Corn pith senescense

Corn is an annual plant, programed to senesce at the end of the season.  Humans have selected for the timing in those hybrids that become commercial to have the senescence delayed as long as possible to obtain maximum grain fill, mostly by delaying time to flowering to maximize photosynthesis.  Excess sugars are stored in the pith parenchyma cells, until moved to the developing grain.  As these cells are depleted of sugars, rate of senescence increases and they become more vulnerable to invasion by those fungi, such as Fusarium species, that produce enzymes capable of digesting the cell walls. 
 
Initial depletion of sugars is from the top and bottom internodes of the stem. As the top internode pith parenchyma cells senesce, the tissue is invaded, possible beginning at the internode.  A common invader, especially of the rind epidermis, is Colletotrichum graminarium, the fungus that causes anthracnose.  The result is small black streaks in the epidermis.  The pith tissue, may be invaded by Fusarium species.  The outward sign is the turning of the upper leaf color from green to gray or white.
 
For the most part, this turning of color of the upper leaf is a good sign because it indicates that the grain was pulling available carbohydrate.  Plants that have white-colored, senescent upper leaves, drying and open husks and green lower leaves are expressions of maximum yield for that hybrid for that season’s environment.  Those plants in which all the leaves are brown, and the lower stalk color is brown, indicate the grain fill demand was too great for the photosynthesis products available, with the roots depleted of sugars and consequently dead before completion of grainfill.  The plant wilted because of lack of water uptake through the dead and rotted root tissue.