A Comprehensive Literature Review

person with black gloves harvesting cannabis with silver shears

Tool Sterilization to Prevent Viroid Transmission

Summary

To determine the most effective method of tool sterilization to prevent mechanical transmission of Hop Latent Viroid, we conducted a comprehensive literature review of studies investigating viroid disinfection treatments. Fifty-three different chemicals/treatments were evaluated for the ability to prevent viroid transmission in the referenced studies. Six tested treatments showed 100% effectiveness at viroid deactivation. While most of these chemicals are shown to be effective against specific viroids, one chemical, household bleach, shows broad effectiveness across numerous studies, different plant species and multiple viroids. Based on this observation, we strongly recommend the use of 10-20% household bleach for disinfection of cutting tools, equipment and surfaces to limit the spread of Hop Latent Viroid in cannabis grow facilities.

Careful evaluation of conditions applied in each referenced study indicate that bleach is an effective disinfectant when used at a range of concentrations and for various incubation times. This quality provides flexibility and room for human error when employed as a broad disinfection SOP in large facilities. Because the effective concentration in dilute bleach fluctuates over time, it is recommended to mix a fresh bleach solution at the start of each day or when the solution becomes saturated with plant material. However, studies indicate that 20% bleach solutions can remain effective for up to 30 days at room temperature (Li et. al., 2015) allowing for flexibility in the event of miscommunication among staff.

It should be noted that several common disinfection practices are not broadly effective at viroid decontamination. While alcohol can be effective at deactivation of other plant pathogens, viroids are not deactivated by alcohol and treatment of cutting tools with only alcohol may actually increase transmission of viroid infection (Matsuura et. al., 2010). Viroids show tolerance to heat treatment in multiple studies. Infectiousness persisted following alcohol dip and flaming, propane flame treatment and prolonged heat incubation of contaminated tools. Therefore, we recommend not relying solely on heat treatment as a regular disinfection practice for prevention of viroid transmission.

Tool Sterilization Recommendation

To prevent mechanical transmission of viroids in cannabis facilities, a dilute bleach solution should be used to disinfect tools, surfaces and gloved hands between plants.

To create a 10% dilute bleach solution mix:

• 1 part household bleach (>5% active ingredient -sodium hypochlorite)

• 9 parts water

To create a 20% dilute bleach solution mix:

• 1 part household bleach (>5% active ingredient -sodium hypochlorite)

• 4 parts water

Potentially contaminated tools should remain in the bleach solution for 60 seconds or more to allow complete deactivation. Bleach is corrosive to metal tools so prolonged incubation is not recommended. The bleach solution should be mixed fresh at the start of each day and replaced if the solution becomes saturated with plant material. Dilute bleach in a spray bottle can be used periodically to disinfect gloved hands.

birds eye view of a Clorox bottle on a wooden table

Literature Summary Data

The tables below summarize results from published scientific reports investigating the efficacy of various chemicals and treatments for removal of viroid contamination from tools. Only studies where effectiveness was determined by inoculation of clean plants with sterilized tools are included in this summary. Tables indicate the concentration(s) of each chemical used in the studies, details of the treatment applied and the percent transmissibility that remained after disinfection.

References

Desjardins, P.R., Saski, P.J., Drake, R.J., 1987. Chemical inactivation of avocado sunblotch viroid on pruning and propagation tools. Calif. Avocado Soc. 1987 Yearbook. 71,259 262.

Garnsey, S.M., Whidden, R., 1971. Decontamination treatments to reduce the spread of citrus exocortis virus (CEV) by contaminated tools. Proc. Fla. State Hort. Soc. 84,63 67.

Hollings, M., Stone, O.M., 1973. Some properties of chrysanthemum stunt, a virus with the characterization of an uncoated ribonucleic acid. Ann. Appl. Biol. 74, 333 348.

Li, R., Baysal-Gurel, F., Abdo, Z., Miller, S.A., Ling, K.S., 2015. Evaluation of disinfectants to prevent mechanical transmission of viruses and a viroid in greenhouse tomato production.Virology J. 12, 5.

Mackie, A.E., Coutts, B.A., Barbetti, M.J., Rodoni, B.C., McKirdy, S.J., Jones, R.A.C., 2015. Potato spindle tuber viroid: stability on common surfaces and inactivation with disinfectants. Plant Dis. 99, 770 775.

Matsuura, S., Matsushita, Y., Usugi, T., Tsuda, S., 2010. Disinfection of tomato chlorotic dwarf viroid by chemical and biological agents. Crop Prot. 29, 1157 1161.

Roistacher, C.N., Calavan, E.C., Blue, R.L., 1969. Citrus exocortis virus – chemical inactivation on tools, tolerance to heat and separation of isolates. Plant Dis. Reptr. 53,333 336.

Singh, R.P., Boucher, A., Somerville, T.H., 1989. Evaluation of chemicals for disinfection of laboratory equipment exposed to potato spindle tuber viroid. Am. Potato J. 66,239 245.

Thi Thu, 2018. Studies on transmissibility, cytopathology and control of Tomato chlorotic dwarf viroid and Potato spindle tuber viroid. Doctoral Thesis, Bonn University

Timmermann, C., Mu¨ hlbach, H.P., Bandte, M., Bu¨ ttner, C., 2001. Control of mechanical viroid transmission by the disinfection of tables and tools. Meded Rijksuniv Gent Fak Landbouwkd Toegep Biol. Wet. 66, 151 156.