Diseases
Mildew, botrytis, root rot, viruses and viroids, sanitation.
Botrytis (bud rot / grey mould) in cannabis
Bud rot is Botrytis cinerea rotting dense colas from the inside, favoured by cool-to-moderate temperatures (10–27 °C, optimum near 20 °C), high humidity and free water on tissue. There is no cure in flower: prevent it with RH below about 50 % in late flower, dew-point control at lights-off, airflow, open canopies and fast removal of infected buds.
Fusarium in cannabis: wilt, crown rot and bud rot
Fusarium fungi cause three different problems in cannabis: vascular wilt (F. oxysporum), root and crown rot (mainly F. solani), and a pinkish-white bud rot. There is no cure once a plant is infected; remove it, stop root-zone stress, sanitise pots and water, and source clean clones and media.
Hop latent viroid (HLVd, 'dudding') and other cannabis viruses
Hop latent viroid is a tiny infectious RNA that spreads through sap on tools, hands, clones, recirculating water and seed, and causes 'dudding': stunting, brittle stems and small, low-resin flowers. There is no cure in a plant. Test mothers (roots and leaves) by RT-PCR/RT-qPCR, discard positives, disinfect tools with bleach or heat (not alcohol), and use meristem tissue culture to rescue valuable genetics.
Powdery mildew in cannabis
Powdery mildew is a leaf-surface fungus (mainly Golovinomyces ambrosiae) that thrives in still, humid, moderate-temperature canopies and spreads by airborne spores. Prevent it with airflow, stable humidity, clean clones and resistant genetics; treat early in veg with least-toxic contact options, because once it reaches flowers there is no clean fix.
Root rot and Pythium in cannabis
Pythium root rot is a water mould that thrives in warm, low-oxygen root zones and swims between plants in recirculating water. Brown, slimy roots whose outer layer slides off, plus wilting with a wet root zone, are the signature. Fix the root-zone conditions first: solution at about 18–21 °C (65–70 °F), high dissolved oxygen, clean water and no overwatering; then remove dead roots and re-establish beneficial microbes.
Sanitation and disease prevention for cannabis grows
Most cannabis disease problems are brought in on clones, tools, hands, media or water, and then helped along by humid, stagnant rooms. Quarantine and test incoming plants, work clean-to-dirty, clean then disinfect with the right chemical class for the pathogen (bleach or hypochlorous acid or heat for viroids; alcohol and peroxide do not inactivate HLVd), treat recirculating water, and keep the climate out of the disease zone.
Septoria leaf spot (yellow leaf spot) in cannabis
Septoria leaf spot (Septoria cannabis) starts as small brown spots with bright yellow halos on lower, older leaves and climbs the plant in wet, humid weather by rain and water splash. It is mainly an outdoor and greenhouse problem; manage it with clean lower canopies, dry foliage, mulch, airflow and debris removal, and protectants only where registered.
