Preventive gardening:
Preventative gardening techniques can be effective in defending against powdery mildew.
Dont water plants at night. Reduce or stop watering before the lights have gone out to help evaporate and reduce room humidity.
Reduce plant density. Spread plants apart to improve air circulation. Don’t place plants directly against walls or into corners, typically areas of poor air circulation. Pull plants 6”-1’ away from walls or reflective surfaces.
Pruning. Remove the lowest leaves as the plants mature and prune the bottom 1/3 of the plant during veg to increase airflow inside the lower canopy. Remove all unnecessary growth. Put an oscillating fan down low to blow through this pruned area.
Foliar feeding. Foliar feeding can sometimes cause excessive nighttime humidity levels. Discontinue if mildew appears.
Harvest early if mildew is a problem.
Environmental control:
Improving growroom conditions is an excellent way to passively prevent and minimize damage by powdery mildew.
Monitor humidity levels. A quality humidity gauge should be used to monitor day and night r.h levels. Avoid prolonged high humidity levels: 50-60% r.h is ideal. Humidity must be kept below 70% during the night; levels over 80% will guarantee infection within 48 hrs.
Ventilation. Constant air movement inhibits mildew, and lowers humidity. Use oscillating fans on all sides of a garden to circulate the air. Ventilate air out of the grow room periodically during the night cycle to reduce humidity from irrigation and transpiration.
Note: Once mildew is established, oscillating fans may actually spread spores throughout the garden. Stop fans, treat infected areas, and then resume airflow.
Heat night air. Warm air holds more moisture than colder air. Heat helps dry the air and lower humidity during the night cycle. Heat the room at night and exhaust the room periodically to remove this warm/moist air.
Dehumidifier.Very effective in preventing mildew from spreading. Set controls for 40-60% and let run during night cycle.
Hepa filter. Filter the intake with a Hepa filter to eliminate spores from entering room. Inspect and change filter frequently.
Ionizer / Ozone generators. Leak some output to kill airborne pathogens and spores.
* Sulphur burner. These devices vaporize (not burn) elemental sulphur, coat the room with a fine film of sulphur, inhibiting PM spores from germinating. Also inhibits insects to some degree.
Run for 12 hours initially (at night, exhaust blower off, circ fans on), then 1-4 hrs/night thereafter. Some discontinue when buds begin to form, other let it run up to the last week. Very effective prevention
Chemical control:
Chemical control should be considered a last resort. Chemicals should be sprayed only in veg or early flower to prevent absorption into the buds and burning bud hairs. Chemicals may have to be applied repeatedly to be effective, and may take a few days for noticeable results. Use a surfactant to help adhere chemical to leaf surface. Some chemicals are more harmful than others; follow label directions and observe precautions.
Always spot spray first. Spraying individual leaves can be an option.
Note: many chemicals will leave a residue that appears similar to powdery mildew!
* Alkaline water:
Alkaline water can affect powdery mildew, as mildew cannot grow in basic conditions. Mist plants generously with water at 8.0+ pH frequently; discontinue when buds begin to mature to prevent budrot.
[Editor's note: this does work and is effective in late flower. Alkaline water does not seem to affect buds or bud hairs. The effect is temporary, but drastically slows mildew grow and spread. Reapply every 4-5 days.]
Baking soda: Sodium Bicarbonate
"Sodium collapses the powdery mildew cell wall". Baking soda leaves an alkaline residue on the leaves, which should be washed off with water before more is applied.
Foliar spray: 15ml / gallon
Potassium Bicarbonate:
"Collapses and desiccates the mildew hyphae. Very safe, very effective contact fungicide". Kaligreen and Armicarb100.
Garden sulphur:
A common non-toxic spray, sulfur interferes with mildew cellular respiration. Spray young plants weekly before hairs form (or spray lower leaves only), then discontinue. Do not wash off. Note: dried sulphur spray looks similar to mildew. For best results, prune plants in veg/early flower, then spray lower stems and foliage. Warning: can give a sulphur taste if sprayed directly onto buds!
Foliar spray: 15-20ml sulphur powder/Liter water. Keep well mixed when spraying.
Note: will not wash off buds. Re-application may be necessary.
Neem Oil:
Protects and kills mildew by inhibiting respiration; also protects against mites and may improve plant vigor. Results are noticeable in a couple of days.
Pro-silica: (Soluble Silicon)
”… increases resistance to pathogens by accumulating in…(leaf and root) cells of plants, providing a barrier against penetration by invading fungi such as powdery mildew and Pythium. Foliar applications leave deposits of silicon…on the leaf surface that promote effective physical barriers to…infection.” Pro-silica is alkaline.
Foliar spray: 1 part in 5
SM90:
A natural plant extract in a vegetable oil base.
Foliar spray: 10ml/liter
Copper Sulphate:
"Copper ions inactivate some fungal enzyme systems, killing the mycellium."
Effective one-shot application, but production may be discontinued.
Benomyl: Apply in veg only.
Malatox:
by the chronic:
…”This is a wonder cure. Mildew completely vanishes for up to 7 weeks! Make sure you spray before the first week of flowering.”
Foliar spray: 2.5ml per liter of water.
Biological controls:
Unpasterized milk:
Note: must be fresh, not store bought (which is pasturized). Diltute 10 parts water to 1 part milk. Works, although the milk may smell.
AQ10:
A biofungicide. Ampelomyces quisqualis is a fungus that "parasites the powdery mildew organism. It offers control over a long period of time." Effective only in initial stages of infection.
Plant Shield:
Plant Shield is a foliar spray (General Hydroponics), which kills many types of leaf and root fungus. Trichoderma harzianum strain T-22. Safe to use. Takes 2-10 days.
Serenade:
"The fermentation product of a bacterium, bacillus subtillis, that inhibits cell growth of fungi and bacteria. It is effective and easy to spray or use as a dip". Contact fungicide.