Potyviruses, such as potato virus Y and tobacco etch virus, as well as cucumber mosaic cucumovirus, have the ability to inhibit 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 colored 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. They also suggest that virus infection could lead to loss of agronomically desirable traits obtained by PTGS in transgenic plants.