Short answer. "Fusarium" covers three different problems in cannabis. Vascular wilt (Fusarium oxysporum) blocks the plant's plumbing so it wilts, often one branch or one side first, with brown streaks inside the stem [3]. Root and crown rot (mainly F. solani) rots roots and the stem base [4]. Fusarium bud rot shows as pinkish-white mould in flowers [2]. None is curable in an infected plant. Remove it, fix the root-zone stress that let it in, sanitise, and start from clean clones and clean media.
What's happening
Fusarium species live in soil, plant debris, unpasteurised media and water. They enter through root tips and wounds (transplant damage, fungus gnat feeding, salt burn) [2][5]. - Wilt: F. oxysporum (formae speciales cannabis and vasinfectum reported on hemp) colonises the xylem. Water flow stops; leaves yellow and wilt from the bottom or on one side. There is typically no obvious root rot [3]. - Crown and root rot: F. solani (primarily), F. equiseti and F. graminearum rot the roots and soften and discolour the crown just above the soil line [4]. In hydroponic cannabis, F. oxysporum, F. solani and F. brachygibbosum caused root browning, crown and pith discolouration, stunting and yellowing [1][2]. - Bud rot: F. solani, F. oxysporum and other species were recovered from cannabis buds and can contribute to yeast-and-mould counts [2]. Stress, warm and wet or alternately wet-and-droughty media, and poor drainage favour wilt and crown rot [3][4][8].
How to recognise it
- Wilt: lower leaves yellow and drop; a branch or one side of the plant wilts during the day and doesn't recover; whole plant can collapse in flower. Cut the stem near the base: brown or reddish-brown streaking in the wood (xylem) is the giveaway.
- Crown rot: dark, soft or sunken lesion at the stem base, sometimes with white-pink fungal growth; roots brown and rotting; plant stunted and yellow.
- Hydro root infection: root browning with crown and pith discolouration [1].
- Bud rot: whitish to pinkish mycelium on or inside buds, often in warm, humid late flower.
- Look-alikes: Pythium (slimy roots that slough, no vascular streaking — see root rot); Verticillium wilt (similar streaking, favoured by consistently moist soils [8]; lab test needed); drought or overwatering wilt (recovers when corrected; no streaking); stem borer damage outdoors.
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Root-zone moisture | Even, with dry-back; no waterlogging, no extreme drought cycles | Overly wet or droughty soils favour Fusarium wilt [8] |
| Solution temperature (hydro) | 18–21 °C (65–70 °F) | Reduces root stress and Pythium co-infection (Supported) |
| Disinfection of reused pots/trays | 10–15 % household bleach solution | Utah State recommendation [3]; clean off soil first |
| Field rotation after wilt | 2–3 years (Utah) to ≥ 4 years (Oregon) out of hemp | Sources differ [3][8] |
| Late-flower RH (bud rot) | 40–50 % | Same targets as botrytis (Supported) |
| Pasteurised media | Use new or pasteurised coco/peat | Unpasteurised coco can carry fungal contaminants [2] |
What to check first
- Cut a stem section at the base of a wilting plant (or a lower branch you can spare). Streaks in the wood = vascular wilt. Clean wood + rotten roots = root rot (Pythium or Fusarium).
- Check the root zone: waterlogging, saucers, compacted media, drought cycles, salt burn from high EC.
- Check for fungus gnat larvae and root aphids in the media; both wound roots.
- Check the source: new clones, reused soil, unpasteurised media, untreated surface water.
- Send a sample to a plant diagnostic lab if you need certainty. Fusarium and Verticillium can only be told apart reliably by culture or molecular test.
How to fix it
Infected plants
- Remove wilting plants with their roots and media, bagged, out of the grow. Utah State recommends removing infected plants with roots [3]. Don't compost them on site.
- In recirculating hydro, isolate the plant's site from the loop immediately.
Cultural and environmental
- Water by weight or sensor; allow dry-back; never leave pots sitting in drainage.
- Avoid root damage: gentle transplants, no cultivating near stems, manage gnats.
- Use new pots and trays, or disinfect reused ones [3]. Clean equipment between rooms or fields [3].
- Use pasteurised media and clean planting stock from a reliable source [7].
- Outdoors: avoid heavy, poorly draining soils [8]; rotate out of hemp as above.
- Late flower: open canopy and keep RH below 50 % against bud rot.
Biological and least-toxic options
Utah State and NC State note that no or very few fungicides are registered for these diseases on hemp [3][5]. Registration varies; check local regulations and labels. - Root-zone biologicals applied preventively: Trichoderma spp., Bacillus spp., Streptomyces spp., Gliocladium/Clonostachys. Supported in other crops for Fusarium suppression; limited cannabis-specific evidencebeing reviewed. - Compost-amended or living soils may suppress some root pathogens (Supported in other crops), but poorly made compost can also introduce them. - Resistant genetics: no tested resistant cannabis cultivars are documented in these sourcesbeing reviewed.
What not to do in flower
- Don't try to nurse a wilting, streaked plant to harvest in a shared space; it sheds spores into media and water.
- Don't drench with unregistered fungicides. Residues, legality, and they don't cure vascular infection.
- Don't harvest or process buds with pink or white internal mould. Fusarium species can produce mycotoxins; treat mouldy flower as wastebeing reviewed [2].
- Don't reuse that plant's media in flower or next cycle.
How to confirm it worked
- No new wilting in neighbouring plants over 2–3 weeks.
- Stem cuts from suspect plants show clean, pale wood.
- Next crop in disinfected pots and fresh media establishes without lower-leaf yellowing or crown lesions.
- If you sent a sample, a lab result tells you which pathogen to plan for.
Common mistakes
- Watering a Fusarium-wilted plant harder because it's wilting.
- Reusing soil and pots from an infected crop without treatment.
- Assuming every wilt is Fusarium (or every root rot is Pythium). Both can co-occur [1].
- Buying cheap, unpasteurised media.
- Ignoring fungus gnats.
Evidence
- Established: F. oxysporum, F. solani and F. brachygibbosum infect cannabis roots and crowns; F. solani and F. oxysporum occur in cannabis buds [1][2].
- Established: wilt and crown rot are distinct diseases with different species and symptoms [3][4].
- Supported: poor drainage, overwatering and drought stress increase risk [3][4][8].
- Contested: rotation length outdoors (2–3 years vs ≥ 4 years) [3][8].
- Supported in other crops, limited cannabis data: biological suppression.
