Short answer. Bud rot is the fungus Botrytis cinerea rotting your densest colas from the inside out [1][2]. It needs cool-to-moderate temperatures, high humidity and free water on the tissue: the PNW Handbook gives 10–27 °C (50–80 °F) with an optimum near 20 °C (68 °F) [2]. You can't cure it in flower. You prevent it with humidity below about 50 % late in flower [2][3], no condensation at lights-off, moving air, open canopies, and by cutting out infected buds the moment you see them.
What's happening
B. cinerea is a necrotroph: it kills tissue and lives on the dead remains. Airborne spores land on flowers, germinate in free water or very high humidity, and infect through senescing pistils, damaged tissue and the cosy wet core of a fat cola. By the time you see it outside, the inside of the bud is already gone. Grey fuzzy sporulation then releases millions of spores to the rest of the room. It also causes stem cankers and damping-off of cuttings [2].
Wounds help it. Caterpillar feeding inside colas is a classic entry point outdoors [4] (see caterpillars). Dense, high-calyx, late-maturing cultivars are reported to be most vulnerable (Grower practice).
How to recognise it
- Earliest sign: a single sugar leaf or small cluster of pistils on a cola turns dull, yellow then brown and dry while the rest is green. Tug it: it pulls out easily.
- Inside: pry the bud apart. Brown, mushy or dusty grey tissue at the core, sometimes along the stem inside.
- Late: grey-brown fuzzy mould with a dusty spore cloud when disturbed. Stem above the lesion may wilt.
- Stems: tan, water-soaked cankers that girdle a branch; everything above wilts.
- Look-alikes: normal pistil browning at ripeness (uniform, bud stays firm), nutrient-related leaf yellowing (whole leaves, not inside the bud), Fusarium bud rot (pinkish-white mould; see fusarium), PM on buds (white powder on the surface, no internal rot).
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Temperature favouring infection | 10–27 °C (50–80 °F), optimum ~20 °C (68 °F) | PNW Handbook [2]; cool nights in late flower are high-risk |
| RH, mid to late flower | 40–50 % | Keep below 50 % in greenhouse production [2][3] |
| VPD, late flower | 1.2–1.6 kPa | Higher VPD = drier leaf surfaces (Grower practice) |
| Leaf/bud surface vs dew point | Keep surface temperature ≥ 2–3 °C (4–5 °F) above dew point | Condensation is free water; free water favours infection [2] (Supported, margin is grower practice) |
| Lights-off transition | Ramp temperature down over 30–60 min; dehumidify before and during | Night RH spikes are the most common trigger (Grower practice) |
| Drying room | 15–21 °C (60–70 °F), 55–62 % RH | Common drying targets; above ~65 % RH with poor air exchange, botrytis can continue in drying budsbeing reviewed |
| Outdoor | Rain or fog during the last 2–4 weeks of flower | Highest-risk window (Supported) |
What to check first
- Confirm: pull the suspect bud apart. Internal brown rot = botrytis until proven otherwise.
- Check your climate log at lights-off and the first two hours of dark. RH climbing past 60–65 % or temperature falling toward dew point is the usual culprit.
- Look at the position: biggest top colas near cool surfaces (walls, AC outlets, vents) or wet zones (under leaks, near humidifiers).
- Check for caterpillars or frass in affected colas, especially outdoors.
- Check for overwatering and wet floors raising room RH.
How to fix it
Immediately
- Cut infected buds out with a margin of at least 5–10 cm (2–4 in) of healthy-looking stem below. Bag them at the plant before removing to avoid shaking spores. Sanitise pruners between cuts.
- Inspect every cola of the same cultivar, then neighbours, every day until harvest.
- Drop RH to 40–45 % and increase airflow through, not just over, the canopy.
Cultural and environmental (prevention)
- Open the canopy: defoliate and lollipop before week 3 of flower so air moves through colas.
- Space plants. Support colas so they don't fold into each other.
- Dehumidification sized for the transpiration load (roughly the water you feed per day, minus runoff).
- Avoid cold surfaces, cold air blowing directly onto buds, and late-day irrigation that raises night RH.
- Choose cultivars with looser flower structure for humid climates and greenhouses.
- Harvest promptly. Botrytis risk rises as flowers senesce.
- Sanitise between cycles; remove plant debris (see sanitation).
Biological and least-toxic options
Products and registrations differ by jurisdiction; Utah State notes that no fungicides were available for bud rot there [3], while the PNW Handbook lists a few biological options for hemp in Oregon [2]. Check local regulations and labels before using anything. - Biological antagonists (Bacillus spp., Clonostachys rosea [formerly Gliocladium roseum], Trichoderma spp.) are used against botrytis in other crops and some are trialled on hemp [2]. Supported in other crops, limited cannabis evidence. They are preventive only. - Plant-extract biofungicides (e.g. Reynoutria sachalinensis extract) are listed for hemp in the PNW [2]. Preventive. - UV-C, ozone, air purifiers: may lower airborne spore load; no peer-reviewed cannabis efficacy for bud rot prevention foundbeing reviewed.
What not to do in flower
- Don't spray buds to "kill" botrytis. Wetting dense flowers spreads spores and feeds the next infection.
- Don't keep infected buds. Mouldy flower is not safe to smoke, vape or extract; spores and mycotoxin-producing contaminants are a health concern, and regulated markets test for yeast and mould counts [1].
- Don't shake or blow infected colas before bagging them.
- Don't raise humidity in late flower for "terpenes". Anything above about 55 % RH in late flower is a gamble with dense genetics.
How to confirm it worked
- No new brown sugar leaves or soft spots in daily inspections for 7–10 days.
- Night RH stays within target and surfaces stay above dew point (a cheap IR thermometer plus a dew-point calculator, or a controller that logs it).
- Dried buds break open clean at the core, with no grey dust or musty smell.
Common mistakes
- Removing only the visible brown bit. The rot extends further inside.
- A big night-time temperature drop with no dehumidification.
- Fans blowing across the top of the canopy while the cores stay stagnant.
- Hanging wet, whole plants in a crowded, humid dry room.
- Letting caterpillar damage go unchecked outdoors.
Evidence
- Established: B. cinerea causes bud rot in cannabis indoors, in greenhouses and outdoors [1][2][3].
- Established: cool-moderate temperature, high humidity and free water favour infection; greenhouse RH below 50 % is recommended [2][3].
- Supported: wounds and insect feeding increase infection [4].
- Supported in other crops, limited cannabis data: biological antagonists.
- Grower practice: dew-point margins, lights-off ramping, drying-room numbers.
- Myth: "Bud rot only happens outdoors." It is common in indoor rooms with night-time RH spikes [1].
