Best Practices For Early Management of Harmful Microalgae During Cannabis Cloning

We uncover the interwoven negative economic impact of algae in cannabis propagation and proposed blueprint strategies for management.

Figure 1. Colonization of rockwool by microalgae during cannabis stem cloning can trigger fungi-mediated stem and root rot infection. (A) Untreated commercial rockwool and (B) Hydrogen peroxide treated rockwool. Photo credit: Louis Bengyella

Algae compete with other organisms for nutrients, but some species may also produce toxins that inhibit (or stop) plant growth [1,2]. This phenomenon is of concern to cannabis cloners. The use of sinsemilla cannabis (i.e., female cannabis that does not develop seeds) to produce clones has become more popular among cannabis growers to quickly start the growing season. This cloning approach is profitable among growers within the commonwealth of Pennsylvania. With commercial feminized clones being valued as high as three to five dollars depending on the cannabis genetics, young plant production can serve as a source of income for nurseries and greenhouses involved in cannabis propagation. Grower's love for sinsemilla clones transcends mere uniformity in cannabinoid content. The use of light-sensitive sinsemilla cannabis clones offers some unique advantages to growers such as:

• guaranteed female plants
• crop with near-uniform cannabinoid and terpenoid content
• rapid plant establishment
• shorter maturation time
• increased production turnover

While producing clones is achievable with some modest investments in cloning gel, scissors, blades, incubator, and substrates such as rockwool (or soilless media plugs such as Jiffy pellets and Ellepots), stem and root rot can inflict significant economic damage. The use of microalgae-contaminated media at a pH greater than 5.5 during cloning may result in a loss of revenue for cannabis cloners (Figure 1).

An infestation of the moisture-loving microalgae in plant media could slow (or stop) rooting allowing root rot-causing fungi to infect plant tissue. Algae can be a problem in cannabis propagation under greenhouse and nursery conditions when a hydroponics (or aeroponics) system is used, and the algae must be detected early. The harmful microalgae, such as the Chlorella species, compete with cannabis clones for light, nutrients, and oxygen (Figure 2). The harmful algae take up organic carbon and can perform photosynthesis.

Figure 2. Brightfield microscopic image of microalgae Chlorella sp. isolated from rockwool propagating material at 800X. Photo credit: Louis Bengyella

Microalgae Chlorella species are unicellular and colonial organisms (Figure 2) that produce chlorophyll [3], and so they appear green on rockwool. These opportunistic microbes prevent rooting, and may increase plant's susceptibility to stem rot resulting in plant death (Figure 1). In some cases, depending on the cannabis genetics, microalgae infestation can induce stress leading to stunted clones that might enter into early bloom resulting in a potential loss of revenue (Figure 3).

Figure 3. Green microalgae can induce stress during rooting, stunt growth, and trigger early bloom at 16 h/8 h photoperiod in certain cannabis genetics. Photo credit: Louis Bengyella

Since microalgae produce glycoprotein sex-pheromone(s) during reproduction under heat stress [4], growers should maintain incubation temperatures at or below 25^oC (~77^oF) during cloning to slow algal growth. Furthermore, it is unclear how glycoprotein sex-pheromone affects cannabis phytochemistry and physiology. Thus, early detection and management of microalgae are required during clone production.

Some effective microalgae management techniques during cannabis cloning include the following:

• The use of low electric conductivity (EC, < 250 ppm) NPK solution buffered at pH 5 - 5.5
• The use of incubation mats set at (or below) 25°C
• Use sterilized cloning tools e.g., steam sterilization of rockwool and spraying capric acid on rockwool when algae is detected.
• Sterilization of cut branches in 0.7 %v/v hydrogen peroxide by submersion before cloning
• Periodic ventilation of incubators (~ 4 times per day) to prevent high moisture which favors the growth of microalgae
• Adoption of a 16 h/8 h photoperiod during cloning
• Avoid using non-sterile water from surface sources such as lakes, rivers, streams, ponds, or wells in any cloning step

Among these control strategies, buffering cloning solution with 100 mM citrate phosphate buffer at pH 5.5 is key to controlling algae. This is because the buffer solution reduces pH alterations in the cloning medium when excessive carbon dioxide released by microalgae is converted to carbonic acid.

As nurseries prepare for cannabis clone production to kickstart the growing season, they need to be aware of the negative impact of algae in the cloning process. Furthermore, growers should adopt general strategies to manage harmful algae in the greenhouse and nurseries as previously described [5].

References

[1] Schwarz, D. and Gross, W. (2004). Algae affecting lettuce growth in hydroponic system. The Journal of Horticultural Science and Biotechnology 79:554-559.

[2] Hulzebos, E. M., Adema, D. M. M., Dirven-van Breemen, E. M., Henzen, L., van Dis, W. A., Herbold, H. A., Hoekstra, J. A., Baerselman, R., & van Gestel, C. A. M. (1993). Phytotoxicity studies with Lactuca sativa in soil and nutrient solution. Environmental Toxicology and Chemistry, 12, 1079-1094.

[3] van den Hoek, C., Mann, D.G., and Jahns, H.M. 1995. Algae An introduction to phycology. Cambridge: Cambridge University Press. ISBN 978-0-521-30419-1.

[4] Kirk, D.L. and Kirk, M.M. 1986. Heat shock elicits production of sexual inducer in Volvox. Science. 231 (4733):51-54.

[5] Hendrickson T and Dunn B (2021) Algae control for greenhouse production available September 2021.