Bud Rot: Why Cannabis Flowers Create the Perfect Microclimate for Botrytis

Featured photo: 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.

Bud rot can develop inside a cannabis flower before serious damage becomes visible on its surface. One important cause is Botrytis cinerea, the fungus responsible for grey mould in many crops. Its development depends on the interaction between the pathogen, susceptible tissue, local moisture conditions and the duration of exposure.

Dense flowers can retain a humid microclimate that differs from the surrounding room. Understanding that difference helps explain why an apparently acceptable reading at a wall-mounted sensor does not rule out conditions favourable to infection inside an inflorescence.

Room measurements and flower microclimate

Temperature and relative-humidity measurements in the growing room provide useful information, but do not necessarily represent the interior of a dense flower. In one cannabis study, sensors placed inside mature inflorescences recorded average relative humidity of about 75%, compared with approximately 55% in the surrounding air. Flower interiors were also slightly warmer (Punja et al., 2023).

Those measurements describe the studied conditions rather than a fixed difference for every cultivar or room. They demonstrate why the location of a measurement matters: the air around susceptible tissue may be more humid than the air reaching the main room sensor.

How flowers retain humid air

Leaves and floral tissues release water through transpiration. In an open space, moving air can carry that vapour away, whereas dense floral structures restrict exchange. Sheltered spaces between neighbouring tissues allow moisture to escape more slowly, and temperature changes can bring the local air closer to saturation.

Air moving across the canopy does not necessarily penetrate the centre of every flower. Flower architecture, surrounding foliage and the arrangement of the crop all influence how readily moist air is replaced.

Conditions that favour Botrytis

Botrytis cinerea produces airborne spores that can land on plant tissue. Successful infection requires favourable conditions, with high relative humidity and free moisture supporting germination and infection. Research across crops has linked prolonged humidity above roughly 90–93%, or periods of surface wetness, with increased risk, although infection can occur across a wider range of conditions (Williamson et al., 2007).

That range should not be treated as a universal cannabis safety threshold. Average humidity, the duration of wet conditions, tissue condition and local variation all matter. The average flower-humidity measurement described above is not itself an infection threshold.

Botrytis is a necrotroph: it kills plant cells and uses the dead tissue. Senescing flower parts, wounds and damaged tissues can provide favourable infection sites. Dense, ageing inflorescences can therefore combine susceptible tissue with restricted ventilation.

Recognising a problem inside the flower

Early signs may include a small sugar leaf turning brown, unusual yellowing within an otherwise green flower, soft or discoloured tissue, grey fungal growth or a section that separates unusually easily. By the time grey mould is obvious externally, deeper tissue may already be affected.

These observations warrant investigation rather than diagnosis from one leaf or photograph alone. The broader guide to assessing an unhealthy cannabis plant explains why symptom patterns need to be considered alongside the growing environment and condition of the plant.

Cultivar differences and flower structure

Dense flowers are often a cultivation goal, but their smaller internal air spaces can also restrict ventilation. Cannabis research has identified cultivar differences in flower microclimate and microbial colonisation, so plants sharing a room do not necessarily experience identical risk.

Genotype should therefore be considered alongside the room conditions. A management approach that controls moisture adequately in one flower structure may not provide the same conditions within a denser inflorescence.

Air circulation and canopy spacing

In cannabis experiments, additional airflow around flowering plants reduced temperature and relative humidity within flowers and lowered total yeast and mould populations. These are useful observations about the tissue environment, but a reduction in total microbial counts is not the same as proof of protection against every Botrytis infection.

The practical objective is air exchange throughout the crop rather than maximum wind speed over its surface. Crowded canopies can slow air movement, retain humidity and create sheltered areas. Reducing density may improve exchange, although commercial decisions also need to account for production per square metre.

Canopy management should match the capacity of the climate system. Where leaf removal is being considered, the guide to defoliation and its trade-offs examines that intervention separately. It should not be treated as an automatic substitute for humidity control or adequate air exchange.

Changes after lights go out

Relative humidity rises as air cools if its water-vapour content remains similar. Conditions after lights go out can therefore differ substantially from a daytime reading. Dense flowers can respond more slowly than the surrounding air, and cooler tissues may experience very high local humidity or condensation.

Following both daytime and night-time behaviour is more informative than relying on a single room average. The duration of humid conditions also matters: a brief change and a flower remaining humid for many hours are different exposures.

Handling affected material

Clearly infected flowers can act as a source of spores. Remove affected material carefully and inspect neighbouring flowers rather than assuming that cutting away the visibly brown centre leaves clean tissue behind. Fungal growth may extend beyond the immediately visible damage.

Dead leaves and damaged material should not be left trapped within dense flowers, where senescent tissue can support the pathogen. Environmental correction remains necessary alongside removal; taking out affected flowers does not explain or resolve the conditions that allowed infection.

Reducing risk through the flowering period

As flowers become larger and denser, environmental management becomes especially important. Useful priorities include controlling humidity, maintaining air exchange, avoiding wet flowers, allowing adequate spacing, removing dead tissue, keeping the crop clean and inspecting dense inflorescences regularly.

These practices address interacting risk factors rather than guaranteeing a disease-free crop. Neither a particular room-humidity reading nor visible movement of the outer leaves establishes that the interior of every flower is adequately ventilated.

Moisture management also continues after harvest. The guide to controlled drying covers that separate stage; drying should not be treated as a way to establish the safety of already infected material.

Assessing bud-rot risk means considering the pathogen, tissue susceptibility, local microclimate and time together. Measurements and observations close to the crop help place room-level readings in context, particularly during late flowering and changes between light and dark periods.

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. https://doi.org/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. https://doi.org/10.3390/plants13060786.

Williamson, B. et al. (2007). Botrytis cinerea: the cause of grey mould disease. Molecular Plant Pathology. https://doi.org/10.1111/j.1364-3703.2007.00417.x.