Potyviruses, such as potato virus Y and tobacco etch virus, as well as cucumber mosaic cucumovirus, interfere with post-transcriptional gene silencing (PTGS). When RedStar-type Petunia hybrida cultivars, whose flowers have alternating white and pigmented sectors, were infected with these viruses, each virus induced a different pattern of restoration of floral anthocyanin pigmentation. Local reversion to coloured phenotypes in the white sectors, which occurred through interference with PTGS of the chalcone synthase A (chs-A) gene, was correlated with locally increased levels of chs-A mRNA and virus concentration. Our results show that virus infection can interfere with PTGS of a native plant gene, and that this can have profound effects on symptom expression.