Cannabis Diseases: Bud Rot, Mildew & More

Bud Rot (Botrytis cinerea)

Bud rot is every grower's nightmare. This fungal infection (Botrytis cinerea) develops inside dense buds where moisture gets trapped. By the time you see external symptoms, the inside of the bud is already destroyed.

Identification

• Early signs: Yellow/dying sugar leaves on specific buds, drying pistils while surrounding pistils are still white
• Internal: Gray fuzzy mold inside the bud, brown mushy tissue
• Smell: Musty, damp, "off" odor
• Touch: Affected areas crumble when handled
• Location: Usually starts in the densest, largest colas

What to Do

1. Remove immediately: Cut 2+ inches below visible rot with sterilized scissors
2. Don't shake: Spores spread through air movement
3. Bag and dispose: Never compost infected material
4. Sterilize tools: Isopropyl alcohol between each cut
5. Check everything: Inspect all buds—rot spreads fast
6. Consider early harvest: If widespread, harvest healthy buds before infection spreads

Prevention

• Humidity: Keep below 50% RH in late flowering, 40% in the final weeks
• Airflow: Strong air movement around and through the canopy
• Defoliation: Remove inner fan leaves to improve airflow
• Inspection: Check dense buds daily by gently squeezing—look for soft spots
• Harvest timing: Don't let plants go past peak ripeness in humid conditions
• Genetics: Choose mold-resistant strains for humid climates

Powdery Mildew (PM)

Powdery mildew appears as white, powdery spots on leaf surfaces that spread rapidly. Unlike trichomes, PM has a flat, matte appearance and wipes off easily. If untreated, it can cover entire plants and spread to buds.

Identification

• Appearance: White/gray powdery patches on leaves—like someone dusted flour on them
• Texture: Flat and matte (trichomes are shiny and crystalline)
• Test: Wipes off easily with your finger (trichomes don't)
• Location: Usually starts on lower leaves with less airflow
• Spread: Rapid—can cover a plant in days

Treatment

1. Remove infected leaves (bag before removing from grow space)
2. Potassium bicarbonate spray: 1 tbsp per gallon water—most effective
3. Neem oil: Works in veg stage (never in flower)
4. Baking soda spray: 1 tsp per quart water + drop of dish soap
5. Milk spray: 40% milk / 60% water—surprisingly effective
6. Commercial options: Green Cure, Regalia, sulfur-based fungicides

Prevention

• Maintain consistent temperature and humidity (fluctuations trigger PM)
• Strong airflow to dry leaf surfaces
• Space plants for air circulation
• Don't let leaves touch wet soil
• UVC light treatment (commercial operations)

Root Rot (Pythium)

Root rot is caused by overwatering and poor drainage. When roots sit in waterlogged soil, oxygen-deprived conditions allow Pythium and other pathogens to thrive. The roots turn brown and slimy, losing their ability to absorb nutrients.

Identification

• Above ground: Plant wilts despite wet soil, leaves yellow, growth stops
• Roots: Brown, mushy, slimy (healthy roots are white and firm)
• Smell: Foul, swampy odor from root zone
• Hydro systems: Water turns murky, algae may develop

Treatment

1. Let soil dry completely—this is critical
2. Remove dead roots: Gently trim brown/mushy roots with sterilized scissors
3. Hydrogen peroxide drench: 3% H2O2, 1 tsp per cup water
4. Beneficial microbes: Add mycorrhizae and beneficial bacteria
5. Transplant: Fresh, well-draining soil if severe
6. For hydro: Change reservoir, clean system, lower water temp, add air stones

Prevention

• Don't overwater: Let top inch of soil dry between waterings
• Ensure drainage: Pots must have drainage holes, never sit in runoff
• Air pots/fabric pots: Prevent waterlogging
• Hydro: Keep water temps below 68°F, use air stones, beneficial bacteria

Damping Off

Damping off is a fatal fungal disease affecting seedlings. The stem rots at the soil line, causing the seedling to collapse overnight. Once you see symptoms, the plant is already dead—prevention is the only cure.

Identification

• Stem: Thin, pinched, water-soaked appearance at soil line
• Plant: Suddenly collapses and falls over
• Timing: Usually within first 2 weeks after sprouting

Prevention (Only Solution)

• Sterile medium: Use new, quality seed-starting mix
• Don't overwater: Soil should be moist, not wet
• Airflow: Remove humidity domes periodically for fresh air
• Sterilize containers: Clean pots with diluted hydrogen peroxide
• Warmth: Keep temps above 65°F—cold + wet = damping off

For more on healthy seedling care, see our Seedling Stage Guide.

Prevention: The Environmental Triangle

Nearly all cannabis diseases thrive in the same conditions: high humidity + stagnant air + organic debris. Control these three factors and you'll prevent 90% of issues.