Small-time growers working in grow tents, as well as large-scale producers with acres of greenhouses, both face the same pathogenic threat: powdery mildew. This fungus is a scourge for all types of cultivators, working in all kinds of environments.
But, while powdery mildew was once just a discussion among hobby farmers, it is now a target for serious research projects. Powdery mildew in the cannabis grow room or greenhouse is a primary concern for growers who care about the bottom line and the safety of the final product.
What is Powdery Mildew?
As per a definition from Advances in Botanical Research, “Powdery mildew fungi (Ascomycota phylum) are obligate biotrophic plant pathogens that can only grow and reproduce on living host cells. They infect a wide range of plants, including many crops, and the diseases they cause are common, easily recognizable, and widespread.”
Powdery mildew infects cannabis through the germination of microscopic spores. Carried by the wind, contaminated surfaces, or the plants themselves, these spores rapidly propagate in the right environment to slow down growth and damage harvests.
In the Canadian Journal of Plant Pathology, authors Cameron Scott and Zamir Punja explain how colonies of powdery mildew “limit photosynthesis and reduce nutrient availability to the plant, causing premature leaf drop as well as reducing overall vigor and potential yield of plants.”
If uncaught and untreated, the fungi responsible for powdery mildew may eventually spread into the flowers during the final weeks of cultivation. Bud rot can set in before or after harvest, destroying months of work and making the end product unfit for human consumption.
Treatment Options for Powdery Mildew
In 2020, authors Scott and Punja published “Evaluation of disease management approaches for powdery mildew on Cannabis sativa L. (marijuana) plants.” Using 12 strains of cannabis and three rounds of tests, the authors tried several chemical and non-chemical treatments for powdery mildew within a controlled environment.
Following inoculation with powdery mildew, each round tested ten chemicals for their effects on disease progression. Out of the ten products used, researchers determined the following options offered the most protection against this fungal pathogen (chemicals listed with primary active ingredient):
● Luna Privilege SC (40.98% fluopyram)
● Regalia® Maxx (20% extract of Reynoutria sachalinensis)
● MilStop (85% potassium bicarbonate)
● Rhapsody ASOTM (QST 713 dried Bacillus subtilis)
● Neem oil (70% cold-pressed neem oil)
● Stargus (96.4% Bacillus amyloliquefaciens strain F727 cells and spent fermentation media)
Notably, several of these products are already approved for use on cannabis in Canada, including MilStop and Regalia Maxx. Neem oil is a ubiquitous natural product for disease and pest treatment across the industry, approved in the U.S. but not in Canada. The most effective product as per the results of this study, Luna Privilege SC, is not approved for use on cannabis in Canada or the U.S.
Non-Chemical Treatment Options
To avoid excessive or potentially dangerous applications of chemicals in the grow room, there is a push to develop technological options for treating cannabis diseases.
One such option is UV-C and UV-B ultraviolet light. The study highlighted above also explored a non-fungicide treatment: a CleanLight Pro, which uses UC-V light.
Researchers exposed inoculated plants to UV-C light using this device for three to five seconds a day throughout the study. While this was a handheld device and manually operated, “treated cannabis leaves showed that mildew spores had not germinated, and mycelial growth was inhibited, suggesting a direct toxic effect.”
Other options (not tested within this study) include gamma radiation, e-beam (beta) irradiation, and cold plasma inundation. About 80 percent of licensed producers are already working with irrigation technologies to kill harmful pathogens post-harvest and before consumption. However, there are growing concerns based on recent studies indicating irrigation damages terpene content.
For both UV-C and irradiation, much more research is required to determine feasibility at a large scale and effect on the final product.
Prevention the Most Effective Against Powdery Mildew
While there have been advances in cannabis-approved fungicides and tech-based treatment options, prevention is always the first line of defense. As Scott and Punja detailed, prevention starts as early as strain selection. Some strains are genetically resistant to powdery mildew, and worth testing should powdery mildew become an ongoing issue.
Other preventions require investments into facility infrastructure, including advanced humidity control systems like the MSP dehumidification system. This is one of many innovative HVAC technologies coming online, all of which strive to improve filtration and reduce energy consumption while offering total environmental control to the cultivator.
Prevention First, Treatment Second
While there seem to be several useful fungicides capable of reducing the spread of powdery mildew, not all are suitable for cannabis. Many of those tested within the 2020 research paper is not approved for use in Canada or the U.S. It remains to be seen whether these products linger on the plant material once dried, cured, extracted, and consumed.
Developing commercial-scale and automated mechanisms to deliver UV-C rays in short spurts may be a more effective and safer treatment option with little impact on the final product's chemical profile.
Yet, the most effective way to reduce the risk of powdery mildew is through preventative measures. This is largely about facility design, proper HVAC, and total environmental control in the grow room.