Mould on CBD flower: the signs and their limits

Some signs are enough to reject a product, even without an analysis. Work by Zamir Punja and his team, published in 2019 in Frontiers in Plant Science, describes grey mould caused by Botrytis cinerea: the bracts turn brown and decay, until the whole head is destroyed in advanced stages. The same authors describe powdery mildew as a white felt of mycelium that gives leaves a powdery look, and Penicillium colonies as blue-green.

Two points to add. First, on a dense bud the outside does not show everything: break it open through the middle before you decide. Second, the grey-brown fuzz Punja and his colleagues describe appeared after tissues had been incubated for 48 hours in a closed plastic bag. That is a laboratory method, not a shop-counter test.

Trichomes are the tiny mushroom-shaped glands that cover the bracts and produce resin. To the naked eye they give the flower a frosted, white or silvery look. Early powdery mildew also leaves a white film. We found no study validating a visual method that lets a consumer tell the two apart.

A jeweller's loupe helps a little. Trichomes have a distinct stalk and round head, and they sit mainly on the bracts. A felt of mycelium looks more like a network of tangled filaments, which also spreads over the leaves. But reliable identification requires a microscope or a culture on growth medium, as in the 2019 work where researchers placed 2 mm fragments on agar to see what grew within seven days. Our article on trichomes shows their shape at different stages.

Above all, remember the reverse rule: no visible sign proves nothing. Spores can be present on a flower that looks perfect.

Water activity, written aw, measures the "free" water in a product, the water micro-organisms can use. It runs from 0 (dry) to 1 (pure water). Multiplied by 100, it gives the relative humidity of air in equilibrium with the flower: an aw of 0.62 corresponds to 62% relative humidity in a closed jar.

A 2023 review in Frontiers in Microbiology, by Kimberly Gwinn, Zamir Punja and two co-authors, sets out the rule: sufficient drying brings water activity below 0.70, which prevents most fungi from growing. It immediately adds a caveat: Penicillium and Aspergillus can survive between 0.62 and 0.70. The ASTM D8197 standard recommends a range of 0.55 to 0.65 for dried flower, which keeps a margin below that threshold.

In practice, three habits follow from this threshold. Keep flower in an airtight container, away from heat. If you use a humidity-control pack, choose one between 58 and 62%, inside the ASTM range. And never put a flower that is still damp back in a jar. Temperatures and containers are covered in our article on storing CBD flower, and the step where it is all decided, upstream, in our article on drying and curing.

The 2023 review lists more than 100 fungal species associated with hemp and cannabis, whether pathogens, epiphytes or endophytes. The toxin-producing genera it names first are Aspergillus, Penicillium, Fusarium and Mucor, and Penicillium is the toxigenic fungus most often recovered from the tissues analysed.

The most telling figure concerns ochratoxin A, a mycotoxin produced by some Aspergillus and Penicillium species. According to the same review, it was detected in one third of 142 illicit cannabis samples, and in none of more than 9,000 flower and extract samples from about 300 producers on regulated legal markets. For context, the EFSA panel on contaminants concluded in 2020 that the tolerable weekly intake of 120 ng per kilo of body weight set in 2006 was no longer valid. That opinion concerns food, not cannabis.

Then there is the inhalation of spores. A CDC study of 2016 insurance records found that people who used cannabis were 3.5 times more likely to have a fungal infection than others (confidence interval 2.6 to 4.8). The authors describe an association, not a proven cause: among the patients, cannabis users were more often immunocompromised (43% vs 21%), smoked tobacco more often, and the route of use was unknown. The risk falls first on people whose immune defences are weakened.

In France, no text sets a microbiological limit specific to CBD flower. Quality therefore depends on the testing chosen by the producer and the seller. On a certificate of analysis, look beyond the cannabinoids for:

• a total yeast and mould count, expressed in CFU per gram (colony-forming units);
• testing for named pathogens, for example Aspergillus or Salmonella;
• a mycotoxin screen, aflatoxins and ochratoxin A, with the limit of quantification;
• a batch number identical to the one on the pouch, and a date of analysis.

A certificate that only mentions CBD and THC says nothing about mould. Our guide to reading a certificate of analysis goes through each line, and the laboratory page explains the measurement methods. Pesticide residues, the other contaminant to watch on flower, are covered in our article on MRLs.