The cannabis industry’s understanding of pathogen risk has matured significantly over the past five years. Hop latent viroid (HLVd) went from an obscure concern to a recognized threat estimated to cost the North American cannabis industry up to $4 billion annually. But as the industry’s testing capacity has grown, it’s becoming clear that HLVd is not the only pathogen hitching a ride in propagation material, and seeds – once assumed to be a “clean” starting point – are proving to be a significant transmission pathway.
Recent research has demonstrated that HLVd can be present both on the seed coat and within the embryo of infected cannabis seeds, and these seeds can produce infected seedlings at a high frequency. This finding has significant implications for licensed producers, nurseries, and breeders who use seed-based propagation, whether for breeding programs, genetic preservation, or commercial production.
The Seed-Borne Transmission Pathway
For years, the working assumption in cannabis pathology was that viroids and most viruses were primarily transmitted through vegetative propagation – cloning, grafting, and the sharing of cuttings. Seeds were considered a lower-risk starting material because the processes of pollination, seed development, and germination were thought to filter out most pathogens.
That assumption has been challenged by accumulating evidence. Studies published in 2024 and 2025 demonstrated seed transmission of HLVd in cannabis at rates that make this pathway epidemiologically significant. When infected mother plants are used for seed production, a substantial proportion of the resulting seeds carry the viroid, and seedlings grown from these seeds develop systemic infection.
The mechanism involves the viroid integrating into developing seed tissues during formation on the mother plant. Unlike many plant viruses that are excluded from gametic cells, small RNA pathogens like viroids can evade these barriers. The result is that seeds from infected parents carry the infection forward to the next generation.
Beyond HLVd: Other Seed-Borne Threats
While HLVd has dominated the conversation, cannabis is susceptible to a growing list of pathogens, several of which have seed-borne transmission potential. Cannabis cryptic virus (CCV), lettuce chlorosis virus (LCV), beet curly top virus (BCTV), and tobacco mosaic virus (TMV) have all been detected in cannabis. TMV is particularly relevant to seed transmission discussions because it’s known to survive on seed surfaces in other crops for extended periods.
For Canadian licensed producers operating under Health Canada regulations, the expanding list of potential pathogens makes a comprehensive testing strategy essential. Relying on a single-pathogen test, even for the most economically important threat like HLVd, leaves gaps that other pathogens can exploit.
Building a Comprehensive Seed Testing Protocol
A robust seed testing program for cannabis should address multiple pathogen categories. For viroids, RT-PCR or RT-LAMP assays targeting HLVd remain the gold standard. For viruses, multiplex PCR panels or immunological assays can screen for multiple targets simultaneously.
Rapid immunological tests provide a practical first-line screening option. ImmunoStrip technology has been widely adopted in the cannabis industry for testing tissue culture material and cuttings. While seed testing presents some unique sample preparation challenges – seeds need to be germinated or the seed coat needs to be removed and the embryo extracted – the underlying immunological and molecular detection methods are applicable.
For facilities processing large numbers of seed lots, ELISA-based batch testing offers efficiency. Pooling samples from a seed lot and running them through an ELISA plate can screen hundreds of seeds in a single assay run. Positive pools are then followed up with individual seed testing to identify which specific seeds are infected.
Implications for Breeding Programs
The confirmation of seed-borne HLVd transmission has particularly significant implications for cannabis breeding programs. Breeders create seeds intentionally through controlled pollination, and if either parent plant is infected with HLVd, the resulting seed lot is at risk of carrying the viroid. This means that breeding mother plants and pollen donors must be tested and confirmed pathogen-free before being used in crosses.
For breeders exchanging genetics through seed – a common practice for introducing new cultivars or maintaining genetic diversity – every incoming seed lot should be treated as a potential pathogen introduction event. Testing before germination, or at minimum testing seedlings soon after emergence, prevents viroid-infected material from entering the breeding pipeline.
Clean Stock Programs: Starting Right
Canadian cannabis licensed producers increasingly recognize the value of clean stock programs, where all propagation material is derived from tested, pathogen-free mother plants. The discovery of seed-borne transmission means that clean stock programs must extend their scope to include seed-derived material, not just clones and tissue culture plantlets.
A practical clean stock workflow for seed-derived plants might look like this: source seed from tested parent plants, germinate under isolated conditions, test seedlings at the 2-4 leaf stage using molecular or immunological methods, and only transfer confirmed clean plants into the production facility. This adds time and cost to the propagation process, but the alternative – introducing an infected plant that then spreads the viroid through mechanical transmission during daily handling – is far more expensive.
Tissue culture remains the most reliable method for producing pathogen-free starting material, particularly when combined with thermotherapy or chemotherapy to eliminate viroids. However, not all facilities have tissue culture capabilities, and seed propagation will continue to play a role in the industry. Making that role safer requires systematic testing.
Hydroponic Transmission: Another Pathway to Address
Research has also demonstrated that HLVd can spread through root contact in hydroponic cultivation systems. Shared nutrient solutions create an opportunity for the viroid to move from infected to healthy plants through root-to-root transmission. This finding is relevant because many cannabis operations use recirculating hydroponic systems where nutrient solution flows past multiple plants.
The combination of seed-borne introduction and hydroponic spread creates a scenario where a single infected seed could ultimately lead to facility-wide contamination. One infected seedling, placed into a recirculating system, can silently transmit the viroid to neighboring plants through the shared nutrient stream before symptoms become apparent.
Mitigating this risk requires both testing at the propagation stage and system design that limits transmission opportunities. UV treatment of recirculating nutrient solution, strategic isolation of new plant introductions, and regular surveillance testing of production plants all contribute to a layered defense.
The Path Forward
The cannabis industry’s approach to pathogen management is maturing from reactive to proactive. Testing is no longer something you do when plants look sick – it’s something you do before plants enter your facility, at regular intervals during production, and especially when introducing new genetics through any propagation method, including seed.
For Canadian licensed producers, the investment in comprehensive pathogen testing pays for itself many times over. A single HLVd outbreak can reduce cannabinoid yields by 30 to 50 percent across affected plants, with the economic impact multiplied across an entire production cycle. The testing tools exist. The scientific understanding of transmission pathways is growing. What remains is implementing systematic testing protocols that match the sophistication of the pathogens we’re trying to keep out.
