Bud rot (Botrytis cinerea) is the disease growers fear most during cannabis , and for good reason — it's destructive, fast-moving, and easy to miss until the damage is already done. The critical thing to understand: it starts inside the bud and works outward. By the time you spot external signs, the interior is already compromised. It thrives in dense, tightly packed where humidity gets trapped, especially when airflow is poor and stays above 60%. Late-flowering plants with thick, rapidly-stacking colas are the most vulnerable — that last week or two before harvest is also when she's most at risk.
Caterpillar damage is a major entry point — the wounds and frass they leave behind create perfect conditions for Botrytis spores to germinate. If you're finding caterpillars on flowering plants, inspect those buds first. Unlike , which sits on leaf surfaces and can sometimes be treated in place, bud rot penetrates deep into tissue and any infected material has to come off the plant entirely. Diagnostic against PM: PM is white powder on exterior surfaces, while bud rot develops as gray-brown mushy decay inside the bud — pull apart a suspect bud and if the interior is brown and decaying, that's bud rot. Infected buds are unsafe to consume due to fungal toxins and spore contamination.
Solutions
✓Remove affected buds immediately — cut at least 1-2 inches below visible infection with sterilized scissors
✓Bag infected material before moving it through the grow space to prevent spore spread
✓Inspect all remaining buds carefully, especially dense colas — gently open them to check inside
✓Quarantine any buds that were directly adjacent to infected ones — they likely harbor spores even if they look clean
✓Drop humidity below 50% RH, ideally 40-45% during late flower
✓Improve air circulation between and within plants — oscillating fans plus defoliation of interior growth
✓Check for caterpillar damage as a Botrytis entry point — treat with Bt (Bacillus thuringiensis) if found
✓Consider early harvest if environmental conditions cannot be controlled and infection is spreading
✓Never consume any bud that has had mold contact — fungal toxins and spores persist even after drying
Plant Overview
How it appears across the plant
Bud rot's signature is hidden. It starts inside dense colas, and the plant-level picture is one of localized damage on specific buds while the rest of the plant looks fine. The earliest signs are subtle: a wilting in the middle of an otherwise healthy cola, or a faint off-smell near a specific bud. As the infection progresses, individual colas show clear external damage, and the gray fuzzy mold eventually breaks through to the surface.
Early. A few sugar leaves poking out of buds turn yellow or brown and wilt — often the very first visible sign. Individual shrivel and brown prematurely in random clusters (unlike natural aging, this affects discrete spots rather than spreading uniformly). A single section of a bud feels slightly damp or spongy compared to the rest of the cola. A faint musty or off smell when you get close to a specific bud. The rest of the plant looks completely healthy.
Mid. Multiple sugar leaves on the same cola yellow and wilt — not just one isolated spot. Damp, spongy patches become unmistakable on the bud surface. Pulling apart a suspect bud reveals visible brown discoloration spreading from the core outward. The musty smell is now reachable from a foot or two away. Adjacent buds start showing the same early signs — spores have already moved.
Late. Gray or brown fuzzy mold clearly visible on the bud surface. The interior is brown, gray, or black when pulled apart, with cobweb-like mycelium strands throughout. A strong musty or rotten smell — sometimes with an ammonia note if bacterial decomposition has joined in. Bud sections become mushy and fall apart when touched. The mold spreads via airborne spores to adjacent buds, and entire colas can be destroyed within 2–3 days. Visible spore clouds puff out when an infected bud is disturbed or squeezed.
Early — how it first appears on a single bud
On a single suspect bud in the early stage, look first for sugar leaves wilting where they shouldn't — a small leaf poking out of an otherwise healthy cola that's turned yellow or brown. The pistils right next to the wilting sugar leaf may be browning prematurely while pistils elsewhere on the cola still look normal. Touch the bud gently in that area — it feels slightly damp or spongy compared to the surrounding firm structure. A faint musty smell may be detectable when you bring the bud close to your nose. If you gently pry the bud apart at that section, the interior looks darker or wetter than it should.
Mid — when interior decay becomes visible
By the mid stage, multiple sugar leaves on the same cola show the wilting pattern, and damp spongy patches are now unmistakable on the bud surface. Pulling the bud apart reveals visible brown discoloration spreading outward from the core — what was hidden in the early stage is now obvious the moment you open the bud. The musty smell is detectable from a foot or two away without leaning in. Adjacent buds on the same plant begin showing the same early signs as the spores spread.
Late — when the cola is unsalvageable
By the late stage, the bud carries visible gray or brown fuzzy mold on the surface. The interior is brown, gray, or black soft decaying tissue with cobweb-like mycelium strands running through it. The smell is strong and rotten, sometimes with an ammonia note from bacterial decomposition. The bud is mushy and falls apart when handled. Disturbing it sends a visible cloud of spores into the air. Here's the rough part: the entire cola is unsalvageable, and any bud that was directly adjacent is contaminated by spores even if it looks clean.
Commonly Confused With
Powdery Mildew
Powdery mildew sits as white powder on exterior surfaces. Bud rot develops as gray-brown mushy decay inside buds. Pull apart a suspect bud — if the interior is brown and decaying, it's bud rot.
Both cause browning during flowering. Nutrient burn produces uniform tip browning across the plant. Bud rot causes browning from specific bud cores outward — pull apart the bud to check inside.
Both can cause wilting. Bud rot causes localized wilting on specific colas where infection is present. Overwatering causes whole-plant droop with wet soil.
