Gray and Brown Molds

Photos courtesy of Ed Rosenthal unless otherwise noted

Pests and diseases are a fact of life in agriculture. Between the 1950s and the 1990s, the go-to solution for pest and disease management was almost exclusively eradication with pesticides. What is now commonly known as the organic agriculture movement arose from the desire to protect the consumer, the farm worker, and the environment. Then in 1959, Vernon Stern introduced the concept of integrated pest management (IPM) in the landmark paper “The Integration of Chemical and Biological Control of the Spotted Alfalfa Aphid: The Integrated Control Concept.” His ideas are still the basis of modern agricultural pest management, whether conventional or organic.

Using IPM for cannabis crops is especially important. As more states legalize cannabis, the idea that a product that is primarily consumed through inhalation should be clean of toxic residues guides much of the legislation. Presently, the use of all cannabis crop protection materials is tightly regulated. Many of the pesticides and fungicides that informal market cannabis growers use have been outlawed for legal cannabis production 

IPM is an approach to pest management in which the grower relies on several control strategies. As many strategies as can work together are employed, hence the term “integrated.” These different approaches can be broken down into four principles of IPM: resistance, exclusion, disruption of the pest life cycle, and eradication. They are not mutually exclusive principles, and strategies often fit into more than one category. But above all: Believe your eyes and act quickly

Diseases can strike cannabis plants at any stage. A garden’s susceptibility is often traceable to environmental imbalances in temperature, moisture, airflow, and pH, among other things.

Looking specifically at gray and brown mold (Botrytis), the indoor garden is easier to control. Since molds thrive in high humidity and cool temperatures, lower the humidity to 50%, and raise the temperature to the 70s. Add fans to increase airflow. Outdoor growers need to bring in the harvest before cool, damp weather sets in. Strategies include planting cultivars that flower early in the season, growing autoflowers, and using light dep methods.

The following excerpt from Cannabis Grower’s Handbook follows the IPM approach. The solutions listed for eradication can also be used preventatively and applied on a regular basis to repel spores before they set in. — Ed Rosenthal

Gray Mold & Brown Mold (Botrytis)

Gray mold, Botrytis cinerea, is found almost everywhere and can cause disease on most plants, including cannabis. It causes damping off, stem canker, and bud rot. It is one of the most common fungal diseases that attack cannabis.

The fungus can germinate only on wet plant tissue when the temperature is between 55 and 70°F (13-21°C). This often happens in dry weather as dew accumulates on the leaves. Once it starts growing, it can tolerate a wide range of humidity and temperatures, but high humidity and cool temperatures help it thrive. Lowering the humidity often stops it from continuing to grow.

Gray mold, like most other fungi, enters and easily infects any part of a plant that is either wounded, damaged from pests and pruning, or beginning to die. Thus, it is very important to sanitize pruning equipment between cuts. Cuts and lesions are a normal part of plant life, so all plants are subject to attack when conditions are favorable to the mold. Unhealthy or shaded areas of plants or crevices in buds are ideal conditions for mold. Spores travel mostly via wind and rain and even in tap water, but they can be brought into gardens on clothing and pets. Shaded areas of the plant that do not get a lot of light are usually infected first. Then the disease spreads quickly through growth and spores.

Gray mold does the greatest amount of damage during flowering. It attacks the flowering tops, leaves, and stalks. Seedlings and seeds can also be infected and killed. The mold starts out whitish like powdery mildew, but then darkens to a smoky gray or brown. It has a fuzzy appearance, and light to dark brown rot forms in the damaged tissue. Leaves and buds turn yellow. In higher humidity, the gray mold leaves a brown slimy substance on the leaves and turns the bud to rot, especially when the tissue is dense late in flowering. Stems with unhealed breaks can be infected with B. cinerea, causing stem cankers, which then affect the rest of the plant by depriving it of nutrients and water.

Monitoring

• Signs: spore-trap sampling of the air and surface swabs. There are numerous methods to identify and quantify the spore load in an air space or on surfaces, which have been developed for other industries. Some of these use spore identification by microscopy; others require the captured spores to be cultured on a nutrient media for identification; and others use PCR technology. 
• Signs: transmission-light microscopes can be used to identify botrytis. The structure to look for is a branched reproductive mycelia with oval spores in clusters on the tips of the branches. They are reminiscent of grape bunches. 
• Symptoms: gray or brown fuzz inside buds that eventually advances to the outside, turning green buds gray or brown.
• Infected area turns brown and dries up. It may be soft if wet or brittle if it has dried.  
• Stem canker or stem rot. 

Resistant Varieties

Although this pathogen can be devastating to crop yield, several resistant cultivars are available. Varieties that produce smaller flowers are likely to be more resistant than those that produce large, compact flowers that create a micro-environment conducive to fungal germination, proliferation, and sporulation. Cultivars that have a much more open branch architecture may allow for better airflow and humidity control. It’s quite possible that cultivars which prefer tropical environments will be less susceptible to this pathogen.

Exclusionary Practices

• Use HEPA filtration of air inlets.
• Decontamination of the grow environment air and incoming air can be accomplished using peroxides, ultraviolet light, chlorine dioxide, ozone gas, ionizers, and antimicrobial fogging.  Hospitals, food processors, and laboratory clean rooms use different technologies and materials to reduce pathogen loads in the air. 
• Create work procedures that forbid going from infected to uninfected rooms, and general worker hygiene to prevent movement of spores from infected areas to clean areas.
• Sanitize hands and tools. 
• Fungicidal clone dips.
• Remove plant debris post-harvest and around the vicinity of the garden to exclude aerial spores from disseminating to clean plants. Botrytis can form a protective structure called a sclerotium, which allows it to survive in debris and soil. 
• Rogue infected tissue: Carefully remove infected flowers to prevent spore dissemination.

Disruption of Life Cycle

• Climate control is especially important as harvest approaches. The best strategy to control Botrytis is to keep the temperature and humidity outside the range that allows for spore germination, mycelial proliferation, and sporulation. This includes the macroclimate of the garden and the microclimate in the canopy and inside the flower. The temperature should be raised to a minimum of 75°F (24°C) day and night if possible. Outdoors, heaters can be used to keep small gardens warm.
• Control air movement: air currents can disseminate spores to plants in areas of the garden that haven’t been infected or to younger plants.
• Use bio stimulants: numerous substances are known to elicit defensive responses that cause plants to thicken their cell walls. This thickening of the cell walls can make it much more difficult for a spore that has germinated to penetrate the flower tissue and infect it.

Eradication

• Preventative biological control with numerous competitive microorganisms (B. subtilis, B. amyloliquefaciens, S. lydicus, U. oudemansii).
• Use fungicidal sprays: some materials that can help control the spread of spores are herbal sprays containing cinnamon or clove oil. These kill the organisms and dry the infected areas in place. 
• Use a 10% milk solution (32 ounces per gallon) to prevent mold. It can be used after rain or during humid weather. 
• Use a 1% potassium bicarbonate (KHCO₃) solution (1.25 ounces per gallon). Sodium bicarbonate (baking soda, NaHCO₃) can also be used, although it delivers sodium, which presents problems if it builds up.

Chlorine Dioxide for Pest & Disease Control

Chlorine dioxide (ClO₂) is a yellowish-green gas that has antimicrobial properties and shows great potential as a disinfectant in cannabis cultivation, drying, curing, and trim rooms. It is less corrosive than other gaseous disinfectants such as ozone; however, it has been shown to neutralize fungal spores for crops other than cannabis (Lee et al. 2020). This paper shows that ClO₂ is very effective in killing Aspergillus spores for storing fresh coffee beans. Plant pathogens such as powdery mildew (PM) are also susceptible to ClO₂ (Sharma et al. 2017).

Many cultivars are especially susceptible to PM infection, and one of the most effective preventative measures against infecting a crop (applying sulfur to the canopy) should not be used once the flowers appear on the stems. Sulfur application will impart an unpleasant aroma and flavor on cannabis flowers. The application of sulfur does not kill PM spores; however, it creates an environment that makes it difficult for the spores to germinate. 

Using a slow-release ClO₂ product, such as ProKure D, in combination with sulfur can be the one-two punch that prevents PM from rearing its ugly head. A slow-release ClO₂ product sanitizes the air and the plant tissue, so that the PM spores are neutralized. Once the flowers develop in the canopy, stopping the application of sulfur and switching to hanging ClO₂ packets above the canopy at a regular interval will prevent PM without leaving a residual aroma or flavor on the cannabis flower. ClO₂ is gaseous and breaks down into chloride ions and oxygen. Chloride is naturally found in plants and does not impart any flavor. This product is OMRI listed and registered with the EPA as a fungicide, disinfectant, algaecide, virucide, and deodorizer, so it goes beyond just eliminating PM from the canopy.

This article excerpted from Ed Rosenthal’s new book Cannabis Grower’s Handbook, which will change the way people grow for years to come. Stay in touch with Ed @edrosenthal420.