Cannabis Bud Rot Prevention Guide
Learn how to prevent cannabis bud rot and keep your THC products fresh and potent.
Short answer
Powdery mildew on cannabis buds cannot be cured at harvest. The fungus grows on the plant surface and sends feeding structures into tissue. A wash removes surface spores and some hyphae. It does not clean the inside of a bud. Buds with visible mildew on the calyxes should be discarded. Buds with a small patch on outer sugar leaves can be trimmed, washed, dried, and tested.
Cannabis Bud Rot Prevention Guide
What powdery mildew is
The species on cannabis is Golovinomyces ambrosiae in most North American reports. It is an obligate parasite. It needs living plant tissue and does not need free water on leaves. It grows at 60 to 80 F and 40 to 60 percent relative humidity when airflow is poor.
Powdery mildew is not bud rot. Bud rot (Botrytis) turns tissue brown and soft. Powdery mildew looks like white or gray flour on leaves, stems, and calyxes. Both can appear at the same time.
Cannabis Botrytis Prevention Tips: A Buyer's Guide to Mold-Free Buds
Pre-harvest treatment
Options for flower still on the plant:
cannabis bud rot early signs and treatment
- Potassium bicarbonate: contact killer, no residue. Mix per label. Reapply each 7 to 10 days.
- Hydrogen peroxide 3 percent: 1 part peroxide to 9 parts water. Kills spores on contact. Use in the last two weeks.
- Bacillus amyloliquefaciens or Bacillus subtilis: bacteria that colonize leaf surfaces.
- Sulfur: works in the vegetative stage. Do not use within two weeks of harvest. Do not use above 85 F.
- Neem oil or horticultural oil: not for late flower. Oils leave taste and residue.
Remove and bag infected leaves. Lower humidity below 50 percent. Add airflow between plants. Keep night temperatures above 60 F.
Post-harvest wash
A wash reduces spores on the outside of buds. Steps used by growers:
- Bucket 1: plain water, 65 to 70 F.
- Bucket 2: water plus 1 cup 3 percent hydrogen peroxide per 5 gallons.
- Bucket 3: water plus 1 teaspoon baking soda per gallon.
- Bucket 4: plain water rinse.
Dip each branch for 30 to 60 seconds per bucket. Agitate. Hang and dry with a fan. Drying must finish fast. Wet buds grow mold.
Limits: the wash does not kill powdery mildew inside the bud. It adds water. If the room is humid, the wash can make the problem worse.
Trim and discard rules
- Mildew limited to fans and outer sugar leaves: trim those leaves, wash the bud.
- Mildew inside the calyxes or on the stem: discard.
- Mildew plus rot: discard. Do not compost near the grow.
Extraction and use
Solvent and CO2 extraction with a 0.22 micron filter removes spores and hyphae from the oil. Rosin pressing does not. Solventless hash made from tumbled trim can concentrate spores. Mold counts in edibles still face state limits.
Testing and safety
State labs test flower for total yeast and mold. Limits differ by state. Inhaling mold spores can cause coughing, sinus problems, and lung irritation. Risk is higher for people with asthma or weak immune systems. Do not smoke or vape bud with visible mildew. When in doubt, throw it out.