Identifying and controlling powdery mildew on cannabis

Powdery mildew is an obligate biotrophic fungus that grows on living cannabis tissue and forms a white, powdery coating.

It is one of the most common and fastest-spreading cannabis diseases and can infest entire rooms if sanitation and climate are not right.

Unlike most fungi, powdery mildew spores germinate even without free water; high relative humidity is enough. That makes it hard to control in many setups.

The fungus draws nutrients from living cells via haustoria, weakening photosynthesis and vitality.

Stage 1: Individual small, circular white powdery spots on upper leaf surfaces, easily wiped off.

Stage 2: Spots merge into a continuous white coating, leaves yellow beneath it.

Stage 3: Infestation on stems and buds, growth disorders, quality and yield loss; infested buds are unsellable.

White, powdery, wipeable coating ON leaves → powdery mildew.

Gray, cottony mycelium INSIDE the bud, on the other hand, is botrytis (bud rot), not powdery mildew.

White speckles that do NOT wipe off and are pinpoint-shaped point more toward pest damage (spider mites) than powdery mildew.

1. Carefully remove infested leaves (into a bag) without stirring up spores; disinfect hands/tools.

2. Adjust climate immediately: lower relative humidity, increase air movement and fresh air.

3. Thin out plants so light and air reach all leaves.

4. During the vegetative stage/early flowering, use approved contact agents on a rotation basis; no sprays on buds close to harvest.

Quarantine all new plants/clones — powdery mildew is often brought in from outside.

Keep relative humidity moderate and ensure constant air movement.

Adequate plant spacing and defoliation so no humid microclimate develops in the canopy.

High relative humidity combined with stagnant air is the main trigger; good circulation is the most effective long-term prevention.

Dense canopies and large day-night temperature fluctuations favor infestation.

Powdery-mildew-weakened plants are more susceptible to secondary pathogens like Botrytis.

Shake/tear off infested leaves in the room, spreading spores.

Only remove individual leaves, but don't change climate and hygiene.

Spray near-harvest buds, jeopardizing quality and consumption safety.

Some genetics are significantly more powdery-mildew-resistant; in problem rooms, strain selection pays off more than any spray regimen.

Since the fungus sits on living tissue, consistent room hygiene between cycles (cleaning, spore removal) is crucial against reinfestation.