Fungal appresorium

​Corn leaf epidermal cells are protected from the many fungi that would benefit from the cells’ nutrients.  Major protection comes from the hydrophobic wax deposited on the exposed surface of the epidermis.  A few fungal species have evolved methods to overcome the waxy protection. The spore of such a fungus germinates when moist, growing a hypha strand on leaf surface. Adhering with some mucilage released from the germ tube tip. It chemically detects and links to molecules in the wax.  At that point, the cytoplasm of the spore flows into the swelling, special hyphae now called an appresorium.   Now a barrier (septum) develops cutting off the spore from the growing appresorium.
 
Turgor pressure in the appresorium hyphae increases as more water moves by osmosis into the structure. The region of the appresorium adjacent to the leaf cell wax produces a specialized hypha called a penetration peg through the wax and into the host epidermal cell.  From there haustoria develop, penetrating cells and absorbing more nutrients. The northern leaf blight corn pathogen Exserohilum turcicum uses this basic method to infect leaves as the hyphae branch to spread elsewhere in the leaf. Most corn leaf pathogens use this basic method to enter the plant, although the rust fungi enter the stomata instead of drilling through the wax.
 
This first line of the corn plant’s defense is effective against the many saprophytic fungi of the corn plant’s environment but more interactive defense systems are stimulated into action after the pathogens have penetrated the leaf epidermal wax.