How to Save Your Buds from Grey Mould
It’s the nightmare just before the finish line: your flowers look magnificent, but suddenly you spot a suspicious grey inside the densest buds. Botrytis cinerea, better known as grey mould or bud rot, is one of the most destructive fungal diseases in cannabis cultivation.
In this article on weedx.ch you’ll learn how to spot the fungus early, what to do in an emergency and how to make your next grow mould-proof – no matter whether your plants come from cannabis seeds or hemp clones.
What is Botrytis?
Botrytis is a parasitic fungus spread through the air. It loves high humidity and stagnant air. On cannabis it prefers to attack the inside of the colas, where it’s damp and dark. Because it eats the plant from the inside out, it’s often only noticed when it’s already too late for the affected bud.
The symptoms: what to look out for
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Discoloured leaves: if small leaves growing straight out of the flower (sugar leaves) suddenly turn yellow or brown and pull out without resistance, be careful.
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Grey fuzz: a grey, dusty or cotton-like coating forms inside the bud.
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Mushy spots: the bud structure turns soft and dark and looks almost “rotten”.
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Spore dust: touching an infected spot releases a fine cloud of dust – millions of fungal spores.
The main causes of mould
Why do some plants get hit and others don’t? It usually comes down to the environment:
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High humidity: levels above 55–60% in late flowering are an open invitation to Botrytis.
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Poor air circulation: when the air between plants stands still, humid “hotspots” form.
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Dense buds: high-yielding indicas with very compact buds are more vulnerable, as hardly any air circulates inside.
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Injuries: open wounds from insect bites or training (LST/HST) give the fungus a way in.
Emergency plan: what to do if mould strikes
If you discover Botrytis, every minute counts to save the rest of the harvest:
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Isolate: don’t touch infected spots more than necessary so you don’t spread spores.
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Cut generously: remove the affected bud plus at least 2–3 cm of healthy tissue below it. Important: clean your scissors with alcohol after every cut!
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Lower the humidity: switch on dehumidifiers and maximise air circulation.
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Consider an early harvest: if the mould keeps spreading, harvesting the healthy parts early is often better than losing everything.
Important: mouldy flowers must never be consumed, vaporised or processed! The spores can cause serious lung problems.
Prevention: stop mould before the first sign
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Control humidity: in late flowering, relative humidity (RH) should stay below 45%.
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Airflow power: position fans so the air keeps moving under and between the buds as well.
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Defoliation: remove unnecessary fan leaves in the middle of the plant to improve airflow.
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Hygiene: keep your grow room clean and change your clothes before going from the garden to your plants.
Conclusion: vigilance is the best protection
Botrytis is treacherous, but manageable with the right climate control and genetics. If you inspect your plants closely every day – especially inside the thick main colas – you can react fast enough when it matters.
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Sources & further reading
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Royal Queen Seeds: Cannabis mould: identification and prevention. (Practical guide.)
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Canna: Botrytis cinerea: all about grey mould. (Botanical details on the fungus.)
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Leafly: How to prevent and identify bud rot (Botrytis) on cannabis.
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University of Hohenheim: Phytomedicine – Botrytis cinerea. (Scientific background.)
