POWDERY MILDEW
the most perfect
“even
conditions of low humidity
and warm, breezy air
spreads existing mildew
spores all over.”
Often indoor growers who believe
they are controlling their humidity
and temperature will have a drop in
temperature down toward the 60-64˚F
range when they go to lights out. This
is a problem that can be managed
with the right equipment and control.
However, the powdery mildew problem
is more complex.
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Maximum Yield USA | June 2016
Leaves transpire moisture, so even
if the air-measured humidity is
low there can still be moisture at a
microscopic level on the leaf surface.
It is important to realize that the
world in which fungi live is at the
microscopic level, so undetectable
moisture on a leaf surface and the
micro-climate at that level might
be the perfect breeding ground
for powdery mildew, even though
the temperature and humidity we
monitor seems to be in control.
Even if a grower really is controlling
their temperature at a steady level of
75˚F and appears to be controlling
humidity at 55-65% or lower, the
surface temperature at the microclimate level on the leaf surface can
be lowered basically by the wind
chill effect. A surface will lose heat
through convection. The insulating
layer of warm air that forms at the
leaf surface is disrupted by moving
air, allowing cooler air to replace the
warm air at the leaf surface.
Unfortunately, the practice of
providing a constant breeze blowing
across plant surfaces may actually be
contributing to the growth of fungi
by lowering the surface temperature
and helping to create a perfect
micro-climate for powdery mildew to
germinate, and then the breeze itself
spreads the live spores throughout
the growing area.
Air circulation is important as
it mixes the air and prevents
temperature layering. It is also
necessary to draw overall humidity
away from plants and to move the
excess heat generated by artificial
lights. But rather than blowing
directly across or through plants, it is
my opinion that it should be moving
air around the plants.