Fungal penetration of corn leaves

​Corn leaves are constantly surrounded by fungal spores during the whole season.  The waxy leaf cuticle and tight compaction of the epidermal cell walls inhibit penetration by the vast majority of fungi.  Even entrance into the leaf through open stomates is restricted by antimicrobial fumes from the cells inside the stomata.
 
A relative few fungi have methods to overcome these defense systems. Spores of Exserohilum turcicum, the pathogen causing northern leaf blight, germinated within a few hours of exposure to moisture on the leaf. The hyphae growing from the spore responds to the surface hardness producing mucilage promoting adhesion to the leaf surface.  Soon the tip of the hypha forms a special cell called an appresorium.  Exposed cell wall of the appresorium cell thickens but the wall adjacent to the leaf surface remains thin.  The cell gains turgor pressure from moisture as cytoplasm in appresorium cell absorbs glycerol from the fungal spore hyphae. Turgor pressure results in formation of new cell that forces through the leaf cuticle and through the epidermal cell walls. 
 
Penetration into the corn leaf by the pathogen causing northern corn leaf blight occurs about 10-12 hours after the spore and moisture on the leaf surface. Favorable environment for spore germination and appresorium development usually is present in the whorl of pre-flowering plants, with fog, high humidity and dew formation. 
 
The battle between plant defense systems and pathogen offense continues.