Powdery Mildew (PM)
Definition: Powdery mildew is a fungal disease that coats cannabis leaves, stems, and eventually buds in white, flour-like patches. Unlike many molds, it doesn't need standing water to spread, and its airborne spores move easily between plants. Left unchecked, it reduces photosynthesis and can make harvested flowers unusable.
How Powdery Mildew (PM) works
Powdery mildew on cannabis is usually caused by fungi in the Golovinomyces group. It lives on the leaf surface and feeds on the cells underneath, gradually covering more of the plant.
How to recognize it
- Small, round white spots on the upper side of fan leaves, often first on lower or inner growth.
- Spots that wipe off with a finger but return within days.
- Patches that spread and merge, eventually reaching stems and sugar leaves.
Trichomes are glossy and attached to flowers and sugar leaves; mildew is flat, dull, and powdery. Dust can be wiped away permanently; mildew comes back.
What encourages it
Moderate temperatures, high humidity especially at night, crowded plants, stagnant air, and low light in the lower canopy. Infected clones are a common way it enters an indoor grow, because spores can ride in on cuttings that look clean.
Responding to it
- Remove and bag affected leaves as soon as you see them.
- Lower relative humidity and increase air movement.
- During veg, many growers use products labeled for powdery mildew, following the label exactly and checking which inputs your state allows.
- Avoid spraying buds in flower; at that stage, environmental control is your main tool.
Prevention
Space plants so leaves don’t overlap, keep oscillating fans running, quarantine new clones for at least a week, and choose resistant genetics if you grow in a humid region. Mildew that reaches the buds is a reason to discard them, just like bud rot.