Viroids are a group of plant pathogens that consist of a small, circular chain of RNA. They are different from viruses in that they have no protein coat and contain no readable genes. Once introduced into host plant cells, the infected cells produce millions of copies. The mechanism of how HLVd causes symptoms is not yet entirely understood but interference with the expression of certain host genes has been shown.
HLVd has rapidly spread throughout the US and Canadian Cannabis Industries, causing annual losses of billions of dollars. Viroid-positive plants were found in most US cultivation facilities that submitted samples.
HLVd often goes undetected in the vegetative grow stage. Depending on the severity of the infection and the strain, visible symptoms range from none to severe stunting, deformed and discolored leaves, and horizontal branches that easily brake. Microscopy showed shriveled glandular trichomes. Clones from infected mothers tend to root poorly. Visible symptoms often appear first in the flowering stage, too late to avoid crop losses. Both yield and potency are reduced in infected plants. A recent Canadian study found average reductions of 27% in yield and 31% in THC content.
Typical HLVd symptomsInfected plant in flower roomHorizontal branching HLVd+Normal branching HLVd-HLVd+/- mother plants from same cloning batch
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HLVd Transmission and Spread
HLVd can persist for days in runoff water or in plant sap. Transmission between plants occurs most commonly through cutting and pruning tools that are not sanitized between plants. Infected mother plants produce infected clones. HLVd is also readily transmitted through seeds. Water-borne root-to-root transmission has also been shown. Transmission through insects is likely but has not shown experimentally yet.
Tissue culture does not necessarily remove HLVd. The type and size of the explants and the severity of the infection of the mother plant are key factors in remediation. We have found both infected and “clean” plants in the same TC batch that started from an infected mother.
HLVd spreads from the site of infection in the same way as plant viruses, following the flow of nutrients and sugars in the phloem. The viroid accumulates in the roots before spreading throughout the entire plant. However, emerging roots from regular clones or TC clones from infected mothers test positive later than the leaves.
