HRT 220 FACTORS IN PREVENTION OF CANNABIS
DISEASES AND PESTS
WEEK 2
ABIOTIC FACTORS
 An abiotic factor is a non-living part of an
ecosystem that shapes its environment.
 In a terrestrial ecosystem, examples might include
temperature, light, and water.
 In a marine ecosystem, abiotic factors would
include salinity and ocean currents.
 Abiotic factors affect the growth, yield and
populations of microorganisms living on plants, as
well as vector populations, impacting the spread of
plant pathogens.
We must pay
attention to abiotic
factors in the
environment so that
we do not create a
favorable
environment.
ABIOTIC FACTORS INFLUENCING CULTIVATION OF CANNABIS
 The relative humidity of the air
 The temperature of the air
 The movement of the air
 The temperature, pH, and amount of water
 The pH of the media and leaf surface
 The temperature of the media
What IPM tactics can we
use to make the
environment unfavorable
to pathogens?
Air
DESIGN FACTORS
IN PREVENTATIVE
MAINTENANCE
Temperature
Water
Media
Growing in indoor spaces provides the luxury of
being able to control your atmospheric conditions.
AIR
The air in the room needs to be managed in order
to create the best environment to ward off pests
and diseases.
Wind strengthens stems and causes evaporation
which keeps things dry and helps to reduce
favorable conditions for molds. Wind needs to be
mimicked indoors.
Humidity is the amount of water vapor present in the air.
Water vapors are the gaseous state of water and are invisible.
Humidity can be measured by three ways:
1. Absolute humidity is the water content of air at a given temperature. It is
measured in the gram per cubic meter.
2. Relative humidity is expressed in percentage, and it measures the current
absolute humidity relative to the maximum humidity for that temperature.
3. Specific humidity is expressed as the ratio of the mass of the water vapors
to the total mass of moist air parcel.
The glass is at 50% of its
capacity. That is similar to
relative humidity. If RH was
at 50% the air would be at
50% of its capacity to hold
water. At 100%, condensation
occurs.
6 ounces is 50% of 12 ounces
and 8 ounces is 50% of 16
ounces.
6:12, and 8:16 can be thought
of as the specific humidity.
When temperature
increases it means
energy increases
which allows more
water molecules to
release their bonds
and float with air
molecules.
As temperature rises
so does the ability for
air to hold more
water.
At 20 C, the air can
only hold up to15
grams of water.
At 40 C, the air can
hold up to 50 grams of
water.
15 grams of water
vapor at 20 C creates
100% RH,
15 grams of water
vapor at 40 C creates
25% RH.
When Night
falls and the
colder air
can’t hold the
water,
condensation
forms.
•Stomatal closure
•Reduced CO2 uptake
RELATIVE HUMIDITY EFFECTS TRANSPIRATION AND NUTRIENT
ABSORPTION
 If the temperature in your grow room or greenhouse lowers, you need to bring humidity down with it or the
dewpoint will be reached and condensation will form.
Condensation creates a favorable environment for some diseases and pests.
 Transpiration increases at lower RH.

With low RH, plants need more water and less fertilizer.

A high amount of nutrients with low RH can cause salt build up and lock-out because it draws nutrient filled water through
the plant at a fast rate, bringing high amounts of nutrients into the tissues of the plant.
 Transpiration slows with high RH.

With high RH, plants need less water.

Increases heat load of plants, stomatal closure, and reduces CO2 uptake
WHAT’S THE CORRECT HUMIDITY LEVEL NEEDED?
 To figure out where you want the relative humidity level at in your grow room, you must relate it to the
temperature.
 The higher the temperature in the room, the higher you want the humidity in the room.
 The higher humidity slows the rate of transpiration so that large doses of nutrients do not get pulled into the
plant.
 If you don’t lower the rate of transpiration, than the high temperatures will cause stomates to close in order to
slow transpiration. With no water vapor being released on the surface of the leaf, the temperature of the leaf
surface with rise and cause curling.
 The lower the temperature in the room, the lower you want the humidity in the room in order to avoid
condensation forming on the plant.
WHAT’S THE CORRECT HUMIDITY LEVEL NEEDED?
 Growers have the benefit of a premade chart that they can consult to figure out what their ideal temperature and
humidity should be at to create the ideal transpiration rate, which will help the plant stay healthy with the right
amount of nutrients, water and cool leaf surface.
 The first and best defense against plant diseases is a healthy plant,
VAPOR PRESSURE DEFICIT (VPD)
 VPD measures the difference between the pressure inside the leaf and the pressure of the air outside.
 Gives us an idea of how easy it would be for the plant to transpire.
 A high VPD means the plant is transpiring quickly – too high and it pulls nutrients through too fast
 A low VPD means the plant is transpiring slowly – too slow and moisture forms, and low amount of
nutrients are up taken.
 Measured in Kilopascals (kPa) or Millibars (Mbar)
1 kPa = 10 Mbar
 Leaf temperature has an effect to VPD. Leaf temps should read below your room temp by 2 C or 3.5 F
difference. If it is equal to or above, lights are too close.
Unit of measurement:
Kilopascal (kPa)
Millabars (Mbar)
1 kilopascal = 10 millabars
In flower you want to
make lower relative
humidity.
This will create an
unfavorable environment
for mold.
THE RISKS OF HIGH HUMIDITY
 When conditions are too humid, it provides a favorable environment for:
 The growth of mold, mildew, and bacteria
 Conditions like root or crown rot.
 Pests, such as fungus gnats, whose larva feed on plant roots and thrive in moist soil.
 However, spider mites do not like humidity above 60%
AIR MOVEMENT TO REDUCE AND PREVENT PEST AND DISEASE
 Moisture is generally needed for spore germination, the multiplication and penetration of bacteria, and the
initiation of infection.
 Temperature, gas exchange, and humidity can all be affected positively through the air circulation and exchange.
 Circulation is used to keep things moving and fairly equal.
 Air exchange is used to control when the temperature or humidity gets out of hand.
 Circulating and exchanging air replaces the depleted gas areas at the leaf surface with normal air to insure carbon
dioxide and oxygen are available for basic life processes.
AIR MOVEMENT
 Mold, mildew, and insects prosper in stagnant, humid conditions. Airflow from fans creates a steady breeze,
protecting against harmful mites, fungus, and diseases.
 Even at low humidity levels, if you have no air flow and your grow room is stagnant, bud rot can proliferate fairly
easily.
 Provide as much air flow as possible without actually damaging the plant.
 Air flow systems should operate continuously in greenhouses during periods of high humidity, which often occur
at night and during rainy days. Exhaust fans, when present, can be used to circulate air in greenhouses.

https://www.sciencedirect.com/science/article/abs/pii/0261219484900589
 Ventilation in high tunnels is manipulated by opening sidewalls, endwalls, and/or ridge vents, if applicable.
AIR MOVEMENT AND
PROPER PLANT
SPACING
 To decrease humidity provide good
air circulation by leaving space
between plants.
 If plants are close make sure the
breeze is strong enough to penetrate
the canopy.
 Genetics/ lighting/ humidity/ air
circulation/ pruning all influence
appropriate spacing
INSECTICIDAL LIGHTS AND FANS
 Fans help lower humidity and temperatures in the growing area.
 Fans inside the growing area are important for eliminating microclimates. Having microclimates
increases your chances of providing favorable conditions for pests and diseases.
 Insecticidal lights kills fungal spores, bacteria, and insects that come in contact with it. They are often
installed in the vents bringing air into the grow room
 Fans are rated by CFMs. The appropriate amount of CFMs is equal to the area of the grow room.
TEMPERATURE AND PESTS AND DISEASE
High humidity and low
temperatures
encourage some plant
diseases.
Maintain optimum
temperature range 70
to 85 F.
Genetics influence
temperature tolerance.
Afghan-cooler
Indica-hotter
High and low
temperatures weaken
plant defense
responses
Lessen temperature
difference between
night and day to
reduce dew
condensation.
TEMPERATURE AND PESTS AND DISEASE
 Temperature is one of the most important factors shaping the nature of plant pathogen interactions, as well as the
normal development of healthy plants.
 High temperatures facilitate the spread of tobacco mosaic virus by weakening the plant defense responses.
 Powdery mildew proliferates at higher temperatures 68-86 F, but begins to die above 90 F
 Botrytis mold proliferates at lower temperatures 30-68 F, but stops growth at 89 F, and dies above 105 F
 Fusarium proliferates at a wide range of temperature, optimal growth is 59-86 F, growth stops above 107 F
 Use a clean water source.
 Infected water can be sterilized by passing it through a UV
light
 Reverse Osmosis will remove pathogens, dissolved solids,
and harmful chemicals.
 If growing prebiotically, chlorine will kill any beneficial
microbes. Be sure to filter water.
WATER
 Eliminate any leaks in the garden space and make sure
drainage is complete.
 Puddled water can lead to algae growth and support fungus
growth.
AVOID OVERWATERING OR UNDERWATERING
 Careful watering is key to avoiding the growth of fungi and molds.
 Most Fungi responsible for dampening off survive in overly wet environments.
 Be sure planting mix drains well.
 Plant seeds one eight to a quarter inch deep if sowing directly, seeds planted deeper are more vulnerable to
dampening off.
 Drought to the point of wilt weakens plants, making them more vulnerable to disease.
 Roots need a constant supply of oxygen and water. This is achieved with moist but not wet conditions.
BALANCE THE PH
 Hidden pH problems slow your cannabis plants’ growth, make your plants sick, decrease yields and potency, and
can even kill your cannabis plants
 Fungi thrive in pH environments lower than 7 with optimum being 4.5 – 6
 Raising pH on leaf structure can fight or prevent a fungal infection
 Potassium hydroxide (lye) and Potassium bicarbonate (baking soda) are often used to increase pH.
 A pH of 8 is sprayed on foliage to create an inhospitable environment for fungi.
PH LEVELS IN THE SOIL
 Cannabis grown hydroponically or without soil needs an even lower pH than a soil grow.
 If you grow in soil, the optimal pH level for the root zone is between 6.0 and 7.0.
 If you grow in soilless or hydroponics, the optimal pH level for the root zone is between 5.5 – 6.5
 allow the pH level to fluctuate across the acceptable range to support nutrient uptake. For example, in hydro,
calcium and magnesium are mostly absorbed at pH levels above 6, while other nutrients like manganese prefer a
slightly lower pH.
 Pests and diseases not only live on the plant, but they can
also live in the growing media.
 Raised beds can prevent gophers, moles, voles, and rats from
MEDIA
eating your plants in the ground.
 Placing wiring at the bottom of beds in the ground can help
ward off pests.
 Healthy soil contains the right balance of nutrients,
microorganisms and other beneficial organisms that help
fight off disease and pests.
 Mulches, crop rotation, irrigation cleaning, all help to reduce
pest problems.
CONTROL PEST
AND DISEASE BY
KEEPING
PLANTS
HEALTHY
 Get to know pests desired environments and create
unfavorable conditions.
 Enzymatic reactions are limited by temperature. Low
temperatures slow enzymes which lowers the defenses of
the plant.
 High temperatures can retard photosynthesis and create
nutrient deficiencies or toxicity.