Grey, fuzzy mould

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. LikelyBotrytis bud rot (grey mould)

   DiseaseAct nowRead: Botrytis (bud rot / grey mould) in cannabis

   Fix: Act now. Bag infected buds before cutting them out, 3-5 cm (1-2 in) past visible rot, and clean tools; never shake them. Drop RH to about 40-50% in late flower, control the lights-off dew point, move air through dense colas. There is no cure in flower; consider harvesting early if it keeps spreading.

   Recovery: No new rot on weekly cola checks for 2 weeks.

   How to confirm it

   • Open the densest colas. In flower, gently spread the largest, densest colas at the base and look for brown, mushy or grey tissue around the stem, a single wilted sugar leaf that pulls out easily, frass or caterpillars. Check 5-10 colas per plant each week from mid-flower, more when RH is high. Bag any rot before removing it.
   • Check temperature and RH at canopy height with a calibrated sensor. Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.

2. Less likelyHumidity too high / VPD too low

   EnvironmentAct within a day or twoRead: Humidity by stage: RH targets from clone to cure

   Fix: Target about 55-70% RH in veg, 50-60% early flower, 40-50% late flower (leaf VPD 1.0-1.5 kPa in flower). Dehumidify, raise exhaust, keep air moving under the canopy and watch the lights-off spike (dew point). High RH in flower raised bud-rot risk and in one study delayed flowering and lowered cannabinoids.

   Recovery: RH holds in band through lights-off for 3+ days; no new mould signs.

   How to confirm it

   • Check temperature and RH at canopy height with a calibrated sensor. Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.

Look for

Dusty grey sporulating fuzz on brown dead tissue; do not shake it, spores spread..