The organisms, nine of which are harmful to tomato plants, are tiny even when compared to viruses, Fox explained. "They are just naked strands of RNA, but very tough, and can even be transmitted by seed. On seed especially, they are notoriously difficult to identify."
Seed was suspected to be behind an outbreak of Columnea latent viroid (CLVd) in three UK sites in 2007, he said. But much remains unknown about how the pathogens are transmitted.
FERA has since developed a means of testing seed for the presence of CLVd, but more recent outbreaks of other viroids are an "emerging threat" to UK tomato production, said Fox.
"Once you have it, there are no treatments," he warned. "But it can largely be prevented by hygiene best practice, which we recommend anyway."