What causes Botrytis bud rot in cannabis?
Botrytis cinerea is a necrotrophic fungus, it feeds on dying and dead tissue first, then colonizes living plant material as infection advances. In cannabis, inflorescences provide ideal substrate: dense, moisture-retaining bud structure with abundant senescing tissue (sugar leaves, spent pistils, dead lower foliage) that gives the pathogen its initial foothold.
Three environmental variables drive outbreaks: relative humidity above 70%, temperatures between 60–75°F, and limited air movement in the canopy. Most flowering rooms sit inside this window for much of the crop cycle. The pathogen may be introduced on infected cuttings, contaminated growing media, clothing, and tools, or it may already be present at low levels in the facility from a previous crop.
Infection typically initiates inside the dense bud, not on the surface. That internal start is why Botrytis is so difficult to catch visually until it has progressed significantly. By the time gray-brown mycelium is visible from outside the bud, the interior is often already lost.
How fast does Botrytis spread in a flowering room?
Fast enough to justify treating it as a crisis from the moment it's confirmed. Under optimal conditions, a single infected bud can sporulate within 48 hours, releasing millions of conidia into the room air. Those spores travel on airflow, including the very fans your HVAC uses to maintain VPD targets, and settle on adjacent canopy.
Research from Simon Fraser University documents bud rot incidence ranging from 1% to 26% across cannabis genotypes under natural inoculum pressure, with the highest rates in September and October when outdoor humidity pressure peaks. In indoor facilities without active outdoor inoculum pressure, spread is still rapid once a colony is established because the room provides a contained, humid environment.
The practical window between first detection and significant crop loss is short. Operators who respond to Botrytis by watching it for several days typically lose more than the initial affected area.
How do you identify Botrytis before the bud is visibly rotted?
Early Botrytis detection requires looking past the obvious. By the time a bud shows the classic gray-brown collapse and powdery gray spore mass on its surface, the infection is advanced.
Earlier indicators are mostly environmental and structural. Watch for:
- Humidity excursions in the canopy microclimate. Dense buds create a microclimate that runs 5–10% higher in relative humidity than ambient room readings. A room running 62% RH ambient may have buds sitting at 70%+ internally.
- Senescing tissue in the canopy. Dead sugar leaves and spent pistils trapped inside buds are Botrytis entry points. They are also invisible from above the canopy.
- Single-stem wilts without root zone explanation. When Botrytis establishes at a stem node or crown, the plant above that point wilts while the rest remains healthy. This is easy to misread as a watering problem.
- Water-soaked, darkening tissue at a bud's base or midpoint, often the first visual sign before spore production begins.
qPCR environmental monitoring can detect Botrytis DNA in air and on surfaces before symptoms appear, and is worth incorporating into any high-value flowering room protocol.
Can Botrytis be remediated after it's detected in a room?
Once Botrytis is actively sporulating in a room, containment and harvest timing replace remediation as the primary tools. There is no registered chemistry that kills active Botrytis inside a dense flowering bud without damaging the product, and no treatment that reverses already-damaged tissue.
What you can do: remove affected material immediately and dispose of it outside the room, avoid shaking or disturbing infected buds during removal (which releases spore clouds), lower room humidity aggressively, increase air exchange, and consider accelerating harvest timing for the most advanced plants. Post-harvest decontamination of the room before the next cycle is non-negotiable.
State remediation pathways for product that shows elevated mold counts at testing generally require post-harvest treatment of the dried flower, not intervention during the crop. A product with visible Botrytis does not qualify for most remediation pathways, it is destroyed product.
What environmental conditions prevent Botrytis in the flowering room?
The target is to stay below the conditions Botrytis requires, without stressing the plant enough to reduce yield or terpene development. That tension is the core challenge of late flowering environmental management.
Practical targets: relative humidity below 50% in the canopy microclimate (not just the room sensor), temperatures in the low-to-mid 70s°F, strong horizontal airflow through the canopy to prevent stagnant pockets, and zero standing dead material inside the buds. Defoliation timing matters, removing senescing tissue before it dies in place removes Botrytis substrate.
Beyond environmental controls, the load of spores already present in the facility determines how aggressively the pathogen will establish. A room that was contaminated in a previous cycle and not thoroughly decontaminated carries that inoculum into the next crop. Surface sanitation between cycles is the variable most operators underinvest in.