Corn infection chamber

Corn at the V6-V8 growth stage has an excellent infection chamber, the leaf whorl.  This ‘cup’, even in the driest conditions always has some free water due to transpiration in the newly developed leaf tissue.  Although growth pushes the current moist tissue into the drier atmosphere with 12 hours, this is enough time for many fungal pathogen spores to germinate, hyphae to set up their drilling stations (haustoria) and enter the leaf tissue. Evidence shows in the formation of a group of yellow spots in leaf tissue a few inches from the whorl as the plant reacts to the invaders.
 
Rust spores often move to the Midwest on upper winds, initially from the southern overwintering location and then move further by more local winds as they produce spores in other infected fields.  More local fungi, such as Exerohilum turcicum, cause of Northern Leaf Blight, and Bipolaris maydis (Southern corn Leaf Blight) or Bipolaris carbonum (Northern Leaf Spot), often produce spores on moist, infected debris from the previous years that move to the moist whorl, quickly penetrating the leaf before it becomes dry.  Yellow spots are seen within 48 hours.  Typical Northern Leaf Blight lesions are not seen for two weeks but then generally in a band across the leaf that was in the whorl when this infection occurred.  Further spread of the disease depends upon showers allowing for exposed leaf tissue to have a moist surface for spore germination and fungal penetration.
 
Those yellow spots represent the beginning of the plant’s reaction to the pathogen.  Host genetics affect how successful the plant is in preventing further spread of the fungus.