Primary root and mesocotyl infection

 With the primary root growing below the seed and the first node of the stem (mesocotyl) pushing the coleoptile towards the light, vulnerabilities to infection by fungi becomes evident.  Eventually, the stem will produce new roots but until then the mesocotyl is essential to transport of the stored carbohydrates from the endosperm and minerals from the primary roots.  This is all occurring in the environment teeming with organisms dependent upon organic food sources.
 
Soil temperature and moisture plays a big role in the interactions between the corn physiology and the fungus activity.  Low temperatures lengthen the time of exposure of the germinating seed and at the same time favor fungi such as Pythium species.  Warmer temperatures favor rapid emergence and reduce the dependence upon the mesocotyl and primary root systems.  Warmth also increases the metabolic rate in the plant tissue that should improve the plants response to invasion by pathogens. 
 
Significance of fungal invasion of the mesocotyl is somewhat confusing, partly because of the complexity of studying seedling development in soils.  Fungi associated with seedling death such as Pythium species and Fusarium verticilliodesas as well as those involved in diseases that develop much later in the plant life such as Diplodia stalk rot or Head smut have been thought to initially invade the mesocotyl.
 
Corn genetics vary for resistance to these organisms and response to temperatures.  Seed germination quality varies with genetics, seed production environments, planting depth and soil conditions.  The fungi also are affected by their own genetics and environmental factors include temperature, moisture, previous crop and organic matter in the soil.  It is to the credit of everyone involved that we usually get the stands that we desire.