Ultrastructure of the appressorium of Spilocaea pomi

The appressorium, seen after penetration of the apple leaf cuticle, has a two-layered wall, an electron-dense outer layer, and an electron-transparent inner layer. The pore of the appressoriumis bordered by the electron-dense outer layer, which tapers at the margin of the pore. The penetration hypha is derived from the infection sac, a structure or organelle within the appressorium.