Cannabis Plant Pests and Diseases Guide

Spider Mites on Cannabis

Spider mites are the most feared cannabis pest. These tiny arachnids pierce cells and suck out contents.

They leave white or yellow dots on leaf surfaces.

Organic Treatments

Use these methods to eliminate spider mites.

• Neem oil spray: Mix 5ml per litre with dish soap every 3-5 days.
• Insecticidal soap: Kills on contact when applied to leaf undersides.
• Predatory mites: Release Phytoseiulus persimilis as biological control.
• Water spray: A strong blast knocks mites off outdoor plants.

Spider mites thrive above 26°C with low humidity. Lower temperature and raise humidity during veg.

Fungus Gnats in Cannabis Soil

Fungus gnats are small black flies around the soil surface. Their larvae feed on organic matter and young roots.

Breaking the Lifecycle

These treatments target fungus gnats at every stage.

• Let soil dry between waterings: Gnats need moisture to breed.
• Yellow sticky traps: Place near soil level to catch adults.
• Diatomaceous earth: Sprinkle on the soil surface to damage larvae.
• BTi (Mosquito Bits): A biological larvicide added to water.
• Sand or perlite top layer: Prevents egg-laying in soil.

Overwatering is the primary cause. Proper watering prevents most infestations.

Aphids on Cannabis Plants

Aphids are soft-bodied insects that cluster on new growth. They excrete sticky honeydew that attracts mold.

Treatment Options

Control aphids with these proven treatments.

• Manual removal: Squish small infestations or wash off with water.
• Neem oil: Effective as both a contact killer and deterrent.
• Ladybugs: A single ladybug eats 50+ aphids per day.
• Insecticidal soap: Works on contact with thorough coverage.

Canadian outdoor growers often deal with aphids during warm summer months.

Thrips on Cannabis

Thrips are tiny slender insects that scrape leaf surfaces. They can vector plant viruses and resist many treatments.

Control Methods

These are the most effective thrip treatments.

• Spinosad: An organic bacterial toxin highly effective as a foliar spray.
• Blue sticky traps: Thrips are attracted specifically to blue colour.
• Predatory insects: Orius (pirate bugs) and Amblyseius cucumeris eat thrips.
• Neem oil: Less effective than spinosad but helps reduce populations.

Keep the area around outdoor cannabis clear of weeds to reduce pressure.

Powdery Mildew on Cannabis

Powdery mildew appears as white patches on leaves. It is the most common disease in Canadian cannabis during fall.

Treatment and Prevention

Use these strategies to manage powdery mildew.

• Improve airflow: Defoliate interior leaves, space plants, and use fans.
• Control humidity: Keep below 55% during flowering with a dehumidifier.
• Potassium bicarbonate: Mix 1 tablespoon per litre to change surface pH.
• Milk spray: Mix 1 part milk to 9 parts water for antifungal effect.
• Remove affected leaves: Dispose of heavily infected foliage carefully.

Prevention through airflow is far easier than treatment. Choose mold-resistant strains for outdoor Canadian grows.

Bud Rot (Botrytis) on Cannabis

Bud rot is the most devastating disease for outdoor Canadian growers. It attacks dense buds from inside out.

Early detection is difficult because damage starts internally.

Emergency Response

Here is what to do when you find bud rot.

• Act immediately: Bud rot spreads rapidly through spores.
• Remove affected buds: Cut 2-3cm beyond visible rot with clean tools.
• Improve conditions: Increase airflow and shake water off after rain.
• Consider harvesting: If near maturity, harvest everything healthy immediately.

Risk peaks during September and October. Choosing early finishing seeds reduces exposure.

Root Rot in Cannabis

Root rot is caused by fungi attacking roots in overly wet conditions. It is common in hydro and overwatered soil.

Targeted Interventions

Treat root rot with these methods.

• Reduce watering: Allow the medium to dry more between waterings.
• Improve drainage: Add perlite to soil mixes and ensure drain holes.
• Beneficial microbes: Inoculate with Bacillus subtilis or mycorrhizal fungi.
• Hydrogen peroxide (hydro): Add food-grade H2O2 to kill reservoir pathogens.
• Water temperature: Keep hydro reservoirs below 21°C.

Well-draining medium and proper watering prevent root rot entirely.

Organic Pest Prevention Strategies

Preventing pests is far easier than treating infestations. These strategies create a hostile environment for pests.

Essential Practices

Implement these in every grow.

• Cleanliness: Remove dead leaves, spilled soil, and debris.
• Quarantine new plants: Isolate new additions for 1-2 weeks.
• Environmental control: Maintain 20-26°C and below 60% humidity in flower.
• Air filtration: Use HEPA intake filters on grow tents.
• Companion planting: Basil, marigolds, and lavender repel pests outdoors.
• Daily inspection: Check plants every day with a magnifying glass.

Healthy, well-fed cannabis naturally resists pests better than stressed plants.

Biological Pest Control

Biological control uses living organisms to manage pests. This approach is sustainable and leaves zero residue.

Available Beneficial Organisms

These are available from Canadian suppliers.

• Ladybugs: Voracious aphid predators that also eat spider mites.
• Predatory mites (Phytoseiulus): Specialist spider mite predators.
• Amblyseius cucumeris: General predatory mites that feed on thrips larvae.
• Hypoaspis miles: Soil-dwelling mites that eat fungus gnat larvae.
• Bacillus thuringiensis (Bt): A bacterium that kills caterpillars when ingested.

Biological controls work best when introduced preventively or at first sign of pests.

When to Use Chemical Treatments

Sometimes organic methods cannot control severe infestations. Chemical treatments may be needed to save your crop.

Critical Rules

Follow these rules for chemical treatment.

• Never spray in late flower: Residues on buds will be inhaled when smoked.
• Choose cannabis-safe products: Avoid organophosphates and neonicotinoids.
• Follow label instructions: Over-application causes plant damage.
• Rotate active ingredients: Pests develop resistance to repeated use.
• Test first: Spray a few leaves and wait 24-48 hours before full treatment.

The safest options include pyrethrin sprays, spinosad, and potassium salts of fatty acids.