White mold, caused by the necrotrophic fungal pathogen Sclerotinia sclerotiorum, has quietly become one of the most concerning diseases facing cannabis cultivators in North America. A 2026 study published in Scientific Reports used dual RNA sequencing to map exactly how this fungus attacks cannabis tissue at the molecular level, and the findings carry practical implications for every licensed producer running a greenhouse operation.
Why Sclerotinia Deserves Your Attention
Sclerotinia sclerotiorum is not a cannabis specialist. It infects over 600 plant species worldwide, including soybeans, canola, sunflowers, and lettuce. That enormous host range is precisely what makes it dangerous in mixed-crop greenhouse environments. If you grow cannabis near tomato, cucumber, or hemp crops, inoculum can travel between houses on clothing, equipment, or air currents.
Unlike powdery mildew, which stays on leaf surfaces, Sclerotinia is a necrotrophic pathogen. It kills host cells before feeding on them, producing oxalic acid and a cocktail of cell wall-degrading enzymes that break down plant tissue rapidly. Infected cannabis plants develop watery, soft rot lesions on stems and buds that spread quickly in humid conditions. By the time symptoms are visible, the pathogen has already established deep in the tissue.
What the 2026 Dual RNA-Seq Study Found
Dual RNA sequencing captures gene expression from both the pathogen and the host simultaneously during infection. This approach lets researchers see the molecular conversation happening between Sclerotinia and cannabis in real time.
The study confirmed that S. sclerotiorum deploys a coordinated attack strategy. Early in infection, the fungus upregulates genes involved in oxalic acid biosynthesis, suppressing the plant’s oxidative burst defense. It then activates polygalacturonases and other enzymes that degrade pectin in cell walls, creating the characteristic watery lesions.
On the cannabis side, the plant mounts a defense response involving jasmonic acid and ethylene signaling pathways, but the response is often too slow and too weak to stop a well-established infection. The research showed that cannabis genotypes vary significantly in how quickly they activate these defense genes, suggesting that breeding for faster immune responses could improve resistance.
Conditions That Favor White Mold Outbreaks
Understanding the environmental triggers for Sclerotinia is essential for prevention. The pathogen thrives when relative humidity exceeds 80% and temperatures range between 15-25 degrees Celsius. Dense canopy growth that restricts airflow creates microclimates perfect for infection. Overhead irrigation and poor drainage compound the problem by keeping surfaces wet for extended periods.
In Canadian greenhouses, the transition from spring to summer creates ideal conditions. Daytime temperatures warm while nighttime humidity remains high, and growers often increase plant density to maximize yields before outdoor harvest season.
Sclerotinia survives between crops as sclerotia, hard, dark structures that can persist in soil and growing media for years. A single infected plant can produce hundreds of sclerotia, each capable of germinating into apothecia that release millions of airborne ascospores. This means that one outbreak, if not properly managed, can contaminate a facility for multiple growing cycles.
Detection and Testing Strategies
Early detection is the most effective tool against white mold. Visual scouting should focus on the lower canopy and areas near the base of stems, where humidity tends to be highest. Look for water-soaked lesions, white cottony mycelial growth, and the formation of dark sclerotia on or inside stems.
However, visual scouting alone catches infections late. Molecular testing using isothermal amplification can detect Sclerotinia DNA before symptoms appear. Agdia’s AmplifyRP XRT platform provides PCR-level sensitivity without requiring a thermocycler, making it practical for on-site use in greenhouse labs. For broader pathogen surveillance programs, combining rapid tests with environmental monitoring of humidity and temperature gives growers the earliest possible warning.
Air sampling is another approach gaining traction. Because Sclerotinia ascospores are airborne, spore traps placed in greenhouse bays can detect spore loads before they settle on plants. Several Canadian research institutions are developing protocols that integrate spore trap data with molecular confirmation.
Integrated Management for Cannabis Facilities
No single strategy eliminates Sclerotinia risk. An integrated approach combining environmental control, sanitation, and monitoring provides the best protection.
Start with environmental management. Maintain relative humidity below 70% during flowering, use horizontal airflow fans to break up moisture pockets, and avoid overhead watering. Dehumidification systems pay for themselves quickly in facilities with recurring mold issues.
Sanitation protocols should include removing all plant debris between cycles, sterilizing growing media and containers, and training staff to recognize sclerotia. Any plant showing symptoms should be bagged and removed immediately, not composted on site.
Biological control agents containing Coniothyrium minitans can parasitize sclerotia in soil and growing media, reducing the inoculum bank over successive cycles. While not a standalone solution, biocontrol adds a layer of protection that complements environmental controls.
For Canadian licensed producers operating under Health Canada regulations, documenting pathogen testing and management protocols is not optional. Regular testing for Sclerotinia and other fungal pathogens like Botrytis cinerea and Fusarium species should be part of every quality assurance program. Having rapid testing tools on hand ensures that suspect plants can be confirmed quickly, allowing targeted removal before an outbreak spreads.
Looking Ahead
The transcriptomic data from the 2026 study opens new doors for cannabis breeding. Identifying genotypes that mount faster, stronger defense responses against Sclerotinia could eventually give growers resistant cultivars. Until then, the combination of environmental management, strict sanitation, and early molecular detection remains the best defense against this persistent and destructive pathogen.
