Rougeau is leaf reddening, caused by any of several viruses in grapevines. The virus infection seals off the leaf petiole (stem) so that the sugars produced by the leaf through photosynthesis cannot be transported back into the vine. The sugar accumulates there until the leaf, in an effort to get rid of a problem, uses it to make red anthocyanin pigment. Vines infected by these viruses have difficulty in ripening their fruit because much of the sugar the vine produces cannot be used by the vine where it needs it— in the fruit. Leafroll virus is a common example of this type of infection. (Wine/Other)