The most important phytopathogenic agents causing diseases of economic concern to grapevine—often of an epidemic nature—are viruses, bacteria, and phytoplasmas. Such agents are transmitted in nature by animal vectors: viruses by nematodes and mealybugs (pseudococcids and coccids); bacteria and phytoplasmas, the latter with an always growing phytopathological interest, by leafhoppers (cixiids and cicadellids). All the diseases caused by phytoplasmas are described by the term grapevine yellows. The yellows are spread in the main grapevine-growing areas of the world, such as central and southern Europe, the Middle East, North and South Africa, North and South America, and Australia