Pythium

The genus Pythium is usually called a fungus although it and its relatives may not share the same origin as most fungi.  Several species of this genus have the ability to infect corn and soybeans. P. ultimum is one that is found in most fields.  Like many Pythium species, it forms thick walled spores (oospores) that remain dormant in the soil until stimulated to germinate, form a special structure (sporangium) that produces swimming spores (zoospores).  
 
The stimulant to cause germination? Leakage from germinating seed! Which seed has the most leakage, you ask?  It is those with deteriorating membranes, of course. Having cracks in the pericarp near the embryo is also associated with more leakage of metabolites from the germinating seed but most research supports the membrane damage as most significant, partly because leakage occurs through intact pericarp as well.  Pythium ultimum is favored by water logged soils and temperatures around 50°F, the same conditions that slow down the membrane repair in imbibing seed.
 
Pythium is easy to isolate from virtually any soil because of its wide host range.  It must infect roots of more plants than obvious but the plants have some resistance to stop the spread and vigorous seedlings soon outgrow the pathogen.  Drying of soil and higher spring temperatures soon favor the plant. 
 
There is one species of Pythium, P. aphanidermatum, favors higher temperatures and water soaked soils and occasionally can infect green stalks of corn causing severe lodging. It is not common for the conditions favoring this pathogen to occur in U.S.A. corn fields at midseason, and there is resistance among hybrids, reducing the occurrence of his disease.