A) HpLVd infection causing “Christmas tree” looking growth on a mature plant, as compared to B) normal growth in a larger plant.

Note: This is the second post of a three part series. The first is an introduction to the pathogen and its biology, the second covers management practices and identifying it in the garden, and the third covers research Dark Heart Nursery that is doing and some commonly asked questions about HpLVd.

Part Two: Identification and Management

HpLVd in cannabis is mainly known to cause bud quality and yield loss. However, it can cause other issues. In vertically integrated facilities, propagation can take a big hit. Cuttings from infected moms do not develop roots as readily as cuttings from healthy moms. Infected plants will also be more symptomatic when co-infected with other pathogens. HpLVd can also cause brittle stems in some strains, so lateral branches break more easily, leading to poor plant structure. We use the modifiers “may” and “can also” because viroids are known to be highly variable in causing symptoms, both due to environmental conditions and crop genetics.[8] Even though your strain is okay with infection now, later in the year it may respond differently to infection. Or strain A is asymptomatic, but strains B and C are heavily impacted.

Basic sanitation can greatly reduce the spread of HpLVd in the greenhouse. These measures include cleaning pruning shears between plants or plant lots, physically separating plants by lot/strain, and sanitizing greenhouse spaces between production cycles. All workers should wash their hands before entering a greenhouse or bay, and wash hands between bays. Any surface that has plant sap on it can be the source of infection for a clean plant. This also includes tubs of water that hold cuttings and any cloning machinery. Sanitize cutting surfaces between moms when taking cuts, including hands. During vegetative growth and training, make sure everyone who is touching and pruning the plants follow sanitation protocols. A little extra time spent on training and cleaning can make a big difference in the product.

Greenhouse sanitation keeps HpLVd from spreading within an operation, but how do you keep it out? Purchase plants from reputable sources and test new plants. For operations that buy most or all of their production plants as clones, make sure you purchase from a nursery that has robust sanitation practices and follow integrated pest management (IPM). You will get cleaner plants with fewer pathogen and pest problems. HpLVd can be asymptomatic in small plants, so testing is the only way to know if a plant is healthy.

If you do your own propagation and introduce new plants on a regular basis, then you should consider having a dedicated quarantine space for those plants. It should be physically separated from your other plants to allow a few weeks for testing and observation of other pests and pathogens. Test new plants for HpLVd when they arrive, then again at two and four weeks. Any infected plants with a low viroid titer will have detectable quantities by week four. Mother plants should be tested regularly. If you are experiencing dudding, then your moms are likely infected. For initial screening, test a couple plants of each strain. Once you determine where the problem is, test more extensively.

Here at Dark Heart, we test all the plants in the elite mom block monthly, and a percentage of production moms every week. When an outbreak occurs, we test the impacted strains more rigorously. All infected plants are destroyed unless we need to save the strain. In this case we put it through meristem tissue culture. Unlike nodal tissue culture, excising meristem domes can successfully exclude viruses and viroids from plants, a technique that has been in use for viroid exclusion for more than 30 years.[10] Of course, patience is necessary with this technique because it takes about a year to have greenhouse acclimatized plants produced this way.

Why did we say when an outbreak occurs? Because HpLVd is everywhere, and whenever you bring in new genetics there is a risk of those plants being infected and spreading HpLVd within the facility. Staying on top of industry trends means plant material is moving between facilities, with high-demand strains being propagated and distributed fast. The pace of this industry coupled with the fact that a new infection may not be detectable for six weeks means that complete eradication of HpLVd is not likely any time soon.

 

HpLVd infected plant showing leaflets overlapping compared to B) uninfected leaves with normal leaflet spread. C) HpLVd infected plant with very large branch angles, which lead to poor canopy structure and broken branches compared to D) uninfected branching angles. Both of these symptoms can be seen in younger plants.

 

HpLVd infection causing “Christmas tree” looking growth on a mature plant, as compared to B) normal growth in a larger plant. C) HpLVd infected flower showing reduced trichome development and smaller flowers compared to D) healthy flower development. E) On left, healthy plant at flowering stage compared to HpLVd stunted plant with reduced flowers on right.

 

References

  1. Hammond RW, Owens RA. Viroids: New and Continuing Risks for Horticultural and Agricultural Crops. APSnet Featur Artic. 2006. doi:10.1094/apsnetfeature-2006-1106
  2. Viroids – an overview | ScienceDirect Topics. https://www.sciencedirect.com/topics/neuroscience/viroids. Accessed January 13, 2021.
  3. Puchta H, Ramm K, Sänger HL. The molecular structure of hop latent viroid (HLV), a new viroid occurring worldwide in hops. Nucleic Acids Res. 1988;16(10):4197-4216. doi:10.1093/nar/16.10.4197
  4. Chang T-J, Yang D-M, Wang M-L, et al. Genomic analysis and comparative multiple sequences of SARS-CoV2. J Chinese Med Assoc. 2020;83(6):537-543. doi:10.1097/JCMA.0000000000000335
  5. van Bakel H, Stout JM, Cote AG, et al. The draft genome and transcriptome of Cannabis sativa. Genome Biol. 2011;12(10):R102. doi:10.1186/gb-2011-12-10-r102
  6. Owens RA, Hammond RW. Viroid pathogenicity: One process, many faces. Viruses. 2009;1(2):298-316. doi:10.3390/v1020298
  7. Mink GI. POLLEN-AND SEED-TRANSMITTED VIRUSES AND VIROIDS. Vol 31.; 1993. www.annualreviews.org. Accessed October 23, 2018.
  8. Adkar-Purushothama CR, Perreault JP. Current overview on viroid–host interactions. Wiley Interdiscip Rev RNA. 2020;11(2). doi:10.1002/wrna.1570
  9. Pethybridge SJ, Hay FS, Barbara DJ, Eastwell KC, Wilson CR. Viruses and Viroids Infecting Hop: Significance, Epidemiology, and Management. 2008. doi:10.1094/PDIS-92-3-0324
  10. Paduch-Cichal E, Kryczyński S. A Low Temperature Therapy and Meristem-Tip Culture for Eliminating four Viroids from Infected Plants. J Phytopathol. 1987;118(4):341-346. doi:10.1111/j.1439-0434.1987.tb00465.x

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