Short answer. Diseases rarely appear from nowhere. They arrive on clones, tools, hands, shoes, media and water, then the climate decides whether they take off [3]. Quarantine and test anything coming in. Work from clean areas to dirty ones. Clean first, then disinfect with a chemical that actually works on your target: for hop latent viroid that means bleach, hypochlorous acid or heat, because 70 % alcohol and hydrogen peroxide do not inactivate it [1]. Treat recirculating water. Keep humidity and airflow out of the zone where mildews and moulds win.
What's happening
Disease needs three things: a pathogen, a susceptible host, and the right environment (the disease triangle). Sanitation removes the pathogen; genetics and plant health change the host; climate control removes the environment. You need all three because none is perfect.
Main entry routes in cannabis: - Clones and mothers: powdery mildew, HLVd, Fusarium, pests [2][3][8]. - Tools and hands: HLVd and other sap-transmitted agents [1][2]. - Media: unpasteurised coco fibre carries fungal communities that can contaminate flowers [3]. - Water: Pythium zoospores and HLVd in recirculating systems [4][8]. - Air: powdery mildew and botrytis spores [6][7]. - People and pets moving between outdoor gardens and indoor rooms.
How to recognise a sanitation problem
- The same disease returns each cycle in the same room or on the same cultivar.
- Problems start a few weeks after new clones arrive.
- Disease follows the path of a person, a tool or a water line.
- Root problems appear system-wide in a shared reservoir.
Numbers that matter
Disinfectant classes
Always clean first: scrub off soil, root mats, sap and biofilm with detergent and water. Organic matter neutralises most disinfectants, especially chlorine.
| Class | Typical use concentration | Contact time | Works on | Notes |
|---|---|---|---|---|
| Sodium hypochlorite (household bleach) | 10 % v/v of household bleach (≈ 0.5–0.8 % NaOCl depending on starting strength); 10–20 % for HLVd [1][8] | 10 min on hard surfaces; tool dip time for HLVdbeing reviewed | Fungi, bacteria, oomycetes, viruses, HLVd [1] | Corrodes metal; rinse tools after; make fresh daily (degrades in light and with organics). 10–15 % for pots/trays [5] |
| Hypochlorous acid | ≥ 1000 ppm for HLVd on tools [1] | being reviewed | Broad, including HLVd [1] | Less corrosive than bleach; unstable, check strength |
| Heat | Tool blades 160 °C (320 °F) for 10 min [1] | 10 min | Including HLVd [1] | Note: autoclaving did not inactivate HLVd in Oregon State tests [1] |
| Alcohol (ethanol, isopropanol) | 70 % | ≥ 30 s wet | Most fungi and bacteria | Not effective against HLVd [1]; flammable |
| Hydrogen peroxide / peracetic acid | Per label | Per label | Fungi, bacteria, spores, biofilm | Hydrogen peroxide not effective against HLVd [1]; peracetic acidbeing reviewed |
| Potassium peroxymonosulfate | Per label | Per label | Broad | Not effective against HLVd in Oregon State tests [1] |
| Quaternary ammonium | Per label | Per label (often 10 min) | Many fungi, bacteria | Efficacy on HLVd not shownbeing reviewed; leaves residue; don't let it contact roots |
Safety: wear gloves and eye protection when mixing disinfectants; ventilate. Never mix bleach with acids (pH-down, vinegar, peracetic acid, some descalers) or with ammonia or quaternary ammonium cleaners: this releases toxic chlorine or chloramine gas. Don't mix disinfectants with each other at all. Store concentrates labelled, away from nutrients and pH-down. Rinse before plants or roots touch treated surfaces.
Climate guard rails
| Parameter | Target | Protects against |
|---|---|---|
| RH, veg | 55–70 % | PM germination at the high end |
| RH, mid–late flower | 40–50 % | Botrytis, PM [6][7] |
| Night RH spike at lights-off | < 10 percentage points | Botrytis, PM |
| Solution temperature | 18–21 °C (65–70 °F) | Pythium [4] |
| Dissolved oxygen | ≥ 6 mg/L, ideally near saturation (~9 mg/L at 20 °C) | Pythium [4] |
| Airflow | Leaf flutter through all canopy layers | PM, botrytis |
What to check first
- Where did the last problem come from? Clones, media, water, tools, people.
- Which disinfectant do you use, and does it work on what you're worried about? (Alcohol for HLVd is the classic gap.)
- Is there a clean-to-dirty flow? Do you go from flowering rooms back to mothers?
- Are mothers tested?
- What does the night-time RH log say?
How to fix it: a working sanitation programme
1. Quarantine incoming plants
- Keep all new clones and plants in a separate space for at least 2–3 weeks (Grower practice; long enough for a pest generation and latent PM to show).
- Inspect with a 30–60x loupe: leaf undersides for mites, thrips, whitefly eggs, PM.
- Test for HLVd (roots and leaves) before they enter the mother room [2][8].
- Prefer seeds or tested clean stock from tissue culture over untested cuttings.
2. Clean room workflow
- Order of work: clone/propagation → mothers → veg → flower → drying. Never go backwards in a day without changing clothes and washing hands (Grower practice).
- Dedicated tools per room, or disinfect between rooms. For HLVd-sensitive stock (mothers), disinfect blades between every plant with bleach, hypochlorous acid or heat, or rotate several pairs through a dip [1].
- Wash hands or change nitrile gloves between plants when taking cuttings.
- Clean clothing or coveralls; shoe covers or dedicated footwear for mother rooms.
- Keep pets and outdoor garden clothing out of the grow.
- Sticky traps at doors and inside rooms to monitor what comes in (see thrips, fungus gnats).
3. Between cycles
- Remove all plant material, media and debris; bag it out.
- Dry-clean then wash every surface with detergent; scrub biofilm in reservoirs, lines, drippers, trays.
- Disinfect with an appropriate class; respect contact time; rinse.
- Run a disinfectant (or appropriate line cleaner) through irrigation lines; flush with clean water.
- Leave the room empty and dry for several days if possible; a plant-free period also starves some pests (Grower practice; e.g. root aphids — see root aphids).
- Replace filters and clean ducting and fan blades that collected spores.
4. Media and water
- Use new or pasteurised media; don't reuse media from diseased crops [3][5].
- Source water: municipal or well water is usually low-risk; surface water and rainwater collection can carry Pythium and Phytophthora (Supported in other crops).
- Recirculating systems: choose a treatment class: UV, ozone, chlorination, copper ionisation, slow sand filtration or oxidisers [4]. Each can affect pH, micronutrients and beneficial microbes [4]. Don't combine an oxidiser with live biological inoculants in the same water.
- Opaque, lidded reservoirs; no light on solution.
5. Plant-level prevention
- Resistant genetics where data exist (e.g. powdery mildew PM1/PM2 — see powdery mildew).
- Don't overwater; manage dry-back.
- Defoliate for airflow before week 3 of flower.
- Avoid wounding; manage chewing and sucking pests (caterpillars, spider mites).
- Preventive beneficials where legal and suitable: root-zone microbes in non-sterile systems, predatory mites for pests.
What not to do in flower
- Don't spray disinfectants on or near flowering plants. They are for surfaces, tools and water.
- Don't fog or "bomb" a room with plants in it.
- Don't bring clones or mothers into a flowering room to "save space".
- Don't start a deep clean during late flower that raises humidity (mopping a room full of plants at lights-off is a botrytis invitation).
How to confirm it worked
- The repeating problem doesn't repeat next cycle.
- Mothers test HLVd-negative on consecutive tests.
- Sticky-trap counts stay flat after new plants enter.
- Reservoirs stay clear, roots white.
Common mistakes
- Disinfecting dirty surfaces (no cleaning step).
- Using alcohol for viroids.
- Mixing bleach with pH-down or other cleaners.
- Old, diluted bleach that has lost strength.
- Skipping quarantine "because it's from a friend".
- Going from the flower room to the mother room.
- Reusing coco or soil from a crop with root disease.
Evidence
- Supported: 10–20 % bleach, ≥ 1000 ppm hypochlorous acid and dry heat inactivate HLVd; ethanol, hydrogen peroxide, peroxymonosulfate and autoclaving did not [1]. Contact-time details for tool dips need confirmationbeing reviewed.
- Established: clones, tools and recirculating water spread HLVd and root pathogens [1][2][4][8].
- Supported: unpasteurised coco carries fungal contaminants [3].
- Established: bleach mixed with acids or ammonia releases toxic gases (general chemical safety).
- Grower practice: quarantine length, work order, plant-free fallow periods.
