Botrytis, commonly called bud rot, is the most destructive fungal disease in cannabis cultivation. It’s caused by Botrytis cinerea, a gray mold that attacks flowers from the inside out, often destroying entire colas before growers even notice something is wrong. The infection starts deep within the densest part of the bud, making early detection genuinely difficult.
How Botrytis Infects Cannabis Buds
Botrytis cinerea spores are everywhere. They float through the air in outdoor and indoor environments alike, landing on plant surfaces and waiting for the right conditions to germinate. On cannabis, initial infections appear to originate from within the flower structure itself, from spores that landed and germinated earlier in the flowering cycle. The fungus doesn’t start on the outside where you can see it. It begins at the base of the small leaves nestled inside the bud, then works its way outward.
This is why bud rot hits the biggest, densest flowers hardest. Research published in Botany found that large, compact flower structures had significantly higher infection rates than smaller, less dense buds. Those trophy colas that growers prize the most are exactly the ones most vulnerable. The tight structure traps moisture, limits airflow, and creates a sheltered environment where the fungus thrives. Strains bred for dense, heavy flower production are inherently more susceptible.
The conditions that trigger infection are straightforward: high humidity, moderate temperatures, and stagnant air. Botrytis cinerea germinates best when relative humidity stays above 60% for extended periods, particularly at temperatures between 15°C and 25°C (59°F to 77°F). Late flowering is the danger zone because buds are at their densest and most moisture-retentive, and in many climates, fall weather brings cooler nights with heavy dew.
What Bud Rot Looks Like
The first visible signs of botrytis are easy to miss because the damage starts inside the bud. You’ll often notice a single sugar leaf turning yellow or brown and wilting in the middle of an otherwise healthy cola. That dying leaf is a signal that tissue decay is already underway beneath it. If you gently pull the bud apart at that spot, you’ll see brown, waterlogged tissue inside, sometimes with a faint grayish fuzz.
As the infection progresses, necrosis spreads from the base of those interior leaves outward. The pistils (the hair-like structures on the bud surface) darken and shrivel prematurely. Within days, the entire flower structure can be consumed. At advanced stages, the gray mold becomes clearly visible on the exterior as a dusty, grayish-brown coating. By this point, spores are actively releasing into the air and spreading to neighboring buds.
A key distinction: botrytis looks different from powdery mildew. Powdery mildew sits as a white, powdery film on leaf and bud surfaces. Bud rot is darker, wetter, and starts from within. If you’re pulling a bud apart and finding slimy brown decay at the core, that’s botrytis.
Prevention Through Environment Control
Since the fungus needs moisture and still air to take hold, environmental management is your primary defense. The goal during late flowering is to keep relative humidity below 50% at the canopy level, ideally closer to 40% in the final weeks before harvest. Consistent airflow through and around the canopy is just as important as the humidity number itself, because even at moderate humidity, pockets of stagnant air inside dense plant canopies create microclimates where moisture accumulates.
For indoor growers, this means oscillating fans positioned to move air through the lower and mid-canopy, not just above the tops. Dehumidifiers sized appropriately for the space are essential during flowering, when plants transpire heavily and push room humidity upward. Monitoring humidity at canopy level rather than on a wall sensor gives you a more accurate picture of what your buds are actually experiencing.
Outdoor growers have less control but can still make a significant difference. Spacing plants generously, removing large fan leaves that block airflow around buds (a practice called lollipopping or selective defoliation), and shaking dew off plants in the morning all reduce risk. Some outdoor growers erect simple rain covers or hoop houses over plants during the final weeks of flowering to keep direct rainfall off the buds.
Plant Training and Strain Selection
Because dense, compact buds are the most vulnerable, choosing strains with a more open, airy flower structure can help in humid climates. Sativa-dominant varieties, which tend to produce looser bud formations, are generally more resistant than indica-heavy strains with rock-hard colas. Training techniques that spread the canopy out, like screen of green (SCROG) or heavy topping, help improve airflow between bud sites.
Selective defoliation during mid-to-late flower removes leaves that trap moisture inside and around buds. This is a balancing act. You want to improve air circulation without stripping so many leaves that you reduce photosynthesis and stress the plant. Removing the larger fan leaves tucked into the interior of the canopy, particularly those pressing directly against bud surfaces, is a practical starting point.
What to Do When You Find It
Once botrytis is actively growing in a bud, that bud is lost. There is no treatment that will reverse the damage or kill the fungus inside the flower tissue while leaving the bud usable. The priority shifts to containment: preventing spores from spreading to healthy buds nearby.
Remove infected buds immediately by cutting the stem several inches below the visible damage, since the fungus may have spread further down than what’s visible. Do this carefully to avoid shaking the infected bud and releasing spores. Many growers mist the infected area lightly with water before cutting, or place a bag over the infected cola before removal, to trap spores. Seal removed material in a bag and dispose of it away from the grow area. Clean your shears with isopropyl alcohol between cuts.
After removing visible infections, closely inspect every remaining dense bud on the affected plant and its neighbors. Check daily from this point forward. If the infection is widespread or conditions haven’t changed (still humid, still poor airflow), consider harvesting early rather than losing the entire crop. A slightly early harvest with healthy flower is far better than waiting and finding half your buds rotted.
Post-Harvest Risks
Botrytis doesn’t stop being a threat once you cut the plant down. If spores are present on the flower surface, or if early-stage infections are hidden inside buds, the drying and curing process can allow the fungus to continue growing, especially if conditions are too humid or too slow.
Dry your harvest in a controlled environment with humidity between 45% and 55% and temperatures around 15°C to 21°C (60°F to 70°F), with gentle air circulation. The goal is to bring the moisture content of the buds down steadily over 7 to 14 days. Drying too fast in low humidity can trap moisture inside dense buds while the exterior feels dry, creating conditions where hidden botrytis can continue to develop. Check your drying buds regularly, particularly the largest and densest ones, for any signs of browning or off smells.
During curing, when flower is stored in sealed containers and periodically aired out, keep relative humidity inside the jars between 55% and 62%. Hygrometer packs designed for curing containers help monitor this. If you open a jar and notice an ammonia-like or musty smell, inspect the contents closely for mold.
Testing and Regulatory Standards
In legal cannabis markets, harvested flower must pass microbial testing before sale. These tests measure total yeast and mold counts (TYMC), and the thresholds vary by state. Minnesota’s standards, for example, require dried raw flower to contain fewer than 100,000 colony-forming units per gram. Exceeding that limit triggers an investigation, and the license holder must implement a corrective action plan before the flower can be released or further processed. In some cases, failed batches must be destroyed entirely.
Concentrates and edibles often have different or no TYMC limits because extraction and cooking processes can reduce microbial loads. But for flower sold for smoking or vaporizing, mold contamination is taken seriously by regulators. Inhaling Botrytis spores poses real health risks, particularly for immunocompromised individuals, potentially causing respiratory infections or allergic reactions.
For home growers not subject to lab testing, the same principle applies: do not consume visibly moldy cannabis. The gray mold may look minor on the surface, but the spore load throughout the bud tissue is far higher than what’s visible. Trimming away the moldy part and using the rest is not a safe approach, as spores and fungal metabolites extend well beyond the visible infection boundary.