The Effects of Powdery Mildew on Cannabis Plants and How to Prevent It

Effects of Powdery Mildew on Cannabis Plants

In commercial cultivation, risks of fungal infection and plant pathogens come with the territory. One of the most common, powdery mildew, is a fungal disease that infects a variety of plants, trees, and agronomic crops including fruits and vegetables. 

While powdery mildew is caused by multiple types of fungi, infections in cannabis are largely attributed to the pathogen golovinomyces. Contamination from golovinomyces begins with spores of powdery mildew on branches of mycelium called conidiophores, each of which bears asexual spores known as conidia. Chains of conidia get released into the air and carried over long distances by wind, fans, and unsuspecting humans before landing on plant surfaces and starting germination. 

Within 14 days of becoming infected, a plant will start to show the appearance of white powdery spots on its upper leaf surfaces. From there the white powder spreads to lower leaf surfaces, stems, inflorescences – the complete flower head including petioles, bracts, stems, and flowers -- and even trichomes. As the fungal disease progresses, it prematurely impacts photosynthesis and the plant experiences a loss in cell function known as senescence. The plant’s weakened immune system causes a range of symptoms that include fungal lesions, leaf drop, leaf yellowing, and the abnormal growth of new shoots. 

Though rarely fatal, the stunted growth, damaged resin, fungal lesions, and other issues resulting from powdery mildew infection can lead to reduced potency, reduced yield, and impaired quality overall. Consumers who unwillingly ingest or inhale mold spores and other potentially harmful pathogens could experience a variety of health issues ranging from severe allergies to lung and sinus irritation or worse – especially if they have a weakened immune system.

Read more about the effects of powdery mildew on cannabis plants here.

Causes of Powdery Mildew in Cannabis Plants

Unlike other fungi, powdery mildew doesn’t need excessive moisture to initiate germination. High humidity and moderate temperatures create ideal conditions for the pathogen to thrive and spread, which makes indoor and greenhouse facilities particularly susceptible to infection. These are some of the climate and environmental factors that facilitate powdery mildew:

• Humidity: Powdery mildew prefers high humidity levels. It thrives in environments that have a relative humidity (RH) of 95% or greater but has also been shown to spread when RH levels are below 50%. 
• Temperature: Temperatures between 68° – 86°F mark the ideal range for powdery mildew, and for this reason outbreaks are common during mid-to-late summer months. While the disease does favor a moderately warm environment, higher temperatures have shown to be fatal to spores.
• Poor air circulation: When a facility isn’t properly ventilated, low air exchange can lead to stagnant air and create conditions that enable powdery mildew to spread even further.

But indoor and greenhouse conditions are one piece of the puzzle. Growers must also be aware of some of the ways their cultivation practices may also be contributing to powdery mildew outbreaks:

• Poor plant nutrition: Infected clones are a common source of powdery mildew. In an industry that utilizes mother plants and clones for propagation like cannabis does, it’s crucial that growers stay ahead of poor plant nutrition. Powdery mildew needs a living host to survive, a single infected plant can produce spores and begin spreading the pathogen to other plants within 5-10 of infection.
• Overcrowding: If humidity and poor ventilation set the stage for powdery mildew’s germination and survival, overcrowding can make outbreaks even worse. Putting too many plants in a room can make healthy plants more easily susceptible to airborne spores simply due to their proximity to diseased plants.
• Infected soil: When a plant is infected, fungal spores of powdery mildew can survive in its shoots, leaves, and other plant parts. Adding diseased plant debris to compost can result in contaminated growing medium which, if used, will spread the pathogen to new, healthy plants.

Read more about the causes of powdery mildew on cannabis plants here.

Prevention of Powdery Mildew in Cannabis Plants

To prevent a powdery mildew infection from occurring, growers may consider fungicides or the use of biological controls at key times throughout a plant’s growth cycle. Another method of prevention is removing infected plants and their debris to eliminate the source of the fungal disease before it gets out of control and leads to crop loss. But considering humidity and temperature fluctuations are common in indoor and greenhouse facilities, taking a proactive approach toward powdery mildew will go a long way toward preventing the disease.
 

Building regular plant inspections into the workflow can help the cultivation team stay on top of plant nutrition, find early signs of infection, and apply mitigating measures before an infection takes hold. This also allows them to ensure cutting and mothers are pathogen-free before propagation. But proper plant nutrition doesn’t end there – growers must also make sure their cultivation practices aren’t contributing to the problem. Planting disease-resistant strains is one way to ensure the health of the garden. Good plant spacing promotes airflow between plants and minimizes the potential for spores to spread. During post-harvest processing, good pruning techniques can also minimize the damage to buds and trichomes caused by powdery mildew even further.

Conducting ongoing sanitation practices is crucial for the effective prevention of powdery mildew and a variety of other pests and pathogens. From disinfecting seeds and new equipment before it enters the facility; to regularly sanitizing trimmers, air filters, and other tools; to developing decontamination protocols and providing gloves, beard, and hair nets to team members – adopting an integrated pest management (IPM) strategy is ultimately more effective against powdery mildew that one-off exterminations in the long run.

Throughout the facility, bringing in dehumidifiers can keep humidity at moderate levels that don’t promote contamination. Air filtration systems provide proper air circulation and limit the ability of powdery mildew and other airborne contaminants to survive. Fans work with air conditioners to keep indoor and greenhouse temperatures from getting too high, while adding CO2 enrichment can help nurture plant growth in less ventilated areas.

Read more about the Revention of Powdery mildew on cannabis plants here.

Management of Powdery Mildew in Cannabis Plants

Despite powdery mildew being a common and typically non-fatal fungal disease, one outbreak can negatively impact overall plant health and lead to devastating losses. While it is possible to save a plant infected with powdery mildew, the challenge for cannabis cultivators is figuring out a balance between finding the best ways to quickly manage occurrences of powdery mildew with ensuring the utmost product quality and safety for consumers.

Since bringing contaminated cuttings and mother plants into a facility is the most common way the pathogen spreads throughout the garden, removing infected plants is the first step toward mitigating it. Powdery mildew can’t survive without a living host, so keeping an eye out for sources of the infection and then destroying them quickly is crucial. Disturbing infected plants can promote the spread of powdery mildew further, however, so it’s important that growers also adopt protocols that limit the potential for cross-contamination. 

Another way to prevent powdery mildew is by applying fungicides throughout the various stages of plant growth to inhibit germination. Horticultural oils, vaporized sulfur, and potassium bicarbonate are just a few of the options growers have at their disposal to reduce and even eliminate spore development in indoor and greenhouse settings. For optimal consumer safety, growers will need to consider local and state regulations when determining the best fungicidal solution and when to apply it.

Improving growing conditions can also help keep powdery mildew outbreaks at a minimum. The variability of cannabis cultivars gives growers the option of sourcing genetics that show resistance to the fungus, allowing them to shore up the health of their garden by planting disease-resistant strains. Proper plant spacing keeps rooms from getting overcrowded, while regular pruning gives growers a chance to inspect plants and remove visual signs of infection in real-time. And using substrate and environmental sensors helps growers track and monitor humidity levels, temperature ranges, and other data about their grow – giving them the real-time insights they need to be able to take action. 

Read more about integrated pest management here.

Conclusion

In summary…

• Powdery mildew is a common fungal disease that is rarely fatal but can result in reduced potency, yield, and quality in cannabis plants. 
• High humidity, moderate temperatures, and overcrowded rooms with poor air circulation create ideal conditions for the spread of powdery mildew.
• Growers can unknowingly spread powdery mildew by bringing infected cuttings into their garden or incorporating contaminated plant debris into compost.
• Using fungicides and biological controls throughout the growth cycle can aid in the prevention of powdery mildew, but growers who stay on top of sanitation protocols, incorporate air filtration and other systems to support good ventilation, and conduct regular plant inspections will experience greater long-term benefits.
• While removing infected plants is the most immediate way to manage a powdery mildew infection before it spreads, planting disease-resistant strains and using sensors to track and monitor the substrate and environment are effective ways to keep the pathogen at bay.