References

You may enjoy further learning about Botrytis cinerea though these links.