DISCLAIMER: Cannabis cultivation is illegal in many jurisdictions. This content is provided for educational purposes only. Always research and comply with your local laws and regulations before cultivating cannabis.
Diseases in cannabis are caused by fungi, bacteria, viruses, and viroids. Unlike pests, which are visible (with magnification), pathogens operate at the microscopic level and often produce symptoms only after the infection is well established. Early detection and environmental management are critical.
Key Principle: Diseases thrive under specific environmental conditions. Correcting the environment is often more effective than applying fungicides. Prevention through environmental control is always preferable to treatment.
The most common fungal disease in cannabis cultivation. Powdery mildew (PM) affects indoor, outdoor, and greenhouse grows worldwide.
| Attribute |
Detail |
| Pathogen Type |
Obligate biotrophic fungus (requires living host tissue) |
| Primary Species |
Podosphaera macularis (most common on cannabis); Golovinomyces cichoracearum |
| Infection Mechanism |
Spores (conidia) land on leaf surface; germinate; fungal structures penetrate epidermal cells to extract nutrients |
| Survival |
Overwinters as dormant mycelium in plant tissue; spores survive on surfaces for weeks |
| Attribute |
Detail |
| Initial Sign |
Small white powdery spots on upper leaf surfaces |
| Progression |
Spots expand to cover entire leaf; may spread to stems and flowers |
| Leaf Underside |
May show corresponding yellowing or necrotic patches |
| Key Diagnostic |
White powder wipes off with finger (distinguishes from downy mildew, which is systemic) |
| Condition |
Threshold |
| Temperature |
68-77°F (20-25°C) optimal; survives 50-90°F |
| Humidity |
Moderate to high RH (50-70%); spores germinate best at high humidity, but colonies survive at low humidity |
| Air Circulation |
Poor airflow strongly favors establishment |
| Day/Night Swings |
RH spikes at night trigger spore germination |
Critical: Powdery mildew spores are always present in the air. Infection occurs when environmental conditions favor germination. You cannot eliminate spores; you can only create an environment hostile to them.
| Method |
Agent |
Application |
Timing |
Notes |
| Preventive |
Maintain RH <50% in flower; good airflow |
Environmental control |
Throughout grow |
Most effective long-term strategy |
| Preventive |
Bacillus amyloliquefaciens (Serenade, Cease) |
Foliar spray per label |
Any stage |
Colonizes leaf surface; prevents PM spore germination |
| Preventive |
Potassium bicarbonate (MilStop) |
Foliar spray per label |
Any stage (avoid buds) |
Raises leaf surface pH; contact kill of established colonies |
| Curative |
Remove infected tissue |
Prune and bag immediately |
Any stage |
Essential first step; prevents spore release |
| Curative |
Hydrogen peroxide (3% diluted to 1:10) |
Foliar spray |
Vegetative only |
Contact kill; organic; short residual |
| Curative |
Sulfur burners |
Per label rate |
Vegetative only |
Highly effective; never use on flowers (affects flavor, safety) |
If powdery mildew reaches flowers:
- Affected buds are not safe for consumption — PM spores can cause respiratory issues when inhaled
- Remove and destroy affected buds (do not compost near grow space)
- Do not attempt to "wash" or "brush off" PM from buds
- The decision to harvest early or destroy the plant depends on infestation severity
The most destructive flowering disease. Botrytis destroys flowers from the inside out, often going undetected until significant damage has occurred.
| Attribute |
Detail |
| Pathogen Type |
Necrotrophic fungus (kills host tissue, then feeds on dead cells) |
| Infection Sites |
Enters through wounds, pruning cuts, or naturally through dense flower interiors |
| Survival |
Sclerotia (hardened fungal structures) survive in soil and plant debris for years; airborne conidia spread infection |
| Attribute |
Detail |
| Initial Sign |
Small brown or gray area within a cola; often not visible from outside |
| Progression |
Affected bud section becomes mushy and gray; gray fungal growth visible on surface |
| Internal |
When the bud is split open, interior is brown, slimy, and may have visible fungal growth |
| Key Diagnostic |
Affected bud section pulls apart easily; interior is brown and decayed while exterior may appear normal |
| Condition |
Threshold |
| Temperature |
55-70°F (13-21°C) optimal; can grow at 32-86°F |
| Humidity |
>55% RH; >70% RH extremely favorable |
| Air Circulation |
Stagnant air inside dense buds creates microclimate for infection |
| Bud Density |
Denser buds have higher risk (less internal airflow) |
| Method |
Agent |
Application |
Timing |
Notes |
| Preventive |
Maintain RH 40-45% during flowering |
Dehumidifier + airflow |
Weeks 3+ of flower |
Single most effective preventive measure |
| Preventive |
Oscillating fans for canopy air movement |
Multiple small fans |
Throughout flower |
Eliminates microclimates within bud sites |
| Preventive |
Trichoderma harzianum |
Soil drench at transplant |
Vegetative/early flower |
Beneficial fungus that competes with botrytis |
| Curative |
Remove affected buds immediately |
Cut 1-2 inches below visible rot; bag and remove from grow space |
As soon as detected |
Do not attempt to salvage rotted buds |
| Curative |
Lower humidity immediately |
Increase dehumidifier output; increase exhaust |
Emergency response |
Slows spread while affected tissue is removed |
Critical: Botrytis spores from rotted buds will spread to nearby flowers. Remove affected material carefully — do not shake or disturb rotted buds. Bag them before removing from the grow space. There is no chemical cure for established botrytis inside a flower.
Soil and waterborne pathogens that attack root systems, reducing the plant's ability to absorb water and nutrients.
| Attribute |
Detail |
| Pathogen Type |
Oomycete (not a true fungus; more closely related to algae) |
| Primary Species |
Pythium aphanidermatum, P. ultimum |
| Favored Conditions |
Water temperature >72°F (22°C); low dissolved oxygen; overwatering; contaminated systems |
| Transmission |
Contaminated water, growing medium, tools; recirculating hydroponic systems amplify spread |
| Attribute |
Detail |
| Pathogen Type |
True fungus; soil-borne |
| Primary Species |
Fusarium oxysporum, F. solani |
| Favored Conditions |
Warm soil; plant stress; wounds; contaminated medium |
| Transmission |
Soil-borne; systemic in some strains; survives as chlamydospores for years |
| Symptom |
Pythium |
Fusarium |
| Root Appearance |
Brown, slimy, mushy; outer root cortex sloughs off leaving stringy core |
Brown to reddish-brown roots; may show vascular browning in stem cross-section |
| Above Ground |
Wilting despite wet medium; yellowing lower leaves; stunted growth |
Wilting; yellowing; may show one-sided wilting (vascular blockage) |
| Odor |
Foul, decaying smell |
Earthy to mild musty odor |
| System Speed |
Rapid (2-5 days from onset to severe damage in hydroponics) |
Slower (1-3 weeks) |
| Key Diagnostic |
Brown slimy roots + foul smell + hydroponic system |
Vascular browning in stem + persistent despite environmental correction |
| Method |
Agent |
Application |
Timing |
Notes |
| Preventive |
Maintain water temperature <68°F (20°C) |
Water chiller (hydroponics) |
Throughout grow |
Single most important factor for Pythium prevention |
| Preventive |
Maintain dissolved oxygen >6 ppm |
Adequate air stones; water movement |
Throughout grow (hydroponics) |
Roots need oxygen to resist infection |
| Preventive |
Bacillus amyloliquefaciens (Hydroguard, Great White) |
Root zone application at transplant |
Vegetative/early flower |
Colonizes root surface; outcompetes pathogens |
| Preventive |
Avoid overwatering |
Allow medium to dry appropriately |
Throughout grow |
Best prevention in soil/coco |
| Curative |
Hydrogen peroxide (11% for hydroponics; 3% for soil drench) |
Reservoir treatment or soil drench |
At detection |
Kills pathogens and some roots; use carefully |
| Curative |
Sterilize and replace reservoir |
Full system clean |
Moderate to severe |
Essential for recirculating systems |
| Curative |
Remove dead roots |
Trim affected tissue carefully |
At detection |
Promotes new healthy root growth |
Seedling disease complex that kills young plants before they can establish.
| Attribute |
Detail |
| Pathogen Type |
Multiple soil-borne organisms (Pythium, Rhizoctonia, Fusarium) |
| Infection Site |
Stem at or just below soil line |
| Survival |
Soil-borne; survives for years as spores or sclerotia |
| Attribute |
Detail |
| Initial Sign |
Thin, water-soaked area at stem base near soil line |
| Progression |
Stem becomes constricted and pinched; seedling topples over |
| Speed |
Extremely rapid; seedling can collapse within 24 hours of first symptom |
| Key Diagnostic |
Healthy seedling one day, collapsed and dead the next; stem is thin and brown at soil line |
| Condition |
Threshold |
| Overwatering |
Consistently saturated medium |
| Temperature |
Cool conditions (<65°F / 18°C) |
| Contamination |
Non-sterile medium; infected trays or tools |
| Poor Drainage |
Standing water in propagation trays |
| Measure |
Implementation |
| Sterile medium |
Use certified sterile seed-starting mix; do not use garden soil |
| Warmth |
Maintain 72-80°F (22-27°C) with seedling heat mat |
| Air circulation |
Gentle oscillating fan (low setting) |
| Moderate watering |
Water from below (bottom-watering); keep surface slightly dry |
| Mycorrhizae |
Apply at germination; beneficial fungi compete with pathogens |
| Clean trays |
Sterilize propagation trays with 10% bleach solution between uses |
Note: There is no cure for damping off once symptoms appear. Affected seedlings should be removed and destroyed. Focus on preventing infection in remaining seedlings.
Systemic vascular disease that blocks water transport within the plant.
| Attribute |
Detail |
| Pathogen Type |
True fungus; soil-borne; systemic |
| Infection Mechanism |
Enters through roots; colonizes xylem (water-conducting tissue); blocks water flow |
| Survival |
Chlamydospores survive in soil for 10+ years |
| Key Fact |
Once soil is contaminated, it is effectively permanently contaminated for cannabis |
| Attribute |
Detail |
| Initial Sign |
Wilting on one side of the plant or one branch (asymmetric wilting) |
| Progression |
Yellowing and browning of lower leaves; wilting progresses upward |
| Stem Cross-Section |
Vascular tissue (ring just inside the bark) is brown or reddish — healthy vascular tissue is white or pale green |
| Key Diagnostic |
Asymmetric wilting + brown vascular tissue in stem cross-section |
| Approach |
Detail |
| Prevention |
Use sterile, soilless medium; avoid contaminated soil |
| Prevention |
Apply Trichoderma and Bacillus products at transplant |
| Prevention |
Do not reuse medium from infected plants |
| Curative |
No cure exists — affected plants should be destroyed |
| Curative |
Do not replant cannabis in contaminated soil/medium |
Soil-borne vascular disease similar to Fusarium wilt but with different environmental preferences.
| Attribute |
Detail |
| Initial Sign |
Yellowing between leaf veins (interveinal chlorosis) starting on lower leaves |
| Progression |
Leaves turn brown and die; wilting during warm periods, recovery at night |
| Key Diagnostic |
V-shaped yellowing pattern on leaf margins; brown vascular tissue in stem |
- No chemical cure; management is prevention-based
- Avoid planting in soil with history of Verticillium (affects many crops)
- Use soilless medium for container growing
- Resistant rootstocks are not available for cannabis
The most significant viral-like threat to modern cannabis cultivation. HpLVD is a sub-viral infectious agent (viroid) that reduces potency and yield.
| Attribute |
Detail |
| Pathogen Type |
Viroid (circular RNA molecule; smaller and simpler than a virus) |
| Discovery |
First identified in cannabis in 2019; previously known in hops |
| Transmission |
Contaminated tools; mechanical contact; infected propagation material |
| Latency |
Plants can carry HpLVD asymptomatically for months or years |
| Attribute |
Detail |
| Vegetative |
Often no visible symptoms; plants appear normal |
| Flowering |
Smaller, less dense buds; reduced trichome production; reduced terpene and cannabinoid content |
| Overall |
Reduced vigor; shorter internodes; abnormal branching; brittle stems |
| Key Diagnostic |
"Dudding" — plants produce noticeably less resin and lower potency than expected for the cultivar |
| Approach |
Detail |
| Testing |
PCR testing of mother plants; commercial tests available |
| Prevention |
Sterilize tools between plants (10% bleach or 70% isopropyl alcohol) |
| Prevention |
Source genetics from tested, clean mother plants |
| Curative |
No cure — infected plants must be destroyed |
| Reversion |
Some plants show symptom reversion under stress (heat), but the viroid is not eliminated |
Critical: HpLVD is a major concern for commercial cultivators. If plants that normally produce heavy resin and high potency suddenly underperform, consider HpLVD testing. This is especially important for mother plants and commercial propagation.
| Disease |
Pathogen Type |
Primary Location |
Key Diagnostic |
Speed |
Primary Prevention |
| Powdery Mildew |
Fungus |
Leaves → flowers |
White powder on leaf surface |
Fast (days-weeks) |
RH <50%; airflow |
| Botrytis (Bud Rot) |
Fungus |
Flower interiors |
Brown mushy buds; gray mold |
Moderate (days) |
RH 40-45%; airflow |
| Pythium Root Rot |
Oomycete |
Root zone |
Brown slimy roots + foul smell |
Fast (2-5 days hydro) |
Water temp <68°F; oxygen |
| Fusarium Wilt |
Fungus |
Vascular system |
Asymmetric wilting + brown vascular ring |
Slow (1-3 weeks) |
Sterile medium |
| Damping Off |
Multiple |
Seedling stem base |
Collapsed seedling; pinched stem at soil |
Very fast (<24 hrs) |
Sterile medium; warmth |
| Verticillium Wilt |
Fungus |
Vascular system |
V-shaped leaf yellowing; brown vascular ring |
Slow (weeks) |
Soilless medium |
| Hop Latent Viroid |
Viroid |
Systemic |
Reduced resin, potency, and bud density |
Very slow (months) |
Clean genetics; tool sterilization |
When you observe plant decline, work through this sequence:
1. Is there visible mold, powder, or fungal growth on leaves/buds?
├── YES → Powdery Mildew (white powder) or Botrytis (gray/brown on buds)
└── NO → Continue to 2
2. Are roots brown, slimy, or foul-smelling?
├── YES → Pythium or Fusarium root rot
└── NO → Continue to 3
3. Is wilting asymmetric (one side/branch affected)?
├── YES → Fusarium or Verticillium wilt (check vascular tissue)
└── NO → Continue to 4
4. Is it a seedling that collapsed at the soil line?
├── YES → Damping Off
└── NO → Continue to 5
5. Are buds noticeably smaller, less resinous, and lower potency than expected?
├── YES → Consider HpLVD testing
└── NO → Review pest identification or environmental/nutrient issues
→ See [Pest Identification](/cultivation/pest-identification)
→ See [Troubleshooting Guide](/cultivation/troubleshooting)
See Also: /cultivation/pest-identification | /cultivation/pests-diseases | /cultivation/troubleshooting | /cultivation/indoor/environment | /cultivation/indoor/grow-guide