How To Identify and Control Powdery Mildew on Cannabis Plants
Professor DeBacco
Powdery Mildew Identification
White powdery substance that is typically first noticed on leaves, but can also infect buds.
Hemp powdery mildew is caused by the fungus (Podosphaera macularis and Golovinomyces ambrosiae) and is identified by its upright chains of barrel-shaped spores.
Can tend to favor on side (top or bottom) of the leaf early during establishment, so be sure to scout both sides.
Also, check the shady (typically interior) leaves as this is where it is likely to first show up and go unnoticed.
Powdery Mildew Life Cycle
The lifecycle of Podosphaera macularis, the causal agent of hop powdery mildew. Prepared by V. Brewster, Hop IPM Field Guide, 3rd edition.
Powdery Mildew Mycelium Network
Stained for Powdery Mildew. One can see the mycelium network of PM inside the leaf even if powdery mildew is not visible to the human eye. DPI = Days Post Inoculation. HPI = Hours Past Inoculation.
Environmental Impacts on Powdery Mildew
Temperature
Moderate temperatures are required for infection and disease development.
Optimal temperatures for conidial production is 65 to 70 degrees F.
86+ degrees F, infection and spore production are reduced by half
55 to 60 degrees F, disease development can be delayed by one to two weeks
Moisture
Moisture is also required for the pathogen to infect and develop.
High humidity provides sufficient moisture, so air exchange around plants is advised especially for indoor or greenhouse environments.
For outdoor locations shady areas in a field will likely have increased powdery mildew pressure
Hemp Variety Screening for Susceptibility to Powdery Mildew
If growing outdoors where there is less control over the environmental conditions, this may influence your variety selection
30 high CBD cultivars were screened for resistance to powdery mildew
Preliminary data from the detached leaf assay showed some differences in susceptibility
These studies are continuing, and we are screening an additional 37 cultivars.
Powdery Mildew Management
Goal is to initially reduce the favorable environment for Powdery Mildew
Continual scouting is also important to catch potential problems early as this can quickly spread
This may influence plant spacing for outdoor locations and pruning method for indoor locations.
Powdery Mildew Spray Products
In both situations know your current local regulations.
Indoor
In grow rooms daily sulfur burning can reduce likelihood of seeing powdery mildew
More contact products that have low residuals are recommended
Hydrogen dioxide
40% milk + 60% water
Sulfur *can burn plants
Neem * can burn plants
Milstop (potassium bicarb), typically not as effective as the milk and water spay
Outdoor
Typically the pressure can be greater and while the indoor products can also be utilized systemic products may be needed.
Caution when using these at later stages to not impact final product quality. Also, these products have the greatest chance at resistance so rotations are advised if considered.
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