Pythium in corn seedlings

​Corn seedlings are most vulnerable to damage before the emergence of the secondary roots.  Pythium species are the most frequent pathogens at this stage, especially invading the mesocotyl tissue and thus interfering the transfer of minerals and carbs to the coleoptile before in emerges.
 
Pythium looks like a fungus, but it differs in structure and chemical constituents, making it have a closer relationship to brown algae.  Among the distinctive features are swimming spores.  Another feature common to Pythium and its relatives is an oospore produced after fertilization of the egg cell.  Oospores develop thick walls allowing the organism to withstand dry and cold conditions in soil until water returns.  Oospores germinate when exposed to water, producing filaments similar to fungal hyphae except Pythium filaments are mostly without septa that wall off individual cells. While exposed to water, sporangia form within the filaments to produce swimming spores called zoospores.  These spores feature two flagella allowing them to swim towards potential host roots, assumedly stimulated by exudates from potential host roots.  
 
Many Pythium species infect grass roots, including corn with a range of ability to overcome host tissue resistance.  A study in southeastern Iowa in 2012 Showed Pythium torulosumas the most frequent Pythium species found among the nine found associated with seedling disease in season favoring the disease (https://crops.extension.iastate.edu/cropnews/2013/04/nine-species-pythium-associated-corn-seeding-blight-southeastern-iowa).
 
Mesocotyl tissue is generally most vulnerable to attack by Pythium species.  Cool wet weather delays the elongation of the mesocotyl to the surface, prolonging the potential exposure and damage from this pathogen and yet encouraging Pythium to produce zoospores.  After infecting the seminal roots and mesocotyl tissues, basically causing collapse of the tissues.  Results could be seedling death or weak growth.  It is a primary cause of weak corn stands and uneven growth in areas of fields that have prolonged early season wetness.
 
Although Pythium species can invade more older corn plants when flooded for some time, it is mostly a seedling pathogen.  Seed treatments can protect the germinated seedling from Pythium, but there are isolates that are resistant to most seed treatments.  Although these pathogens are common in soils, conditions for significant damage is not common.