Powdery Mildew

Powdery mildew (PM) is the second most damaging cannabis disease after bud rot, distinct in that it can attack any plant tissue at any growth stage — from seedlings through finished flowering — and spreads via wind-borne spores capable of traveling miles. The infection appears as white-grey powdery patches on leaf surfaces, initially small circular spots that expand and merge until entire leaves and even bud surfaces are coated in what looks like flour dusting. Unlike bud rot which colonizes interior bud tissue, powdery mildew grows externally on plant surfaces, making it visible early but also making the spores easily airborne and contagious. Optimal conditions for PM: humidity 60-80% RH (paradoxically PM does NOT require leaf wetness — it thrives in moderate humidity, unlike most fungi), temperatures 60-80°F (16-27°C), poor airflow allowing stagnant air pockets, low light conditions, dense canopy without defoliation, plants grown from clones imported from infected facilities (PM clone-cohort infection is the #1 source in commercial cannabis). Treatment options: organic — potassium bicarbonate sprays (Eagle 20 alternative), milk-water solutions (1:9 ratio, surprisingly effective), neem oil during vegetative only (never on flowering buds), beneficial microbes (Bacillus subtilis); aggressive — sulfur burners during vegetative stage, hydrogen peroxide foliar spray (1:10 dilution), UV-C light treatment. Treatment should never be applied within 2-3 weeks of harvest to avoid contaminating flower with chemical residues. Critical management rule: any cannabis with visible PM on bud surfaces should not be smoked — even brief exposure to PM spores via combustion can trigger respiratory infections, especially Aspergillus complications. Prevention: maintain humidity below 55% RH during flowering, aggressive defoliation, oscillating fans throughout canopy, sanitation between grow cycles (PM spores survive on surfaces for months), mother-plant inspection for clones, and mold-resistant strain selection for humid climates.