Post-transcriptional gene silencing (PTGS) provides protection against viruses in plants by homology-dependent RNA degradation. PTGS initiated locally produces a mobile signal that instructs specific RNA degradation at a distance. Here we show that this signal-mediated intercellular spread of PTGS does not occur after PTGS initiation in cells expressing cucumber mosaic virus 2b protein (Cmv2b), a nucleus-localized plant viral PTGS suppressor. Silencing spread via the signal was also effectively blocked in independent assays by expressing Cmv2b only in tissues through which the signal must travel to induce PTGS in the target cells. Furthermore, the signal imported externally into the Cmv2b-expressing cells was not active in triggering degradation of the target RNA and loss of signal activity in these cells was associated with a significantly reduced transgene DNA methylation. These findings indicate that Cmv2b inhibits the activity of the mobile signal and interferes with DNA methylation in the nucleus. Signal inactivation provides a mechanistic basis for the known role of Cmv2b in facilitating virus spread to tissues outside of the primarily infected sites.