Short answer
Milk and bicarbonate sprays can slow powdery mildew on leaves, but the idea that they cure it is a Myth. They work by contact, need full coverage every 7–10 days, and belong in veg only. Suppression is Supported in other crops; there are no cannabis trials.
Why
Potassium bicarbonate is a recognised contact option for hemp powdery mildew. It raises the pH on the leaf surface and dries the colonies, and it has to be repeated every 7–10 days [1].
Milk has some evidence behind it, on squash. In a greenhouse trial, sprays of more than 10% cow's milk cut zucchini powdery mildew about as well as the fungicides it was compared with [2]. That is a different fungus on a different crop.
Bicarbonate works better with oil. On pumpkin, 0.5% sodium or potassium bicarbonate mixed with 0.5% spray oil beat either one alone, and worked best when applied before infection. Potassium bicarbonate above 0.5% burned infected leaves [3].
None of these sprays reaches the spores already waiting on nearby leaves. The mildew comes back unless you fix still air and humidity spikes, and wet buds invite botrytis [1][4].
The numbers
| Spray | Rate tested | Crop | Result |
|---|---|---|---|
| Cow's milk | > 10% | Zucchini | Severity cut, close to fungicide [2] |
| Na/K bicarbonate + oil | 0.5% + 0.5% | Pumpkin | Better than either alone [3] |
| Potassium bicarbonate | > 0.5% | Pumpkin | Leaf injury [3] |
| Any spray | Every 7–10 days | Hemp | Contact only [1] |
What to do
- Check it is mildew: rub it with a damp finger and see whether it grows back in the same spot.
- Fix the room first: fans above and below the canopy, thin the inner growth, and keep humidity steady at lights-off.
- Bag infected leaves and remove them from the room.
- In veg, spray potassium bicarbonate at label rate. Cover leaf tops and undersides, and repeat every 7–10 days.
- Check the label and your local rules on what may be used on cannabis.
Common mistakes
- Spraying buds in mid or late flower.
- Spraying once and calling it cured.
- Using strong mixes that burn leaves.
- Spraying only the spots you can see.
Related
- Full article: Powdery mildew in cannabis
- Answers: what can I spray in flower? · should I wash my buds?
- Terms: powdery mildew · horticultural oil
