A mosaic virus is any virus that causes infected plant foliage to have a mottled appearance. Such viruses come from a variety of unrelated lineages and consequently there is no taxon that unites all mosaic viruses. All the symptoms of each virus or complex appear the same, so the only way to know exactly which viral subtype is infecting a plant is to send it to a local extension service. Alternatively, commercial testing kits exist for settings like greenhouses.