The plant I have as an example is Phlox paniculata 'Norah Leigh'.
Some plants produce variegated leaves for genetic reasons.
But others, such as abutilon produce leaves with yellow markings because of a virus infection which gets transferred from one to another. Yes, I think as Bob says, the phlox is probably a virus-induced variegation. However, the virus may not be in the roots, the virus is only in the leaves. When I propagate my phlox I tend to take them from the young stem cuttings when they're coming through, and are still quite small, about 50mm long. My stepdaughter is getting married in June, and the reception will be held in our garden.