Penetration and infection of poplar leaves by urediniospores of Melampsora larici-populina and Melampsora medusae

Penetration and infection of poplar leaves by Melampsora larici-populina and M. medusae were studied by light, scanning, and transmission electron microscopy. Germtubes developed from urediniospores via pre-formed germpores and extended enteroblastically. Germtubes closely followed leaf surface features and occasionally penetrated epidermal cells directly causing leakage of cell contents. Such germtubes were not seen to establish infection. Germtubes generally entered leaves via stomata with or without formation of appressoria and formed sub-stomatal vesicles which were anchored in stomata by lugs protruding from the outer vesicle wall. The substomatal vesicle extended and three cells were delimited, the terminal cell being the primary haustorial mother cell. This cell formed an appressorium and established host infection. Later hyphae extended from cells subtending the primary haustorial mother cell and made contact and established infection in host cells lining the substomatal cavity.