The disease in plants that causes a 'mosaic' pattern of discoloration is called "Tobacco Mosaic Virus." It is a viral disease that produces characteristic mosaic-like mottling and discoloration on the leaves of infected plants. This virus affects a wide range of plants, especially tobacco and other members of the Solanaceae family, including tomatoes and peppers. The infection causes patches of light and dark green areas on the leaves, giving them a mosaic appearance. The Tobacco Mosaic Virus was the first virus to be discovered and is one of the most well-known plant viruses causing this characteristic symptom.