Alfalfa mosaic virus (AMV) in cannabis – symptoms, transmission, and relevance

Alfalfa mosaic virus (AMV) is one of the oldest known plant viruses and was first associated with Cannabis sativa as early as the 1970s. It has a broad host range, is globally distributed, and under certain conditions, can also cause yield losses in cannabis plants – primarily through leaf alterations, growth depression, and potential seed transmission.

General characteristics of AMV

AMV is a positive-sense single-stranded RNA virus from the family Bromoviridae, genus Alfamovirus. It has a segmented genome (four RNA segments) and requires a virus-encoded coat protein for full infection. The virus is naturally present in numerous cultivated plants such as alfalfa, potato, pea, pepper, and tomato – making it relevant for cannabis cultivation as well.

Symptoms in cannabis

In the vegetative stage

Vein banding: Noticeable lightening along the leaf veins, appearing in a net-like pattern
Mosaic pattern: Light and dark green spots or mottling on young leaves
Leaf deformation: New leaves often appear wavy, curled, or slightly distorted
Stunted growth: Especially with early infections, plant development may be impaired

In the flowering stage

Persistent mosaic symptoms on newer leaves
Reduced photosynthetic efficiency, leading to smaller inflorescences
Lower overall vigor, especially under additional stress conditions
No direct resin changes, though yields may decline indirectly

Compared to more aggressive viruses (e.g., HLVd or BCTV), AMV has a relatively mild effect, but in mixed infections, it can be a significant co-factor.

Modes of transmission

AMV can be transmitted both horizontally and vertically:

Aphids (e.g., Aphis gossypii, Myzus persicae, Phorodon cannabis) transmit the virus in a non-persistent manner – even a brief feeding probe is sufficient
Seed transmission: In various hosts (alfalfa, pepper), the virus has been shown to be seed-transmitted. While exact data for Cannabis sativa is lacking, the risk is considered realistic
Mechanical transmission is also possible (e.g., via contaminated tools or hands)

Relevance in home cultivation

For hobby growers, AMV poses several risks:

Confusion with nutrient deficiencies: Mosaic or vein banding can easily be mistaken for magnesium or iron deficiency
Hidden spread via seeds: Especially since certified virus-free seed is rarely available in the private sector, AMV may be introduced unnoticed
Infection sources nearby: As AMV has many host plants, weeds or neighboring crops (e.g., peas, clover, tomatoes) can act as virus reservoirs

Diagnostics

RT-PCR (Reverse Transcription Polymerase Chain Reaction) is the most reliable detection method
ELISA (Enzyme-Linked Immunosorbent Assay): Commercial antibody test kits are available for AMV and are also used in vegetable production
Rapid tests: Immunostrip lateral flow assays are available from specialized laboratory suppliers and can also be used in hobby settings

Prevention and management

Preventive measures:

Aphid control: Use of yellow sticky traps, neem oil, beneficial insects such as ladybugs or parasitic wasps
Weed management: Remove potential host plants in the vicinity (e.g., clover, alfalfa)
Tool hygiene: Disinfect tools after each plant (e.g., with isopropanol or Virkon S)
Quarantine new plants: Observe new cuttings or seedlings separately for at least 2 weeks

Handling seeds:

Surface sterilization of seeds: e.g., 10% sodium hypochlorite solution for 1–2 minutes, then rinse thoroughly with water
Virus testing of parent plants: Anyone producing seeds should regularly test mother plants for AMV

No cure – but selection is possible:

Once infected, plants remain systemically contaminated. There is no cure. The only way to eliminate AMV would be via in vitro meristem culture – practically unfeasible for home growers. Instead, affected plants should be discarded and destroyed.

Conclusion

AMV is not a killer virus like HLVd or BCTV, but it is still a serious concern. Due to its potential seed transmission and wide range of vectors, it poses a hidden threat – especially in home cultivation, where standardized virus-free practices are rare. Good hygiene, plant quarantine, and attention to mosaic symptoms are the best tools to prevent the silent spread of AMV.