For many years, cannabis cultivators were puzzled by a mysterious disease that appeared from nowhere. Infected plants showed similar symptoms: abnormal branching, brittle limbs, reduced vigor, decreased flower and trichome production, and reduced potency. Sometimes a single sick plant would lag behind the rest, while other times the pathogen would spread rapidly and ravage entire crops.

Prized heirlooms, mother plants, and clone-only cuts would all slowly diminish in health and performance, despite receiving the exact same inputs in previous cycles.

Exasperated growers everywhere compared notes to no avail.

Soon, the mystery pathogen had spread far and wide enough that it became known as dudding or dudding disease. Infected plants were simply called duds because they weren't as healthy or productive as the rest of the plants. Putative Cannabis Infectious Agent (PCIA) is another name that was used to describe this pathogen.

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In 2018, Dark Heart Nursery officially confirmed that the hop latent viroid (HpLVd) is the cause of dudding. This made it possible to rapidly test for the presence of hop latent viroid and jumpstart long overdue research into HpLVd as a cannabis pathogen.

It's believed that HpLVd jumped from hops to cannabis in the Pacific Northwest, a popular region for both hops and cannabis cultivation. This is possible in part because hops and cannabis belong to the same family, Cannabaceae. There are similar types of viruses/viroids that infect specific crops such as cucumbers, lettuce, beets, and tobacco.

The hop latent viroid is tiny. Smaller than a virus, it is impossible to detect without a diagnostic test. What's especially challenging with HpLVd is its latent, asymptomatic nature. A latent organism means it is capable of lying dormant or hidden until circumstances are suitable for development or manifestation; asymptomatic means “without symptoms,” so a very healthy-looking crop could be infected.

HpLVd can move at different speeds. In perpetual gardens with infected mom populations, infected plants can slowly go from asymptomatic to symptomatic over several crop cycles. Uninformed growers may initially write a small decrease in yield as a one-off, not a big deal, just part of growing.

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Eventually they realize something's wrong as the garden slowly (or quickly) gets worse with each crop. It sometimes takes years for cultivators to realize that a strain just doesn't yield like it used to. It's now thought that genetic drift may actually be a viral or viroid load on the plant.

Today, there is still little awareness about how widespread this problem is in the industry and the scale of its impact: Dark Heart has found infected plants in every grow it's tested in the United States. It's estimated that HpLVd reduces yield by up to 20 per cent, a severe economic impact.

Dark Heart's research into the latent nature of HpLVd is ongoing as it is unclear when or why HpLVd comes out of dormancy and becomes active, or vice versa. One thing that is clear is that some strains and genetic lines seem more vulnerable than others.

One area of research focus is on HpLVd vectors and transmission methods. The most common way plants get infected is by mechanical transmission — the use of tools between plants. When an infected plant is cut, that tool is contaminated and transmits HpLVd to other plants.

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Cloning, trimming, deleafing, and pruning are all ways HpLVd can spread. Once a cutting tool is used on an infected plant, the tool is contaminated and should be sterilized with virkon or bleach. Failure to follow SOPs can result in widespread infection in a short time, especially in larger commercial operations that propagate in-house or frequently prune and cut plants.

Another way HpLVd can spread is by seed. An infected mother can pass HpLVd to offspring through seed. Root aphids are also believed to be a vector as they spread disease by piercing roots to feed.

The only way to detect HpLVd is an RT-PCR assay test performed by a plant diagnostic lab. Tests are inexpensive and provide fast results. Due to the latent nature of HpLVd, it is recommended to use a lab that will test plants at least three times to be considered negative.

HpLVd cannot be removed from infected plants. If a plant tests positive for HpLVd, there are two options a grower can take. They can destroy infected plants and neighboring plants that have had contact, and closely monitor the rest of the garden in conjunction with diagnostic testing.

The second option is tissue culture, which is ideal when the infected plant is a unique strain that needs to be saved. Cells are taken from the plant and put through the tissue culture process. It takes several months for the cells to grow into rooted plants. After three negative results, the strain is cured. It is important to note that meristem tissue culture alone is not always sufficient to cure plants of HpLVd and that pre-treatment and multiple rounds of testing are needed to confirm a plant has been cured.

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As more research unfolds and new discoveries are made about HpLVd and other cannabis pathogens, the importance of proper SOP and sanitation measures remain crucial. Growers should ensure that their plant stock comes from professional nurseries that utilize tissue culture as part of a clean plant program and regularly test their populations for HpLVd.