Contributed by:
snoofer
Images archived 2002
Introduction:
Powdery mildew is a common fungal disease that can
seriously damage indoor and outdoor crops. Powdery
mildew can rapidly infect crops in both vegetative and
flowering stages, coating leaves, stems and buds in
fungus.
Powdery mildew typically thrives in
cool, damp, shaded and poorly ventilated areas. Airborne
spores brought into the grow room land on leaf surfaces
and will germinate given favorable conditions. High
night humidity levels often trigger the growth of mildew
spores. Powdery mildew can attack indoor crops year
round.
Powdery mildew is almost impossible to
stop in late flowering, so early detection and control
is essential. Perpetual harvest, dense scrog/sog
systems, and damp basement grows are particularly
vulnerable to powdery mildew. Note: strains vary in
their susceptibility.
This FAQ focuses on indoor
prevention and treatment options. Please read
Bongaloids
Powdery Mildew FAQ for more information.
Identification, Symptoms and damage:
Early signs of powdery mildew include white
powder/fuzzy patches on leaves (usually low in the
canopy) and a fuzzy white coating on lower stems. Note:
powdery mildew can be wiped off the leaves for a quick
visual check.
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| (photo provided by turtle power) |
(photo provided by just one
blunt) |
These fuzzy
mycelium patches produce airborne spores that rapidly
attack adjacent plants; mildew will eventually coat
leaves and entire plants, reducing photosynthesis, plant
vigor and bud quality.
Plants on the edge of a
garden, in corners and under stress are attacked first;
infection usually starts in the lower canopy where
conditions are optimal. As infection progresses, mildew
will spread to the top of the plants and finally attack
the buds.
Infected buds may appear normal; but
are internally dusted with white powder (which cannot be
removed by drying), and have a stale, musty/moldy smell
when dry. Smoking or trimming infected buds can cause
sickness and lung infections, and is not recommended.
Infected leaves should be discarded. Lower buds are the
most susceptible.
Powdery mildew is difficult to
100% eradicate; control requires prevention, early
detection, and pro-active measures.
Preventive gardening: Preventative
gardening techniques can be effective in defending
against powdery mildew.
Maintain healthy
plants. Stressed plants are often attacked first, so
it is important to monitor and remove unhealthy plants.
Detection. Inspect corners, edge and
lower portions of the garden frequently. Remove infected
leaves, or move infected plants out of the main garden.
Dont water plants at night. Reduce or
stop watering before the lights have gone out to help
evaporate and reduce room humidity (thnx dutchmaster).
Reduce plant density. Spread plants apart
to improve air circulation. Dont place plants directly
against walls or into corners, typically areas of poor
air circulation. Pull plants 6-1 away from walls or
reflective surfaces, and blow air to these areas.
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: Note: Chemical information sourced
from maximumyield.com and cannibisculture.com
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 siliconon the leaf
surface that promote effective physical barriers
toinfection. 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.