Viroids are the viruses of plants, and quite a curious one has been discovered in hops, beloved in both the cannabis world for their terpenes and in the beer one, being one of its staple ingredients.
This viroid, named hop latent viroid (HLV), is not the first one to be found in hops. The hop stunt viroid (HSV) is well-known by this point. However, while the HSV is “only present in Japanese hops,” HLV seems to occur “worldwide in most of the hop cultivars tested so far,” the scientists who discovered it reported. 
The good news is, HLV is latent. This appears to be the reason why it hasn’t been detected so far despite being so widespread, contrary to HSV on both levels. However, its latency can even make it a silent threat in the big picture.
“Latent viroids should not be underestimated as pathogens because they are a potential hazard for other crop plants to which they can be easily transmitted by various means and then cause severe damage. This is clearly demonstrated in the previously reported cases of columnea latent viroid and of HSV as one of the latent grapevine viroids,” which become dangerous once they’re not confined within their non-reactive hosts. The authors report that latent viroids may exacerbate disease when other infections from microbes or viruses are present.
Moreover, since hops are propagated vegetatively (grown from a cutting) and inspected visually, latent viroids can’t be detected in the symptom-free carriers, which go on to be distributed as healthy plants.
Another finding worth noting is that HLV can be characterized as the first member of a new viroid group, defined by their sequences.
So far, there are five viroid groups, their prototypes being: potato spindle tuber viroid (PSTV), coconut cadang cadang viroid (CCCV), hop stunt viroid (HSV), apple scar skin viroid (ASSV), and avocado sunblotch viroid (ASBV).
“With the exception of ASBV, whose viroid nature may be questioned, the sequence similarity between the four group prototypes ranges between 39-60%, whereas it ranges between 65-100% when the members within a group are compared.” Since HLV’s sequence similarity falls in the 36-54% range, the aforementioned criteria classify it as a new viroid group.
This groundbreaking study highlighted the universality of HLV on hops across the globe, as well as the viroid’s latent nature which caused it to go undetected. These researchers also advise that the viroid can be transferred from infected to healthy plants via the tools used cutting and pruning.
The study also illustrates rapid diagnostics to assess whether a plant is indeed infected, including “return”-electrophoresis (a way to separate DNA, RNA, or proteins) and molecular hybridization. These methods enable mother plants and root stock to be used for cuttings and propagation to be screened for HLV to ensure they are viroid-free.
1- Puchta H, Ramm K, Sänger HL. The molecular structure of hop latent viroid (HLV), a new viroid occurring worldwide in hops. Nucleic Acids Res. 1988;16(10):4197-4216. [journal impact factor = 16.97; times Cited = 112]
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