InMatthiola incana R. Br. the phenomenon of so called "breaking of flower color" is observed which is manifested in white areas dilution or intensification of the normally pink or violet flower colors. This phenomenon is shown to be caused by a virus of the turnip-mosaic-type. There is an interaction between the virus and the genotype of the host, in which are involved only genes controlling the quantity of the anthocyanin pigments but not genes responsible for alteration of the molecule structure of the anthocyanins.