Melanins are dark-brown to black pigments found in animals, plants, and microorganisms. These pigments are not essential for growth and development, but rather they enhance the survival and competitive abilities of species in certain environments. The association of melanins with immune responses has been noted for plants, animals and fungi. In fungi, melanin provides the mechanical strength to the appressorium for penetration of the leaf surface and it increases the virulence and pathogenicity of the fungus. Also melanins act as body armour protecting fungi against environmental stress or unfavourable conditions like extreme temperatures, UV-, ionizing and gamma radiations, and compounds secreted by microbial antagonists. Compounds which inhibit the biosynthesis of DHN-melanin, such as tricyclazole, phthalide, chlobenthiazone and pyroquilonare used as “antipenetrant” fungicides. They do not kill plant pathogens, but they cause the development of defective appressoria lacking melanin; these appressoria are unable to penetrate the plant epithelium and establish infection.