Two plants of the same strain and age and inputs show the visibly reduced yield, trichomes, and potency of an infected plant, left, versus a healthy one. Photo courtesy Dark Heart Nursery, a pioneer in identifying the hop latent viroid in cannabis plants.
Known colloquially as “dudding disease,” the hop latent viroid is estimated to cause the cannabis industry more than $44 million in losses every year.
Recognized through symptoms like stunted growth, brittle stems, reduced trichome and oil production, reduced flower mass and a malformation of the leaves, the infection — which can stay dormant in plants for long periods of time before showing symptoms — was found in every California garden tested in 2019 by Dark Heart Nursery.
To learn more about the hop latent viroid, how it is spread and what can be done about the potential crop-killer, Marijuana Venture contacted Bryce Falk, distinguished professor emeritus in plant pathology at the University of California, Davis. Falk received his B.S. degree in biology from the California Polytechnic State University at San Luis Obispo and his M.S. and Ph.D. in plant pathology from the University of California, Berkeley, and has been working with plant viruses since 1978, including research at the UC Davis lab that now bears his name.
Marijuana Venture: What is a viroid?
Bryce Falk: Viroids are really interesting. They were originally discovered because the diseases that they cause look very similar in plants to some that are caused by viruses, but they’re not viruses. They’re the smallest of the infectious agents of plants in terms of the size of their genetic information. They, in fact, have very little genetic information at all.
And they don’t have any gene, they just take over the plant’s transcriptional machinery and then they can cause disease. There are many of them that are very important, in potatoes, in coconuts and tomatoes and in a lot of different plants. And, of course, the hop latent has become important in the cannabis and hemp industries.
MV: And how is it detected? How does it present?
BF: Well, that’s a good question. When you see symptoms, when you see these plants that are affected and stunted and don’t grow normally, that could be due to a variety of different causes. But you can detect viroids and particularly the hop latent viroid by using contemporary genetic testing approaches, the same types that are used for us for COVID: PCR-based tests. Those are used to specifically identify in the RNA material that is, in fact, the infectious hop latent viroid. And we know that the hop latent viroid causes this disease because it’s such a small RNA, you can have it made by a company, and some scientists did. And then they put that back into cannabis plants and got the same symptoms. So we know that it’s this infectious RNA viroid that’s causing the disease.
MV: Is this dangerous to humans at all?
BF: Oh, no, no, no, we see these things all the time. We eat plant viruses all the time, too, and there’s absolutely no effect on us.
MV: So this is just more of an agricultural issue, affecting production?
BF: Yes, and yes, it’s affecting yield quantity, but specifically yield quality. And the problem with viroids is that they’re so highly infectious, and they’re really easily spread. If you go to a leaf of a plant that’s infected, and you cut off a few leaves with a scissors, and then you take that same scissors and cut off a few leaves on the next plant, you actually transmit the viroid to that next plant. And so then you spread it that way. And that’s a big problem, as you would guess, in the cannabis industry.
MV: If this is found inside a grow or a garden, is there any action that can be taken?
The hop latent viroid caused this plant to fail to produce strong roots. Courtesy Giving Tree Farms.
BF: Well, there’s nothing to do to cure it, no. If it’s only found in a couple of plants, obviously, you have to get rid of those plants or isolate those plants from others and not spread it. The best thing is really being very careful horticulturally in handling the plants. That’s the best way to really prevent spread. And you have to start with material that’s not infected.
As we know, so many plants, not just cannabis, are vegetatively propagated by cuttings; we make little cuttings and reroute them or graph them. And if we have a viroid-infected plant, and making cuttings, we’re just spreading it. And so really, you have to test ahead of time, make sure you have clean material. And then just be very careful with your sanitation so that you don’t come in from another place and introduce it and spread it.
MV: So if there’s nothing that can be done once it’s discovered, how can it be prevented? Is it just testing ahead of time?
BF: Yeah, making sure you buy clean plants and start with clean plants. Now, for most viroids, and I’m not 100% positive on hop latent in cannabis, but most of them don’t go through the seed. So if you actually plant new plants from seed for many crops, you don’t transmit the virus. I’m not 100% positive on hop latent, but many people purchase plants right for use in the cannabis industry and they’re not starting with seed. When you purchase plants, you don’t know how those have been handled and what’s been done. So you have to make sure that they’re clean plants.
MV: Can you talk a little bit about where this comes from originally and how it found its way to cannabis?
BF: Well, we don’t know the answer to that. We know that many pathogens have a wide host range. And so the hop latent viroid, we know for years it’s been in hop production. And so probably someone — and I’m making the wild guess here — just accidentally spread it from infected hop to some cannabis. And once it’s in there, without you being aware and being careful of the sanitation, you’re going to spread it for sure.
MV: You say that it spreads it spreads from hops. Eastern Washington is a huge producer of hops and is also now an area where cannabis is grown. Should cannabis farmers be worried if they’re located next to a hops farm?
BF: I don’t think so. I think if it’s going to spread, humans are going to spread it. I think this is a good example of how important it is to really be careful with the way that we grow plants, because there are lots of pathogens that are spread this way. This is one example of why we have to start with pathogen-free material. And more can come along if we’re not careful and spread them.
This interview has been edited for length and clarity.