HLVd has caused significant crop loss for Cannabis farmers in the form of lower yields (translating to a possible 33% loss financially across some state markets),8 lower trichome content, and reduced terpene profiles.3
HLVd is not only highly infectious but it remains hidden or dormant (latent) to observation until the cannabis plant experiences a secondary stress such as heat, over watering, pest infestation etc. that causes visual symptoms to occur.4
Sometimes HLVd can start showing symptoms during the flowering stage, even if not visually detectable during the entirety of the vegetative growth. It can cause the plant to develop small, abnormal (sometimes spiky), and less potent buds all over resulting in loss of yield and potency.3,8
Some infected cannabis plants never show symptoms throughout their entire lives and this is another reason why HLVd can be spread so easily as growers may sell or trade cuttings or clones with others and not have any suspicion that those pieces of plant material may be harboring the viroid.4 Improper sterilization of cutting tools and the reuse of gloves across different plants can quickly spread the viroid throughout your grow room as well.3,5
HLVd can exist in most of a plant’s tissues (stems, leaves, roots, etc.) but largely resides in the roots of cannabis plants. This does not mean leaves, stems, and buds are unaffected. The Viroid can take time to migrate through the entire plant as it enters the phloem (the vascular tissue responsible for conducting sugars and other metabolic products downward from the leaves) through a cut or tear in a stem or leaf, travels to the roots, and then spreads throughout the rest of the plant.6
Left: a young veg plant showing normal leaf distribution with common morphology.
Right: An HLVd positive plant early in its vegging stage, showing clustering of new leaf growth and aberrant leaf morphology.
- Shorter internodal spacing
- Smaller leaves (sometimes with higher rates of chlorosis)
- Stunting of bud size and morphology
- Reduction of trichomes
- Reduction of terpenes
- Reduced overall plant vigor (weaker apical/main and lateral stalks)
The most accurate way to assess the status of cannabis plants is through one of two processes known as Reverse Transcription PCR (RT-PCR) and Quantitative PCR (qPCr) which can be performed by many commercial labs across the US.3,9
Routine testing of mother plants, as well as any new clones or mothers introduced from outside your facility, is highly recommended. A schedule of up to 4 tests per nursery plant over a period of a month and a half has been suggested by one source.8 Another source has suggested isolating new plants for 30 days and testing on the third week of isolation.3 Regardless, testing is critical to ensuring the prevention and/or spread of existing HLVd in your grow facility.3,6
Left: A plant from the same pack of seeds at the same time in flower (week 7). Note the greater yield and more normal spacing on the leaves. This is closer to the normal presentation for this strain when uninfected by HLVd.
Right: A plant late in flower (week 7) showing greatly reduced yield from the norm. Note the small bud size and the tight spacing on the clusters of leaves. Reduced yield and leaf bunching are two of the most common observable effects of HLVd or the ‘dudding virus.’
HLVd can also be spread through aqueous transmission (through water). For example, a non-infected clone can become infected if it is in the same cloner as an infected clone. Even if the two clones never have root-root contact, HLVd can move through the water from one infected plant to the roots of all the other clones in the same cloner. This also means plants that share drip trays or saucers could potentially end up infecting healthy plants when water run off from the bottom of an infected plant’s pot can mix with the roots and soil of the healthy plant.5,6
There are no published reports currently that show insect transmission of hop latent viroid in cannabis. However, many other viroids are spread by insects, making insect transmission of HLVd a high possibility. It is good practice to limit your cannabis plants exposure to bugs (best pest management practices, healthy plants, good environmental controls, etc.) as many other viruses and pathogens are spread by insects.7
Change gloves when working with multiple plants. One pair of gloves per plant.
Clean cutting tools with at least a 10% solution of bleach that contains 5.25% sodium hypochlorite for at least 10 seconds in between taking cuttings from one plant to the next. Also clean any work surfaces that come in contact with plant material. 3,4,5 Isopropyl alcohol will not kill the viroid so bleach is required for efficient removal of HLVd from cutting/workstation surfaces.10
Avoid having plants share drip trays/saucers if possible (due to aqueous transmission).5,6
Destroy any plants that are infected or remove them from your grow to be treated for HLVd through tissue culture (only recommended if the cultivar is critical to your business due to time and financial cost of tissue culture).
Summary: HLVd can be a devastating issue for any grower whether it’s the home or commercial cultivator. HLVd can remain dormant for a long time and may never show symptoms while it spreads throughout a grow. Mechanical transmission through dirty/used cutting tools and gloves is the number one method of transmission of the viroid. Through routine testing, via RT-PCR or qPCR methods, and employing best practices in cleanliness all growers can limit the spread of this pernicious pathogen and get the quality flower we all love.