Recognizing and preventing bud rot (Botrytis) in cannabis
Gray, cottony rot inside the bud with wilted individual leaflets is the warning sign. Bud rot is not curable — this climate strategy prevents it before it breaks out.
Pathogen
Botrytis cinerea
First symptom
Wilted individual leaflet in the bud
Risk RH (flowering)
> 55–60 %
Target RH late flowering
40–50 %
Curable?
No — only remove & prevent
Key points
- Botrytis cinerea starts inside dense buds — the first sign is individual wilted, dried-out leaflets in the middle of an otherwise healthy bud.
- Infested buds are lost and hazardous to health; there is no cure, only removal and prevention.
- The main lever is climate: relative humidity below ~50% in late flowering, avoid temperature drops, ensure air movement.
Note
- Never consume buds infected with botrytis — the spores are harmful to health.
- Do not cut open infected buds inside the grow room; this spreads spores onto healthy plants.
Definition and classification
Bud rot (botrytis) is caused by the fungus Botrytis cinerea. It preferentially infects dense, moist buds and decomposes them from within.
Infested material must not be consumed; the spores are a health concern. That's why prevention is the only sensible strategy.
Scientific background / pathogen biology
Botrytis is a necrotrophic fungus: it kills tissue and lives off the dead material. Spores are nearly ubiquitous and germinate in moisture on injured or weakened tissue.
Dense buds trap moisture inside and offer an ideal microclimate — that's why rot starts at the core, not on the outside.
Damage pattern and symptoms by severity
Stage 1: A single wilted, dried-out sugar leaflet in the middle of an otherwise healthy bud — the most important early warning sign.
Stage 2: When you open the bud, gray-brown, cotton-like mycelium and brown, mushy inner tissue.
Stage 3: The bud disintegrates, gray spore mass puffs out, spreading to neighboring buds.
Checklist
- Look for wilted individual leaflets in dense buds
- Carefully open the suspicious bud — gray/brown inside?
- Daily visual inspection during humid phases
Diagnosis — differentiation
Gray, cotton-like mycelium inside the bud → Botrytis (bud rot).
A white, powdery coating ON leaves/buds, on the other hand, is powdery mildew, not Botrytis.
A single wilted leaflet in a plump bud is nearly pathognomonic for developing bud rot — check the bud immediately.
Control / immediate actions
1. Remove the infested bud generously (with a margin into healthy tissue), into a bag — do not cut it open in the room, to avoid spreading spores.
2. Disinfect tools and hands to avoid carrying spores over.
3. Dehumidify the room climate immediately and increase air movement.
4. In case of heavy infestation shortly before harvest: harvest earlier if necessary to salvage remaining material.
Checklist
- Remove infected material in isolation
- Disinfect tools
- Lower RH, increase air movement
Prevention (the real lever)
Keep relative humidity in late flowering at 40–50% and avoid nighttime humidity spikes.
Ensure even air movement through the canopy; stagnant, humid air in dense buds is the main risk factor.
Defoliate/thin heavily so buds can dry out (defoliation during flowering), and avoid temperature drops that cause condensation.
Checklist
- Keep late-flowering RH at 40–50%
- Ensure air movement in the canopy
- Thin out dense bud zones
- Avoid temperature drops (condensation)
Environmental factors and interactions
High relative humidity (> 55–60%) and large day-night temperature differences lead to condensation on the buds — the core risk for Botrytis.
Dense strains and overfertilization with nitrogen (soft tissue) increase susceptibility.
Pre-damaged tissue (pest feeding, mechanical damage) offers Botrytis entry points — IPM and mold prevention are connected.
Common mistakes
Cutting open infested buds in the room, thereby spreading spores.
Monitoring RH only during the day, but ignoring the humid nights.
Hoping for 'healing' — infected tissue is lost.
Advanced considerations
Genetics matter: airy, less dense phenotypes have a clear advantage in humid climates.
Controlled drying/harvest planning around weather changes (e.g., in outdoor fall) significantly reduces the botrytis risk.
Frequently asked questions
Can I save a bud with bud rot?
At what relative humidity does it become critical?
Sources
→ Full registerRelated articles
Safety & Education
Mold and mycotoxins in cannabis
Why microbiological safety does not end at the visible bud and which information gaps are particularly risky.
Diagnosis & Troubleshooting
Identifying and controlling powdery mildew on cannabis
White, powdery spots on the upper leaf surface are the key symptom. Unlike most fungi, mildew doesn't need moisture — this climate and hygiene strategy stops it.
Tools & Calculators
VPD explained simply
What vapor pressure deficit means and how it governs growth, transpiration, and mold risk.
Diagnosis & Troubleshooting
Identifying and fixing calcium deficiency in cannabis
Stunted, spotted new shoots and brown leaf margins reveal Ca deficiency. This shows you that the problem almost always lies in pH and water quality — not in the Ca dose.