Corn seed environment. Mycorrhiza.

A few of the more beneficial fungi in soil form a symbiotic relationship with roots as mycorrhiza.  Ectomycorrhizal fungi tend to grow on the surface of the root tissue and between cells inside the root.  These fungi are most common among tree species and often revealed when they form mushrooms around near trees. 
 
Corn and other annual crops often have symbiosis with endomycorrhizal fungi that penetrate the root tissue, initially growing between cells as filaments (hyphae) and then forming special membrane-enclosed structures called haustoria that push against the cellular membranes. This allows efficient movement of minerals such as phosphorus, copper, and zinc from the fungus into the corn root cell and sugar molecules into the fungus.  Increased corn root water uptake also is associated with this symbiosis.
 
Many endomycorrhizal fungi produce spores that exist in soil until stimulated to germinate and grow towards roots.  They cannot live as saprophytes and are dependent upon on living root tissue.  This has made studying these fungi difficult because independent artificial culture is not easy. Most of these fungi are classified as a member of the Glomus genus and several species may be in the same field.  There are indications that the same species can infect corn and soybean, suggesting no reductions with crop rotation between these two crops.  Apparently, these fungi are more available to early corn infection in no-till than in deep tillage fields.
 
Mycorrhizal fungi are among the unseen environmental factors that contribute to the small variations that occur within the corn field.