Localized melanization of appressoria is required for pathogenicity of Venturia inaequalis

During formation of appressoria produced from conidia and ascospores of Venturia inaequalis, a dark brown ring structure was detected at the base of appressoria. This melanized appressorial ring structure (MARS) was attached to the leaf surface like a sealing ring and formed the fungus-plant interface; it is believed to be required for pathogen penetration of the cuticle. Neither germ tubes nor infection structures beneath the cuticle were found to be visibly melanized. MARS were formed not only on apple leaves but also on leaves of nonhost plants and artificial surfaces differing in hydrophobicity; the formation of appressoria and MARS was confined to hard surfaces. The melanin nature of the ring was confirmed by using melanin biosynthesis inhibitors. Applications prior to inoculation largely inhibited the melanization and reduced infection rate by 45 to 80%; curative applications were not effective. Transmission electron microscopy verified a localized melanization of the cell wall around the penetration pore, and melanin was incorporated into all layers of the fungal cell wall. Appressoria without MARS were not able to infect the plant, suggesting that this structure can be considered to be a pathogenicity factor in V inaequalis.