Cannabis Bud Rot: Why Room Humidity Can Mislead You

The humidity measured in a grow room can differ considerably from the conditions inside a dense cannabis flower. Sheltered tissues may remain humid even when the surrounding air seems well controlled, allowing bud rot to develop before damage becomes visible on the outside. One important cause is Botrytis cinerea, the fungus responsible for grey mould in many crops.

The hidden humidity inside cannabis flowers

Punja et al. (2023) compared temperature and relative humidity inside mature cannabis inflorescences with the surrounding greenhouse air. In the cultivar ‘Pink Kush’, flower interiors averaged 75% relative humidity and approximately 26°C, compared with 55% and 23°C outside. Measurements taken four times daily over two weeks showed a daily pattern: humidity inside the flowers was highest in the morning and lowest in the afternoon. Shorter measurements of other cultivars showed similar patterns.

These averages describe the studied crop, not a humidity level shared by every cannabis flower. For growers, the finding exposes a limitation of room monitoring: a sensor outside the flowers does not necessarily describe the conditions around the tissues where infection develops.

A cannabis bud is an inflorescence, a cluster of small flowers, bracts, leaves and supporting tissues. As it develops, neighbouring structures create sheltered spaces where water vapour released by living tissues can accumulate. Air moving through the canopy, the layer of leaves and stems, may exchange poorly with air deeper inside a dense flower. Differences in flower structure between cultivars therefore matter when assessing how well a ventilation strategy reaches susceptible tissues.

Cooling adds to the problem because relative humidity rises as temperature falls if the amount of water vapour stays similar. Conditions during the transition from light to darkness deserve attention alongside daytime readings.

Infection can begin before damage is visible

Airborne spores of Botrytis cinerea can settle on plant tissues and initiate infection under favourable conditions. Ageing flower parts, wounds and sheltered areas within the inflorescence offer opportunities for establishment. Botrytis is a necrotrophic pathogen: it kills plant cells and obtains nutrients from dead tissue.

Humidity, surface moisture and exposure time influence infection. Williamson et al. (2007) reviewed how very high humidity or surface wetness favours Botrytis in different crops, with requirements varying between host tissues and experimental systems. The 75% average measured inside cannabis flowers should not be read as an infection threshold.

Buirs, Lung and Punja (2025) investigated susceptibility at different stages of cannabis flowering. In their greenhouse inoculation experiments, spores introduced on days 14, 21 or 28 generally produced more disease than those introduced on days 7 or 35, while visible fungal growth appeared later in flowering. An infection can therefore precede obvious external symptoms; the timing reported in these experiments belongs to the studied genotypes and greenhouse conditions rather than a universal bud-rot schedule.

What air circulation can change

Punja et al. (2023) found that additional airflow lowered temperature and relative humidity inside cannabis flowers and reduced total yeast and mould counts. Those counts measure a broader microbial population, however, rather than Botrytis disease alone.

The 2025 study tested the disease outcome more directly. Across three greenhouse comparisons involving the cultivar ‘OG’, enhanced air circulation reduced bud-rot incidence by 66–92% relative to plants without the additional fans. Minor drying damage occurred on tissues nearest the fans, showing why fan placement matters as well as air movement. The results support improving air exchange within and around the crop, but do not provide a universal fan speed or an expected reduction for every grow room.

Recognising and responding to bud rot

A small leaf emerging from a flower that suddenly turns yellow or brown deserves inspection, especially after humid conditions. Discoloured or soft flower tissue, a section that separates unusually easily, and visible grey fungal growth are other warning signs. A single brown leaf does not establish the cause, but local deterioration within an otherwise healthy-looking flower should not be dismissed. By the time mould is visible outside, deeper tissues may already be damaged.

For a broader approach to distinguishing symptoms and possible causes, see What’s Wrong With My Cannabis Plant?

If bud rot is present, carefully remove and discard affected flowers, inspect neighbouring material and address conditions favouring further infection. Trimming out the visibly brown centre does not establish that the remaining flower is safe to consume. Nor do drying and curing make mould-infected cannabis safe; How to Dry Cannabis Flowers Without Ruining Them discusses moisture management after harvest.

Following conditions as flowers mature

Increasing flower density can leave parts of the crop poorly represented by a room sensor. Compare humidity near and within the canopy at different heights, using comparable sensors at the same time, and include observations after lights go out. These measurements help identify variation around the crop; they are not measurements of the air inside a flower.

Look for sheltered areas created by crowded plants or dense foliage, and check whether circulation and moisture removal reach those areas. Reducing canopy density can improve ventilation, but removing leaves also changes photosynthetic capacity and may affect production. Those trade-offs are discussed in Defoliation: Does Removing Cannabis Leaves Actually Increase Yield?

Avoid leaving dead leaves trapped among developing flowers, and give large, dense inflorescences particular attention as harvest approaches.

References

Punja, Z. K., Ni, L., Lung, S., & Buirs, L. (2023). Total yeast and mold levels in high THC-containing cannabis (Cannabis sativa L.) inflorescences are influenced by genotype, environment, and pre-and post-harvest handling practices. Frontiers in Microbiology, 14, 1192035. DOI: 10.3389/fmicb.2023.1192035.

Buirs, L., & Punja, Z. K. (2024). Integrated Management of Pathogens and Microbes in Cannabis sativa L. (Cannabis) under Greenhouse Conditions. Plants, 13(6), 786. DOI: 10.3390/plants13060786.

Buirs, L., Lung, S., & Punja, Z. K. (2025). The epidemiology and management of Botrytis cinerea causing bud rot on greenhouse cultivated cannabis (Cannabis sativa L.). Canadian Journal of Plant Pathology, 47(5), 443–463. DOI: 10.1080/07060661.2025.2478250.

Williamson, B., Tudzynski, B., Tudzynski, P., & van Kan, J. A. L. (2007). Botrytis cinerea: the cause of grey mould disease. Molecular Plant Pathology, 8(5), 561–580. DOI: 10.1111/j.1364-3703.2007.00417.x.

Featured photograph: cropped from Figure 14e in Buirs & Punja (2024), Plants, 13(6), 786. Licensed under CC BY 4.0. Cropped from the original figure; no other changes.