Cannabis Pests & Diseases — Field Guide
Even the best grower can lose a harvest if they react too late. Early diagnosis and biological prevention are key.
TL;DR: Top pests: spider mites, thrips, fungus gnats. Top diseases: powdery mildew, botrytis, root rot. Biological treatments: predatory mites (Phytoseiulus), Bacillus (BTi, subtilis), neem (only in veg).
The 5 Most Common Pests
Spider mites: spots on leaves, fine webs. Treatment: Phytoseiulus persimilis.
Thrips: silvery scratches. Treatment: Amblyseius cucumeris, blue sticky traps.
Fungus gnats: small black flies. Larvae eat roots. Treatment: BTi in the irrigation water.
Whiteflies: yellow leaves, honeydew. Treatment: Encarsia formosa.
Aphids: ladybug larvae, neem.
The 3 Most Dangerous Diseases
Powdery mildew: white coating. Prevention: RH <60%, Bacillus subtilis. Remove immediately if infected.
Botrytis / grey mold: brown flower core in late flowering. Prevention: RH 40–45%, air circulation. In case of infection: cut away affected flowers.
Root rot (Pythium): brown roots. Prevention: oxygen in the water, Trichoderma.
FAQ
Spray neem during flowering?
No. Neem leaves residues and tastes bitter when smoked. Only use in the vegetative stage.
Infestation shortly before harvest?
Generously remove affected flowers. In case of severe mildew/botrytis: use the rest for extraction, not as smokable flowers.