Biology of corn seedling diseases

​Corn seedlings face soil environments with multiple potential pathogens and saprophytes.  If stressed by cold wet weather that favor pathogens such as Pythium species, stimulated to grow towards seedlings leaking metabolites.  If warmer, Fusarium species likewise are attracted to the living plant tissue.  Resistance mechanism in the seedlings includes structures of the plant tissues and production of anti-fungal metabolites produced in the root and hypocotyl cells.  The latter is influenced by heat energy, cell vitality and seedling ‘vigor’.
 
If an individual plant survives these early potential problems to emerge with 2-3 leaves, as nodal roots take over and the hypocotyl and earlier roots decline, the plant will usually not show the early wilt symptoms.  Fusarium may have successfully penetrated earlier and make its way to the apical meristem without causing direct damage.  It has been shown that even if the Fusarium hyphae were in the seed before planting, it does little visible damage until perhaps making its way to the new kernels by harvest.
 
When seedlings wilt in the field, it is difficult to assign a single cause.  It is usually seen scattered among undamaged seedlings. Was it biological vigor or quality of that individual seed, microenvironment of that individual seed or scattered presence of a pathogen. Samples of dead seedlings will frequently show presence of Fusarium species but was it a cause of the seedling death or simply a quick invader of weakened plant tissue? This is not an easy problem to correctly analyze.