Committee Blog: Hop Latent Viroid (HLV) – Overview of Pathogen Biology, Spread, Control, and Testing

by Sarah Taylor-Laine (NCIA Education Committee, NCIA Cultivation Committee)

Hop Latent Viroid (HLV/HpLV), sometimes referred to as “dudding” or “stunting”, is one of the most pressing biological threats facing the cannabis industry worldwide. In the United States, HLV was first detected in California in 2017. Subsequent research by Dark Heart Nursery indicated that 90% of Californian facilities tested at the time were positive for the pathogen. The Dark Heart Nursery research team estimated that an HLV epidemic could result in up to US$4 billion in losses to the cannabis industry annually. To add insult to injury, HLV-infected cannabis may be linked to cannabis hyperemesis syndrome (CHS), which affects both adults and children.

As the name suggests, HLV was first detected in hops (Humulus lupulus; Cannabaceae). Outside of the United States, HLV has been detected in hops in New Zealand, the United Kingdom, South Korea, Japan, Brazil, the Czech Republic, Poland, Slovenia, Germany, China, Belgium, Turkey, and Canada. Within the United States, HLV has been detected in hops in Washington, and in cannabis in California, Colorado, Nevada, Oregon, and Washington. However, these are only officially-published reports and given the worldwide occurrence, HLV is likely to be endemic to many locations not listed here.

Viroids are subviral pathogenic RNA molecules which replicate within host plants and are only approximately 1/50th the size of the smallest viruses. Specifically, HLV is a circular RNA molecule of approximately 256 nucleotides in length, which can assume a rod-shaped secondary structure. In hops, HLV has been shown to interact with other viruses and alter the gene expression patterns of host plants. HLV has been found to not only infect hops and cannabis, but also Dianthus deltoides, Chenopodiastrum murale, Dysphania pseudomultiflora, cowpea (Vigna unguiculata), common bean (Phaseolus vulgaris), and stinging nettle (Urtica dioica). Notably, by subjecting hop plants to heat stress, researchers have been able to induce “thermomutant” HLV variants which could then infect both tomato (Solanum lycopersicum) and Nicotiana benthamiana, opening up the possibility that HLV could spread to new plant species under climate change-induced warming scenarios.

HLV often does not produce obvious symptoms in hops, and where they do occur there may be a long latency period, making control difficult. In hops, HLV is often detected early in the season at the plant base, where it gradually spreads up the plant and can be detected in all aerial tissues by mid-season during the flowering period. In cannabis, HLV causes brittle stems, a horizontal growth habit, and the destruction of flowers and trichomes. In both hops and cannabis, HLV may cause pathology in host plants through RNA interference (RNAi). HLV infection is reported to reduce THC content by 50-70%, which is not surprising as glandular trichomes are the source of THC in cannabis. In hops, HLV infection has been shown to reduce the content of terpenes and other secondary metabolites in susceptible varieties. It is currently unknown whether HLV infection decreases the terpene content in cannabis, but it is very likely.

In hops, HLV is not readily transferred by seeds, but may be transferred in uninucleate pollen. However, activating pollen nuclease HBN1 was shown to eliminate HLV in mature pollen. “Real world” transmission of HLV appears to primarily occur through the use of contaminated cutting tools, the use of infected plant materials, or from plant-to-plant. However, as an obligate pathogen, HLV requires living host tissue in order to survive and propagate. HLV has been confirmed to be transmitted between hop plants by potato aphid (Macrosiphum euphorbiae), green peach aphid (Myzus persicae), and hop aphid (Phorodon humuli), and may also be transferred by other herbivorous insects. Planting cannabis on sites previously planted with infected host plants is likely to be a source of infection, although this has not been experimentally verified.

In hops, chilling plants at 2-4 °C (approximately 35-40 °F) for 8-21 months prior to harvesting meristem tips for propagation has been shown to reduce or eliminate the spread of HLV, although the effect may be variety-specific. In addition, subjecting hop mericlones to heat treatment cycles (25-35 °C/77-95 °F for two weeks) has been shown to reduce HLV incidence by 70-90%. In terms of waste management, anaerobic fermentation at 70 °C (158 °F) causes HLV degradation, although standard ensiling does not appear to be effective. As with most viral and viroid diseases, there is currently no cure available to disinfect mature host plants. However, researchers are currently studying whether RNAi may have utility in cannabis crop protection against viral and viroid infections. Therefore, it is currently recommended to test all stock plants to confirm that they are pathogen free and to destroy all infected materials.

Plant materials can be tested for HLV infection using several molecular techniques, including DNA barcoding, RNA sequencing (RNA-seq), recombinase polymerase amplification (RPA), quantitative real-time polymerase chain reaction (qRT-PCR), reverse transcription PCR (RT-PCR), dot-blot hybridization, tissue print hybridization, in situ hybridization, and enzyme-linked immunosorbent assay (ELISA).

It is unknown how many, and which, lessons learned in hops will be transferable to cannabis. Both susceptibility and control of HLV in cannabis is likely to be variety-specific. To enable the breeding of HLV-resistant cannabis varieties, future research should focus on characterizing the molecular mechanism underlying HLV pathogenicity and host susceptibility. However, the situation remains that HLV is likely endemic in many cannabis growing regions and cultivators should focus on targeted biosanitation efforts to thwart HLV infection in their crops. These efforts should include testing all propagation materials for infection, destroying infected materials, sanitizing cutting tools and work surfaces, and limiting herbivory by sap-sucking insects such as aphids.

Sarah Taylor-Laine is a plant science polymath. Sarah has taught and developed college-level courses related to general biology, plant biology, and cannabis sciences at several institutions, most recently at Beal University. She has also worked across many industries including agriculture, horticulture, and arboriculture, performing diverse work in research, quality control, product development, regulatory affairs, scientific communication, and field-based consulting.