Cannabis Pests - Botrytis

Finding out your precious flowers are being destroyed by mold is never a nice thing. Botrytis is a destructive fungus that can affect all kinds of plants, including our beloved cannabis. This article helps you on your way to dealing with botrytis, and also brings to your attention the importance of prevention. 

What Is Botrytis?

Botrytis, also known as grey mold or bud rot, is a type of greyish white fungus (mycelium) which generally affects cannabis flowers from the inside where air has become stale, however, it can affect all areas of the plant. Botrytis consumes plant matter, turning it into a horrible, degraded mush.

When the conditions are in its favour, this airborne pest can wreak havoc on your harvest and make a large portion of the weed completely unusable. 

Depending on the strain and the formation of its flowers, the risk of botrytis is particularly high at the end of flowering when the main colas grow dense in warm environment. As it develops, it begins to spread spores to surrounding plants, where they can lay silent until the conditions signal them germinate and multiply. For anyone who has ever experienced a loss of yield because of botrytis, we feel you.

The Causes Of Botrytis

Botrytis is most often caused by warm, humid air sitting stagnant in small pockets of space within and around the flowers. Cannabis buds can get fairly dense too, which means getting air to circulate on the insides of the bud is not really possible. Usually, an unstable, moist environment with poor ventilation during flowering is the reason botrytis appears.

The thing is, because botrytis tends to grow from within the flowers, it can be quite hard to notice in it's early stages. You may reach the end of flowering and begin harvesting before you even realise there is a grey mush consuming the insides of your buds. The flowers and leaves can look relatively healthy and appear as if they are maturing as they should.

The discolouration of the leaves surrounding the bud can also be an indication of botrytis. Although the sugar leaves may seem like they are yellowing naturally as the plant reaches the end of its cycle, they may be weak and pull out very easily.

If botrytis infests your cannabis plants earlier in flowering and the leaves around the bud can begin to yellow (also to do with overheating/humidity), be careful not confuse it with a nutrient deficiency and double check the state of the buds to ensure no mold is growing. Remember, if the environment is too warm and humid, and there isn't enough air circulation, you already know there is a risk.

CBD run

Tip: Botrytis can also arise from other damages the plant experiences. Open wounds or broken tissues can be an inviting setting for botrytis to start feeding and breaking down plant matter.

Symptoms Of Botrytis

Unfortunately, the symptoms of a botrytis infestation are not easily noticeable and by the time you realise it's there it may have already spread to other plants. If you're lucky and manage to spot a small cluster, however, you may be able to remove the affected area and control the environment to ease your plants through to the end of their cycle.

Botrytis appears as a white, grey or brown fluffy mold and if it's growing on the outside of the flower, will most often be seen in a crevice between sugar leaves or the calyxes, close to the main structure of the bud. It can also grow on stems and leaves, particularly where there is already decaying material.

When the flowers are heavily affected, sugar leaves will turn yellow/lose colour and can be pulled out even with a gentle tug. If you suspect your plants have bud rot, you'll probably want to give all your buds a thorough check. If one of your top buds has shown symptoms, it's very possible the other colas have already become infected. 

Symptoms of a botrytis infestation:

• Yellowing, weak sugar leaves
• Smelly, stale air around the plants
• Grey, white or brown rot developing close to flowers in crevices between leaves and calyxes
• Slow growth and poor bud development
• Browning around damaged areas of the plant.

Tip: Routinely check your flowers with a loupe or magnifying glass so you can get familiar with how they should look on a macro level.

Treating Botrytis

Most of the time, treating botrytis isn't really an option and the infected weed should be removed and discarded. Because botrytis tends to affect late flowering plants, the bud cannot recover or be saved, and any treatments we apply will only ruin the flavour and quality of the final product.

Remove affected areas - Be extremely careful when removing the infected sites because the spores can spread very easily, even with the slightest movement. You may also consider wearing a facemask and gloves to. Make sure to discard them safely far away from the rest of your cannabis plants.

Clean the air - You may want to consider installing an air purifier to remove an fungal spores from the air around your grow space. This is beneficial for you too so you don't end up breathing in spores, which can be harmful for one's health.

Correct the environment - This is likely the primary cause of the botrytis problem so should be corrected as soon as possible. Install proper ventilation, get the air circulating efficiently, reduce the humidity levels, check and adjust temperatures. If you haven't got the right equipment, keeping molds at bay becomes a real challenge.  

Tips To Prevent Botrytis

Preventing botrytis is relatively simple if you follow the right practices and keep the grow conditions at the optimum levels. Like many other molds, botrytis will only colonize when the conditions allows it do so. Fortunately, there is plenty we can do to maintain the correct environment so molds like botrytis don't have an opportunity.

Tips for preventing botrytis:

• Maintain grow room temperatures between 20-26°C
• Do not let relative humidity exceed 65%
• Replenish the air in the grow room at least once per minute. Filter it through a purifier to ensure removal of fungal spores.
• Place oscillating fans in the grow room to prevent stale air building up. Make sure they circulate the air from all angles but do not have them pointing directly at your plants on a high power.
• Regularly clean up and dead or dying leaves.
• Be cautious if your cannabis plant is damaged (e.g. Broken stems or cuts)
• For outdoor plants, shake off excess water from the rain  and treat/monitor damage from storms.
• Sterilise your grow equipment between uses.

Treating bud rot by NanoGrow from GrowDiaries.

Also be sure to follow the correct practices post harvest, as cannabis buds can still develop botrytis during the drying and curing stage if the conditions are not right. Focus on controlling the moisture levels and relative humidity so you can be confident your buds won't end up turn moldy. 

Conclusion

It is never nice to learn the hard way and put months of work into your grow project, only to have it ruined by botrytis. Do everything you can to set up your grow environment correctly so you can avoid this one like the plague.

If you found this article useful or have any tips regarding botrytis prevention and treatment please feel free to drop us a comment down below!

External References

The diverse mycoflora present on dried cannabis ( Cannabis sativa L., marijuana) inflorescences in commercial production. Canadian Journal of Plant Pathology. - Punja, Zamir. (2020). 

Three Botrytis Species Found Causing Gray Mold on Industrial Hemp ( Cannabis sativa ) in Oregon. Plant Disease. - Garfinkel, Andrea. (2020).

Expression of Putative Defense Responses in Cannabis Primed by Pseudomonas and/or Bacillus Strains and Infected by Botrytis cinerea. - Balthazar, Carole & Cantin, Gabrielle & Novinscak, Amy & Joly, David & Filion, Martin. (2020).

This article was updated January 2021.