The short answer
Powdery mildew on cannabis gets managed with contact treatments, not a systemic cure. Nothing you spray kills the colony inside the leaf tissue. In veg you have real options: potassium bicarbonate, horticultural oil, sulfur, and biological sprays such as Bacillus amyloliquefaciens or Streptomyces lydicus. In flower that list shrinks fast. I would stay with potassium bicarbonate, a milk or whey spray, or low-strength hydrogen peroxide, fix the airflow, and accept that a badly infected plant should come down early instead of getting drenched in oil you cannot rinse off a bud.
Whatever you pick, treat it as one part of a plan. Sprays knock the visible colony back. Airflow, humidity, and spacing decide whether it returns next week.
Confirm what you are looking at
Powdery mildew starts as pale, dusty patches on the upper leaf surface, usually low and shaded first. It wipes off with a finger and leaves green tissue underneath. That is different from downy mildew, which sits on the underside and leaves yellow patches on top, and from a calcium or pH problem that yellows leaves without any fuzz.
Check the underside of a few lower leaves with a flashlight before you choose a product. The two mildews want different treatments, and spraying the wrong one wastes two weeks.
Veg stage options
- Potassium bicarbonate. My default. It raises surface pH and breaks the mildew's cell walls on contact. Mix per label, add a surfactant so it spreads, and cover both leaf surfaces. It leaves a white residue, so plan a rinse before flip.
- Horticultural oil (neem, mineral, or plant-based). Smothers spores and works well in rotation. Skip it in heat above roughly 85F and never tank-mix with sulfur.
- Sulfur. Effective and cheap, but it burns plants in hot weather and should not go on within a few weeks of harvest. Some people react badly to sulfur residue, so keep it for early veg.
- Biologicals. Bacillus amyloliquefaciens strain D747 and Streptomyces lydicus colonize leaf surfaces and crowd the mildew out. They prevent more than they cure, so start them before you see spots.
- Milk spray. One part milk to nine parts water, applied in daylight. Cheap, mild, and useful on young plants or as a stopgap.
Flower stage options
This is where growers get into trouble. Oils and sulfur late in flower leave taste and residue you cannot remove, and buds hold moisture in places a spray cannot dry out. Stick to potassium bicarbonate at a low rate, diluted milk, or 3% hydrogen peroxide, and spot-treat instead of fogging the whole room. Remove the worst leaves by hand and bag them, do not drop them on the floor.
If the infection is on the colas themselves, cutting the plant early usually beats a rescue spray. A reduced harvest you can sell with a clean test beats a full one you cannot.
Environment does most of the work
Powdery mildew germinates in still, humid air and struggles where leaves move and dry. Keep night humidity under about 55 to 60%, run enough oscillating fans that every leaf flutters, and defoliate the lower third so air reaches the soil line. Water in the morning, not at lights-off. A 10F swing between day and night makes condensation worse, so tighten that gap before you buy another bottle.
Rotate, do not repeat
Mildew adapts. If you use bicarbonate every three days for a month, expect it to stop working. Alternate modes of action: bicarbonate one week, a biological the next, oil only when temperatures allow. Keep a spray log with dates and rates. It sounds fussy until you need to remember what you used.
After harvest
Wash your hands, your scissors, and the tent walls with a dilute bleach or peroxide solution between runs. Mildew spores live on fabric and equipment for a long time. If you grow hemp for CBD, the same rules apply, and a third-party COA on the finished flower tells you more about quality than the spray label did.