IPM Pointers to Prevent Disease in Greenhouse Cannabis

Cannabis Buds

Plants should be inspected daily for the presence of powdery mildew and botrytis. For large operations, the greenhouse can be divided into a grid so that the entire crop is surveyed in less than a week.

For years, Cannabis sativa has been a Controlled Substances Act Schedule I substance, and growth of both hemp and adult-use and medicinal cannabis has been illegal. Now, things are changing. A federal law permitting hemp production was passed on Dec. 20, 2018. Cannabis (adult-use and medicinal) is still illegal under federal law, but 11 states have legalized recreational use, and growth for medical use is permitted in 31 states. Growers who read this article are encouraged to check on the legal status in their location.

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Scout Regularly and Employ Physical Controls

A recurring problem with greenhouse cannabis production is plant diseases. Because of high humidity and tightly packed growth, plants are especially subject to foliage diseases. Common cannabis diseases are powdery mildew caused by Podosphaera macularis, and gray mold caused by Botrytis cinerea. Cannabis diseases can be managed by an integrated pest management (IPM) program that includes monitoring, physical controls, resistant species, biopesticides, and least-toxic chemical controls. The emphasis should be on prevention.

Plants should be inspected daily for the presence of powdery mildew and botrytis. For large operations, the greenhouse can be divided into a grid so the entire crop is surveyed in less than a week. Powdery mildew prefers new growth. It starts as bumps on leaves that turn white and spread. Botrytis attacks flowers. Buds are covered with a gray fuzz, and finally slime. A written record can help monitor disease progress and effects of treatment.

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Physical controls include temperature, sanitation, ventilation, air filtration, and irradiation with UV-C light. Powdery mildew and botrytis are encouraged by humidity and keeping humidity below 50% can reduce damage. Fans and good ventilation are essential. Sanitation includes dedicated greenhouse clothing to reduce introduction of spores. Air can be filtered or treated with ultra-violet (UV) light to reduce spore contamination. Powdery mildew thrives in high humidity (>90%) and cool temperatures (60°F) at night, followed by low humidity (40% to 70%) and hot (80°F) daytime temperatures. Keeping temperatures and humidity from fluctuating by heating at night can reduce the incidence of disease.

Low intensity UV-C light (200 to 280 nm) may protect cannabis against powdery mildew and botrytis. For instance, USDA researchers have found that treatment once or twice a week with 20 to 60 seconds of UV-C light at 20.6 microwatts/cm2 followed by a four-hour dark period can control powdery mildew on strawberry. Care must be taken not to overexpose the plants, as that could cause phytotoxicity. The UV light destroys fungal DNA, and fungus DNA repair mechanisms are not active at night. The light may also trigger induced systemic resistance (see below) in the plants and kill the spider mites that plague cannabis growers.

Take Advantage of Resistance

Cannabis strains with high resin content may be more resistant to mold, and mold-resistant cultivars are advertised on the internet. Mexican, Colombian, and Thai strains are more resistant than Afghan cultivars. But economic plant characteristics may be more important than disease resistance. For instance, smaller, thinner buds are resistant to botrytis, but larger buds are favored in the marketplace. For cannabis, important considerations are cannabinoid and terpene content. Products in commerce contain about 5% to 10% tetrahydrocannabinol (THC), but some strains contain 20%. For medical cannabis, equal amounts of THC and cannabidiol (CBD) are preferred. For hemp, biomass, fiber strength, and CBD content are valued.

Resistance can also be induced. Induced resistance occurs when a chemical, insect, or physical challenge initiates a protective response. The plant produces secondary metabolites that prevent infection and spread of a plant pathogen. Among commercially available materials are potassium phosphate and potassium phosphite, silica formulations, and extract of giant knotweed, Reynoutria sachalinensis (Regalia). Growers will need to check with their state agencies to see if these products are available for use on their crops and how they must be used.

Use Biopesticides Preventatively

There are also non-toxic and least-toxic remedies for powdery mildew and other foliage diseases. Powdery mildew and botrytis will respond to oils, potassium bicarbonate, and biopesticides such as Bacillus subtilis and Bacillus amyloliquefaciens. Formulations are often applied to prevent disease, not cure it. All these materials should be tested for phytotoxicity on one plant before application to the entire crop.

The Pest Management Problem

Because federal law prohibits production of adult-use and medicinal cannabis, there is a pest management problem in the states where it is legal. An EPA-registered pesticide cannot have cannabis listed on the label. Thus, EPA-registered pesticides cannot be applied. Yet, many states allow cannabis to be grown, sold, and consumed. To make the best of a bad situation, cooperation between state and federal agencies has led to lists of approved pesticides. Pest management agencies in each state will answer questions about which pesticides are allowed.

For instance, EPA-registered pesticides with broad labeling that are also exempt from food tolerance requirements can be used in California. And 25b-exempt materials such as clove oil are allowed. Pesticides not registered for food use; restricted pesticides such as the insecticides bifenthrin, fipronil, and cyfluthrin; pesticides on the Groundwater Protection list such as imidacloprid; and pesticides labeled Danger are forbidden.

Check State-Approved Least-Toxic Pesticides Lists Carefully

Growers should adhere to the list of approved products for their state. For example, Oregon allows potassium bicarbonate, various seed oils, horticultural oils, neem oil, essential oils such as cinnamon, clove, rosemary and thyme, and microbials including B. subtilis, and B. amyloliquefaciens. California has a generic list that includes horticultural oil, neem oil, sodium and potassium bicarbonate, and B. subtilis.

But the specific formulation is important. Oregon, Washington, and Colorado have lists of permitted formulations, but the lists are not consistent. Oregon and Washington permit Kaligreen potassium bicarbonate; Colorado does not. Colorado lists GreenCure potassium bicarbonate, but only for home use.

Organic Formulations

Organic certification agencies are willing to certify hemp production. But because adult-use and medicinal cannabis is illegal according to federal law, USDA organic certification is not possible. However, growers can still use organic practices and stick to pesticides approved in their state for organic production such as Cease (Bacillus subtilis QST713), Double Nickel (B. amyloliquefaciens D747), Rhapsody (B. subtilis QST713), and Serenade (B. subtilis QST713).

Safety First

Growers should keep in mind that their product may be smoked like a cigarette or ingested orally. Therefore, pesticide use should be minimized. Safety is not guaranteed, but pesticides mentioned here are mostly biopesticides, salts, and oils that have low toxicity to mammals and are approved by many states where cannabis growth is legal. Compliance with state lists is important, because legal cannabis is analyzed for cannabinoid content, illegal pesticides, and other contaminants.

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