Bud Rot in Cannabis: How to Identify and Prevent Gray Mold on Buds
Bud rot in cannabis is an infection of the flowers by the gray mold fungus Botrytis cinerea, the name commonly used for this disease in English-language sources. It appears as brown, water-soaked spots within the flower clusters, dying tissue that turns brown, and later a gray-brown mass of spores. Prevention centers mainly on climate: fact sheets from US universities recommend keeping relative humidity below 50% in greenhouses, together with airflow through the canopy and minimizing how long moisture sits on the plants.
According to NC State Extension, the fungus infects more than 200 plant species, including strawberries, grapes, and vegetables. In cannabis, it is one of the pathogens that destroy the plant's flower clusters directly. A review published in the journal Pest Management Science (2021) puts the economic losses from such flower-infecting pathogens, before and after harvest, at up to 20%.
What bud rot looks like on cannabis flowers
The first signs are brown, water-soaked, translucent spots on the flowers. The tissue then dies, turns brown, and can develop a gray-brown mass of spores, as described in the Pacific Northwest Plant Disease Management Handbook. On the stem, yellowish areas and cankers appear, which in dense, heavy growth can snap branches or split stems. Small black sclerotia, which allow the fungus to survive over time, sometimes form within the stem tissue.
A multi-year study of cannabis grown indoors, in greenhouses, and outdoors (Frontiers in Plant Science, 2019) found brown discoloration, dead bracts and leaves, and decomposed tissue on infected flowers. After the samples were incubated in a plastic bag for 48 hours, gray-brown mycelium grew on them. In severe cases, where up to 50% of plants were infected, leaves also carried spots. According to the authors, such leaf infections occurred only under high spore pressure and in plants close to harvest.
Cannabis industry sources consistently describe the rot as often starting inside dense buds around the stem and only becoming visible from the outside later. Opening such buds reveals soft, mushy tissue discolored gray to brown, according to these accounts. A musty smell is cited as a further sign.
The conditions that let Botrytis infect buds
Botrytis cinerea is a weak pathogen. According to Utah State University, the fungus enters only through wounds or dying plant tissue. The Pacific Northwest handbook names weakened, injured, and aging tissue as entry points. Spores spread by wind or splashing water, and the fungus survives in soil and plant debris as sclerotia.
Infection requires high humidity and free water on the plant surface. Fact sheets differ on the favorable temperature range:
Source | Favorable temperature range | Additional detail |
|---|---|---|
University of Kentucky (2021, Botrytis diseases in general) | 15 to 24°C | infections stop above 30°C |
Pacific Northwest handbook (hemp, 2026) | 50 to 80°F (about 10 to 27°C) | optimum 68°F (20°C) |
NC State Extension (hemp, 2024) | 65 to 75°F (about 18 to 24°C) | cool, humid conditions |
Utah State University (hemp) | 65 to 75°F (about 18 to 24°C) | moisture and wetness from rain or irrigation |
A University of Kentucky fact sheet on Botrytis diseases names 90% relative humidity as the optimum for infection; injured tissue can already become infected starting at 80%. Utah State University considers hemp at risk when greenhouse humidity stays high for a week or longer, or when cool rain falls outdoors.
Why dense buds and wet autumn weather raise the risk
Strains with large, dense flower clusters are more susceptible to botrytis than strains with smaller, loosely structured buds, according to the review in Pest Management Science. The authors attribute this to better airflow in looser clusters, though they describe this explanation as an assumption.
Outdoors, bud rot can be devastating in cool, wet weather. The same review notes that airborne spore counts from plant-pathogenic fungi and molds vary with the season and are higher in autumn, which mainly affects outdoor cultivation. Rain during the period when flowers are ripening can trigger large outbreaks, according to the Pacific Northwest handbook.
Preventing gray mold indoors and in greenhouses
For greenhouses, fact sheets from Utah State University, NC State Extension, and the Pacific Northwest handbook all recommend relative humidity below 50%. The handbook adds warm temperatures and high light intensity. How temperature and humidity interact to affect vapor pressure deficit can be tracked with the VPD calculator.
A 2024 review on disease management in cannabis greenhouses, published in the journal Plants, sets a different range for the flowering stage: 23 to 28°C at 50 to 70% relative humidity. It names 0.5 to 1.0 m/s as the airflow target at which microorganisms are suppressed. According to the same review, lower plant density can significantly reduce greenhouse humidity. The authors recommend daily checks for bud rot from the sixth week of flowering onward.
University fact sheets list further cultural practices:
avoiding injury to plants and flowers, since the fungus enters through wounds
avoiding overfertilization, according to the Pacific Northwest handbook particularly with nitrogen
cleaning tools and equipment with a 10 to 15 percent household bleach solution, according to Utah State University
removing dead and infected plants and plant material from the growing area
Preventing botrytis outdoors
The Pacific Northwest handbook describes canopy management as the single most important preventive measure. Wider spacing between plants and rows, along with suitable row orientation, is meant to move air through the canopy. During and after flowering, watering is kept sparing, applied at the base rather than overhead, and done in the morning so foliage dries before sunset. Plants should not stay wet for 12 hours or longer.
Harvesting earlier to limit losses
Licensed producers remove infected buds and harvest earlier to limit spore formation and spread. The 2021 review considers both measures effective at reducing outbreaks but describes them as labor-intensive. According to the 2024 review, harvesting after 6 weeks of flowering instead of 8 can reduce botrytis infection. In some genotypes, this can come at the cost of yield and active-compound content.
Removing and disposing of infected buds
According to Utah State University, a botrytis infection cannot be cured. The fact sheet calls for removing infected plants immediately to limit spore production. The 2024 review likewise recommends removing and disposing of infected flower clusters.
Germany's Federal Environment Agency (Umweltbundesamt) describes how to handle infected plant material using the example of gray mold on strawberries. Affected plant parts are removed promptly and disposed of with household or organic waste, since not every compost reaches temperatures that kill the pathogen.
Legal rules on growing and possessing cannabis differ from country to country. In Germany, private home cultivation falls under the German Cannabis Act (KCanG). Section 9(2) KCanG prohibits passing homegrown cannabis on to third parties, and Section 10 KCanG requires it to be secured at the grower's home or usual residence against access by others, particularly children and teenagers. The allowed amounts for private cultivation under German law are covered in a separate article on cannabis possession in Germany.
Frequently asked questions
What does bud rot look like on cannabis buds?
Typical signs are brown, water-soaked spots on the flowers, dying tissue that turns brown, and later a gray-brown mass of spores. Cannabis industry sources also describe the rot as often starting inside dense buds and giving off a musty smell.
At what humidity does bud rot develop?
A University of Kentucky fact sheet names 90% relative humidity as the optimum for infection, with injured tissue becoming susceptible starting at 80%. A favorable temperature range of 15 to 24°C is also given.
What is bud rot?
Bud rot is the common English name for the flower rot in cannabis and hemp caused mainly by the gray mold fungus Botrytis cinerea.
Can bud rot in cannabis be treated?
According to Utah State University, an infection cannot be cured. Infected plants are therefore removed immediately to limit spore production.
How should plant material infected with gray mold be disposed of?
Germany's Federal Environment Agency describes, using strawberries as an example, removing affected plant parts promptly and disposing of them with household or organic waste, since not every compost reaches temperatures that kill the pathogen.
Sources
Gray Mold of Industrial Hemp. Shelby Ferrell, Lindsey Thiessen, NC State University Extension
Hemp Bud Rot (Botrytis spp.), Utah State University Extension, Plant Health / IPM
UBA-Fragen: Sind die erhältlichen Mittel gegen Grauschimmel an Erdbeeren …, Umweltbundesamt
§ 9 KCanG – Privater Eigenanbau, Bundesministerium der Justiz
§ 10 KCanG – Schutzmaßnahmen im privaten Raum, Bundesministerium der Justiz
As of: 22 October 2026
This article is for informational purposes only and is not a substitute for medical or therapeutic advice. If you have any health-related questions, please consult a doctor or another qualified professional.