You treated it. You cleaned the room. Plants looked healthy going into the next cycle and then the white spots came back.

Powdery mildew is one of the most persistent problems in commercial cannabis cultivation, and the reason it keeps returning is almost always the same: the visible outbreak was treated, but the environmental pressure causing it was not. When spore pressure is living in your HVAC, your dry room, or your shared equipment, cleaning the flower room addresses the symptom not the source.

This FAQ answers the most common questions commercial growers ask about powdery mildew on cannabis: what causes it, why it keeps coming back, what the white spots actually are, and what a facility-level fix looks like.

White spots on cannabis leaves are most commonly caused by powdery mildew a fungal pathogen that appears as a white or grayish powder on leaf surfaces. It thrives in environments with humidity fluctuations, low airflow, and dense canopy gaps, and it can spread rapidly through a grow room once conditions support it.

White spots can occasionally be caused by other issues spider mite damage, mineral deposits from water, or light bleaching but in commercial cannabis cultivation, powdery mildew is by far the most common cause and the one with the most significant consequences for microbial compliance testing.

Powdery mildew in commercial cannabis grow rooms is almost always caused by a combination of environmental conditions rather than a single isolated factor. The most common contributors are:

• Humidity rising above the threshold powdery mildew favors, particularly during late vegetation and flowering

• Reduced airflow in canopy gaps, corners, and low-circulation zones where spores can settle and establish

• Temperature fluctuations between lights-on and lights-off periods that create condensation on leaf surfaces

• Spore pressure being continuously reintroduced from HVAC systems, adjacent rooms, or shared equipment

• Dense canopy that restricts air movement between plants and creates favorable microclimates

If white spots keep appearing in the same areas of your room cycle after cycle, the pattern usually points to an airflow dead zone or a humidity consistency problem in that specific zone — not random bad luck.

Late flower is the highest-risk point for powdery mildew in the grow cycle and also the most damaging time to deal with it aggressively, since most treatments at that stage carry tradeoffs for product quality and compliance.

The deeper problem is that most treatments address visible symptoms rather than the contamination source. If powdery mildew keeps showing up in late flower cycle after cycle, the spore pressure is almost certainly coming from somewhere in your facility environment HVAC pathways, a connected dry room, shared carts, or surfaces that are not being consistently sanitized between runs.

The room gets treated. The source does not. So pressure rebuilds in the next cycle and shows up again at the same vulnerable stage.