Hop Latent Viroid in Hemp

Hop latent viroid (HLVd) is a pathogenic RNA particle and was first reported in cannabis plants in California in 2019. It has since been reported in cannabis growing in Oregon, Washington, and Canada. Hop latent viroid is common in hop plants and frequently occurs in hop yards across the Pacific Northwest. This viroid can also infect stinging nettles (Urtica dioica), and potentially, other plant species. In 2023, it was reported that 90% of cannabis-growing facilities in California were infected with HLVd in a survey conducted in 2021. Of nearly 16,000 cannabis plant samples submitted to a diagnostic laboratory from provinces in Canada during 2020-2022, 26% tested positive for HLVd. Hemp disease surveys conducted by OSU at 79 hemp production sites in 2021-2022, detected HLVd in eight sites in Oregon and two sites in Washington.

Hop latent viroid consists of a single-stranded, circular form of nucleic acids. Viroids are similar to viruses, except that viroids are much smaller in size and lack a protective protein coat. After particles of HLVd invade a plant cell, they integrate within the plant cells’ nuclei, and ultimately hijack the plant’s genetic machinery in order for more copies of the viroid’s nucleic acid sequence to be produced. Viroids can move from cell to cell in plants and ultimately viroids move into plant sap, where they can then circulate throughout an affected plant, including into plant roots, crowns, and bud portions. Dried plant material from infected plants, including buds, can contain the viroid and may be potentially infectious.

Viroids can be transmitted to other host plants through the movement of contaminated (infective) plant sap, including via farm implements and tools used for field cultivation and pruning practices. Surfaces in production, propagation, and harvesting facilities can also become contaminated with viroids after they come in contact with plant sap of infected plants. Hop latent viroid does not appear to move plant-to-plant by cannabis leaves being in contact with each other. There is currently no evidence that HLVd is transmitted by sap-feeding insects, but research shows that other plant viroids (tomato planta macho viroid and potato spindle tuber viroid) can be transmitted by insect vectors, such as aphids. Recent studies have shown that HLVd can be seed-borne if seeds are produced from infected mother plants and/or pollen produced by infected plants. This viroid can also be propagated through vegetative cuttings if cuttings are taken from infected mother plants. Roots can test positive for HLVd two weeks after cuttings are made from infected mother plants; roots may be more reliable for HLVd testing compared to leaves of young plants.

Knowledge about the range of specific symptoms caused by HLVd in hemp lines is limited, making visual recognition of this disease unreliable. Cannabis plants can remain asymptomatic after infection or exhibit such subtle symptoms that the disease goes unnoticed. Symptoms can vary depending on the viroid strain, plant genetics, environmental conditions, and viroid titer (concentration or number of viroid particles within a plant). Plants growing under hotter conditions, such as outdoor sites in eastern Oregon, southern Oregon, and south-central Washington, may exhibit more obvious symptoms since higher temperatures and brighter light conditions can promote the build-up of viroid numbers in plants. Plants with a yellowish coloration, brittle branches, misshapen leaves, and small buds were originally noticed in cannabis production in California, where the HLVd problem was called dudding (duds) disease. Hop latent viroid can also result in plant stunting. In addition to stunting, HLVd-infected hemp plants may exhibit smaller leaves, shorter stem internodes, and yellow patterns or generalized yellow coloration of leaves. In the Pacific Northwest, foliar symptoms and plant stunting appear to be more obvious towards mid-season on-ward. Vegetative cuttings made from infected mother plants exhibit difficulties with rooting, but young plants/seedlings often do not exhibit any obvious symptoms aboveground. It is unknown whether HLVd affects the chemical composition of hemp, but in THC-type cannabis, HLVd is known to reduce flower mass and trichome numbers as well as reduce cannabinoid and terpene levels.

Recommended management

Recommended management includes exclusion, eradication, and sanitation techniques.

1. Use stock that has been tested for HLVd and shown to be free of this viroid.
   • If producing seed or taking cuttings, test mother plants and/or pollen lines for HLVd before collecting seeds or making cuttings.
   • If growing plants from seeds with unknown viroid status, grow seedlings separate from other hemp plants. Allow them to grow for a few weeks or until they reach about 1.5 foot in height, and then test plants for HLVd before transplanting seedlings to the field or moving them into an indoor production facility.
2. Remove any infected plants from production.
3. Clean tools, surfaces, and equipment that may have been contaminated with infective plant sap. Ideally, clean tools between plants for indoor production operations where cutting of petioles or stems occurs. At a minimum, clean equipment between plots or fields in outdoor production systems. Currently, there is no reliable method that ensures all HLVd particles are completely inactivated on contaminated tools. Practices to remove sap from field equipment should include at least power washing with soapy water in order to clean sap off equipment and implements. For plant propagation or pruning practices where stems and/or petioles are cut and may transfer sap onto tools, the following options may be considered for decreasing HLVd transmission among plants:
   • Heat blades at 320°F for 10 min between plants.
   • Ethanol, Virkon, and hydrogen peroxide are not effective for inactivation of HLVd. Autoclaving does not inactivate the viroid.
   • Bleach (sodium hypochlorite, NaOCl), hypochlorous acid, and skim milk can deactivate HLVd. However, the effectiveness of these treatments is likely modulated by several factors including the amount of infective plant sap present, whether the sap is in wet or dried form, and the presence of infected plant tissue.
   • Treatments where infective sap placed on filter paper disks and then chemicals applied to the contaminated filter paper disks, showed that 10% bleach, 20% bleach, and 1000 ppm hypochlorous acid all can inactivate HLVd.
   • Infective plant sap treatments with 25% household bleach (sodium hypochlorite at 42 g/liter and 4% chlorine) and 20% dried skim milk were both helpful at inactivating a different viroid (potato spindle tuber viroid) in laboratory, non-plant studies. Only the skim milk treatment was found effective at degrading the nucleic acids of the potato spindle tuber viroid, ensuring that viroid particles were no longer infective.
   • Infective plant sap treatments were evaluated by five different research laboratories using potato spindle tuber viroid-infective sap as dried sap droplets that were treated with disinfectants for recommended manufacturer times (10, 15, and 30 minutes) and then used to inoculate plants. Results showed that Virkon, Hyprelva SL, Jet 5, MENNO Clean, Virocid, and 0.8% sodium hypochlorite were not reliably effective at deactivating potato spindle tuber viroid.
   • Tomato chlorotic dwarf viroid-contaminated scalpels dipped in 3% sodium hypochlorite for 15 seconds appeared to inactivate tomato chlorotic dwarf viroid. However, 5% trisodium phosphate, 0.1 N hydrochloric acid, 70% isopropanol, and dips using <1% sodium hypochlorite were ineffective.

References for additional technical information: 

1. Adkar-Purushothama, CR, Sano, T, and Perreault, J-P. 2023. Hop latent viroid: A hidden threat to the cannabis industry. Viruses 15.
2. Atallah, OO, Yassin, SM, and Verchot, J. 2024. New insights into hop latent viroid detection, infectivity, host range, and transmission. Viruses 16.
3. Mackie, AE, Coutts, BA, Barbetti, MJ, Rodoni, BC, McKirdy, SJ, and Jones, RAC. 2015. Potato spindle tuber viroid: Stability on common surfaces and inactivation with disinfectants. Plant Dis. 99:770-775.
4. Matsuura, S, Matsushita, Y, Usugi, T, and Tsuda, S. 2010. Disinfection of tomato chlorotic dwarf viroid by chemical and biological agents. Crop Protection 29:1157-1161.
5. Olivier, T, Sveikauskas, V, Grausgruber-Gröger, S, Virscek Marn, M, Faggioli, F, Luigi, M, Pitchugina, E, and Planchon, V. 2015. Efficacy of five disinfectants against potato spindle tuber viroid. Crop Protection 67:257-260.
6. Punja, ZK. 2024. Transmission, Spread, and Longevity of Hop Latent Viroid Affecting Cannabis. YouTube video.
7. Punja, ZK., Wang, K, Lung, S, and Buirs, L. 2024. Symptomology, prevalence, and impact of hop latent viroid on greenhouse-grown cannabis (Cannabis sativa L.) plants in Canada. Canadian Journal of Plant Pathology. 46:174-197.
8. Sano, T. 2021. Progress in 50 years of viroid research-Molecular structure, pathogenicity, and host adaptation. Proceedings of the Japan Academy. Series B, Physical and biological sciences, 97(7):371–401.